Image forming device

Abstract

To resolve, with a simple configuration, "end dirt" and "image deletion" which are difficult to be prevented at the same time in terms of a discharge amount.SOLUTION: An image forming apparatus 100 has a photoreceptor 1, an electrifying member 2, an electrifying power supply 21, a developing member 4a, a transfer member 5, a transfer power supply 18, acquisition units 19, 40 that acquire information on the electric resistance value of the transfer member 5, and a control unit 40 that can control the electrifying power supply 21. When the electric resistance value of the transfer member 5 indicated by the information acquired by the acquisition units is a first resistance value, the control unit 40 determines an electrification voltage applied in an image forming operation as a first electrification voltage, and when the electric resistance value of the transfer member 5 indicated by the information acquired by the acquisition units is a second resistance value lower than the first resistance value, determines the electrification voltage applied in the image forming operation as a second electrification voltage having an absolute value larger than the absolute value of the first electrification voltage.SELECTED DRAWING: Figure 4

Description

[Technical Field]

[0001] The present invention relates to an image forming apparatus such as a printer, a copying machine, or a facsimile machine that uses an electrophotographic method. [Background technology]

[0002] In a conventional image forming apparatus using an electrophotographic system, the surface of an electrophotographic photosensitive member (hereinafter simply referred to as "photosensitive member") is uniformly charged by a charging unit, and a dark potential is formed on the surface of the photosensitive member. The charged surface of the photosensitive member is then exposed by an exposure unit, and a bright potential is formed on the surface of the photosensitive member, and an electrostatic latent image is formed on the photosensitive member based on the contrast between the dark potential and the bright potential. Toner is then attached to the electrostatic latent image formed on the photosensitive member by a developing unit, and a toner image is formed on the photosensitive member.

[0003] The toner image formed on the photosensitive member is transferred to a recording material by a transfer means. A transfer roller, a roller-shaped transfer member, is often used as the transfer means. The transfer roller contacts the photosensitive member, forming a transfer nip, and rotates while contacting the photosensitive member. The transfer roller conveys the recording material by sandwiching it between the photosensitive member and the transfer roller, and transfers the toner on the photosensitive member onto the recording material. During transfer, a transfer voltage of opposite polarity to the normal polarity of the toner is applied to the transfer roller, and the toner image on the photosensitive member is electrostatically transferred onto the recording material. Note that, although the recording material will be referred to as "paper" below, this is not limited to paper. For convenience, the high / low or increase / decrease in potential or voltage refers to the high / low or increase / decrease in absolute value of the potential or voltage.

[0004] The transfer nip has a region through which the recording material passes and a region through which the recording material does not pass, in a direction substantially perpendicular to the movement direction of the photosensitive member surface (the conveyance direction of the recording material). Here, the region through which the recording material passes in the transfer nip is referred to as the "paper-passing region," and the region through which the recording material does not pass is referred to as the "non-paper-passing region." For convenience, the regions on the photosensitive member corresponding to the "paper-passing region" and "non-paper-passing region" of the transfer nip are also referred to as the "paper-passing region" and "non-paper-passing region," respectively. Non-paper-passing regions generally occur outside the paper-passing region, adjacent to both ends of the paper-passing region in a direction substantially perpendicular to the movement direction of the photosensitive member surface. Because the recording material acts as an electrical resistor when transferring a toner image from the photosensitive member to the recording material, the transfer current tends to concentrate and flow in the non-paper-passing region. Therefore, after transfer, the surface potential of the photosensitive member in the non-paper-passing region is lower than the surface potential of the photosensitive member (non-exposed portion) in the paper-passing region.

[0005] The potential difference between the surface potential of the photoconductor in the non-paper-passing area and the surface potential of the photoconductor in the paper-passing area after transfer is eliminated during the subsequent charging process of the photoconductor surface. However, if the potential difference is large, a potential difference between the surface potential of the photoconductor in the non-paper-passing area and the surface potential of the photoconductor in the paper-passing area may remain even after charging. As a result, during development, a difference in the potential relationship between the photoconductor and the developer in the non-paper-passing area may occur. In particular, in the non-paper-passing area, the potential difference between the non-exposed portion of the photoconductor and the developer may become small, causing a phenomenon known as "fogging," in which toner adheres to the surface of the photoconductor. Furthermore, if the recording material is transported at an angle during transfer, the toner that has adhered to the surface of the photoconductor in the non-paper-passing area due to "fogging" may adhere to the edges of the recording material in a direction approximately perpendicular to the transport direction (the width direction of the recording material), resulting in "edge contamination" that soils the recording material.

[0006] Patent Document 1 discloses a configuration in which a pre-exposure means for exposing the surface of the photoconductor is provided downstream of the transfer nip in the rotation direction of the photoconductor and upstream of the charging position by the charging means. By providing the pre-exposure means, the potential difference between the surface potential of the photoconductor in the non-paper passing area and the surface potential of the photoconductor in the paper passing area, which occurs in the transfer nip, can be eliminated, thereby suppressing "edge contamination." [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Publication No. 6-83249 Summary of the Invention [Problem to be solved by the invention]

[0008] However, if the potential difference between the surface potential of the photoconductor in the non-paper-passing area and the surface potential of the photoconductor in the paper-passing area, which occurs at the transfer nip, is eliminated by, for example, using a pre-exposure device as described above, the amount of discharge to the photoconductor increases during charging. This increased amount of discharge to the photoconductor increases the likelihood of image defects known as "image deletion." "Image deletion" refers to an image defect in which discharge products generated during discharge adhere to the photoconductor. These discharge products absorb moisture, preventing the photoconductor from properly charging, resulting in a decrease in image density. In recent years, image forming devices have become longer-lasting, and the tendency for the amount of discharge to the photoconductor to increase with extended use is also a contributing factor to the likelihood of image deletion.

[0009] In this way, there is a trade-off between "edge staining" and "image deletion" from the viewpoint of the amount of discharge, and it is desirable to achieve both of these suppressions.

[0010] SUMMARY OF THE INVENTION An object of the present invention is to suppress edge staining while suppressing image deletion. [Means for solving the problem]

[0011] The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention provides an image forming apparatus including a rotatable photosensitive member, a charging member for charging the surface of the photosensitive member, a charging power source for applying a charging voltage to the charging member during the charging process, a developing member for supplying toner to the surface of the photosensitive member that has been charged to form a toner image, a transfer member that contacts the surface of the photosensitive member to form a transfer section and transfers the toner image from the surface of the photosensitive member to a recording material that passes through the transfer section, a transfer power source for applying a transfer voltage to the transfer member, and an acquisition section for acquiring information regarding the electrical resistance value of the transfer member. another acquiring unit that acquires information about the electrical resistance value of the recording material; and a control unit capable of controlling the charging power source, wherein the control unit controls the charging voltage to be applied in an image forming operation to form the toner image on the recording material to be a first charging voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a first resistance value, and controls the charging voltage to be applied in the image forming operation to be a second charging voltage having an absolute value larger than that of the first charging voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value. and when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is the second resistance value, if the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a third resistance value, the charging voltage is controlled to be the second charging voltage, and when the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a fourth resistance value lower than the third resistance value, the charging voltage is controlled to be a third charging voltage whose absolute value is smaller than that of the second charging voltage. The image forming apparatus is characterized by the above.

[0012] According to another aspect of the present invention, there is provided a device for controlling a developing power supply, which applies a developing voltage to the developing member when forming the toner image, a transfer member that contacts the surface of the photoreceptor to form a transfer section and transfers the toner image from the surface of the photoreceptor to a recording material passing through the transfer section, a transfer power supply that applies a transfer voltage to the transfer member, an acquisition section that acquires information about an electrical resistance value of the transfer member, and a control section that can control the developing power supply. and a control unit, wherein the control unit controls the developing voltage to be applied in the image forming operation of forming the toner image on the recording material to be a first developing voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a first resistance value, and controls the developing voltage to be applied in the image forming operation to be a second developing voltage whose absolute value is smaller than that of the first developing voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value. [Effects of the Invention]

[0013] According to the present invention, it is possible to suppress image deletion and edge staining. [Brief explanation of the drawings]

[0014] [Figure 1] FIG. 1 is a schematic cross-sectional view of an image forming apparatus. [Figure 2] FIG. 2 is a schematic diagram showing the positional relationship in the longitudinal direction of each part around the photosensitive drum. [Figure 3] FIG. 4 is a graph illustrating the transition of the surface potential of the photosensitive drum. [Figure 4] FIG. 4 is a graph illustrating the transition of the surface potential of the photosensitive drum. [Figure 5] FIG. 10 is a graph showing the relationship between Vback and fog density. [Figure 6] FIG. 2 is a flowchart of control in the first embodiment. [Figure 7]FIG. 10 is a flowchart of the control in the second embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0015] The image forming apparatus according to the present invention will be described in more detail below with reference to the drawings.

[0016] [Example 1] (1) Image forming device 1 is a schematic cross-sectional view of an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic laser printer, and is capable of forming an image on a recording material P such as paper or plastic film in accordance with image information input from an external device (not shown) such as a personal computer. First, the configuration of the image forming apparatus 100 of this embodiment will be described.

[0017] The image forming apparatus 100 includes a photosensitive drum 1, a drum-type (cylindrical) photosensitive body serving as an image carrier, inside the apparatus main body M. The photosensitive drum 1 is constructed by providing a photosensitive material such as an OPC (organic photoconductor), amorphous selenium, or amorphous silicon on a cylindrical drum substrate made of aluminum, nickel, or the like. The photosensitive drum 1 used in this embodiment is a negatively charged OPC photosensitive body with an outer diameter of 24 mm. This photosensitive drum 1 is constructed by a photosensitive layer formed by stacking a charge generation layer and a charge transport layer, in this order, on the surface of a conductive substrate made of an aluminum cylinder.

[0018] The following devices are arranged around the photosensitive drum 1 in the rotation direction Rd. First, a charging roller 2, a roller-shaped charging member, is arranged as a charging device. The charging roller 2 is composed of, for example, a conductive base shaft (core) that also serves as a power supply electrode and an elastic layer cylindrically surrounding the outer periphery of the base shaft. The charging roller 2 used in this embodiment is an elastic roller with an outer diameter of 10 mm, a core diameter of 5 mm, and a thickness of 2.5 mm for the elastic layer. In this embodiment, the core is made of SUS, and the elastic layer is made of a rubber mixture of NBR and epichlorohydrin. The charging roller 2 is pressed against the photosensitive drum 1 and rotates in accordance with the rotation of the photosensitive drum 1. In the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where charging is performed by the charging roller 2 is the charging position Pa. The charging roller 2 charges the surface of the photosensitive drum 1 by discharge occurring in at least one of minute gaps formed on the upstream side and downstream side 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 contact point between the charging roller 2 and the photosensitive drum 1 may be considered to be the charging position Pa.

[0019] Next, an exposure device 3 is arranged as an exposure means. In this embodiment, the exposure device 3 is configured as a laser scanner device (laser optical system). With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where exposure by the exposure device 3 is performed is an exposure position Pb.

[0020] Next, a developing device 4 is arranged as a developing means. In this embodiment, the developing device 4 uses a non-magnetic one-component developer (toner) as the developer. This developing device 4 has a developing roller 4a as a developer carrier (developing member). The developing roller 4a abuts against the surface of the photosensitive drum 1 during development, and supplies toner to a developing section that faces (contacts) the photosensitive drum 1. Note that the developing device 4 may use a magnetic one-component developer (toner) or a two-component developer containing toner and a carrier as the developer. With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the toner is supplied by the developing roller 4a (the position where it abuts against the developing roller 4a in this embodiment) is the development position Pc.

[0021] Next, a transfer roller 5, a roller-shaped transfer member (transfer rotor) serving as a transfer means, is arranged. The transfer roller 5 is urged (pressed) toward the photosensitive drum 1 by a transfer pressure spring (not shown), a biasing member serving as a biasing means, and is pressed against the photosensitive drum 1. This forms a transfer nip (transfer nip) Nt, which is the contact area between the photosensitive drum 1 and the transfer roller 5. The transfer roller 5 rotates in accordance with the rotation of the photosensitive drum 1. The transfer roller 5 conveys a recording material P by nipping it between itself and the photosensitive drum 1, and transfers a toner image from the photosensitive drum 1 to the recording material P when a voltage is applied. The transfer roller 5 is composed of, for example, a conductive shaft (core metal) that also serves as a power supply electrode and a cylindrical elastic layer that surrounds the outer periphery of the shaft. This elastic layer is generally made of a semiconductive rubber material such as EPDM, NBR, urethane rubber, epichlorohydrin, or silicone rubber. The material of the elastic layer may contain an appropriate amount of a conductive agent, for example, an ionic conductive agent. The transfer roller 5 used in this embodiment is an elastic roller with a roller outer diameter of 14 mm, a core diameter of 5 mm, and an elastic layer thickness of 4.5 mm. In this embodiment, the core is made of SUS, and the elastic layer is made of a mixed rubber material of NBR and epichlorohydrin. In this embodiment, the contact pressure of the transfer roller 5 against the photosensitive drum 1 is 9.8 N (1 kgf). In this embodiment, the electrical resistance value of the transfer roller 5 (hereinafter simply referred to as "resistance value") is 2.0 x 10 when the transfer roller 5 is pressed against an aluminum cylinder with a force of 9.8 N, rotated at 50 mm / sec, and +1000 V is applied. 8 Ω. Note that this resistance value of the transfer roller 5 is the resistance value when the transfer roller 5 is left in an environment of normal temperature and normal humidity at the beginning of use (when new). With respect to the rotation direction of the photosensitive drum 1, the position where the toner image is transferred to the recording material P on the photosensitive drum 1 (the position corresponding to the transfer nip portion Nt) is the transfer position Pd.

[0022] Next, a static elimination needle 20 is arranged as a static elimination member that eliminates excess charge on the surface of the recording material P after transfer and reduces potential unevenness on the photosensitive drum 1 caused by peeling discharge. The static elimination needle 20 may be a static elimination needle with a sharp sawtooth tip and made of a thin metal plate such as a stainless steel plate or an aluminum plate that has good conductivity. The static elimination needle 20 is arranged downstream of the transfer roller 5 in the conveyance direction of the recording material P so that the tip of the needle faces the surface of the photosensitive drum 1.

[0023] Next, a cleaning device 6 is provided as a cleaning means for removing deposits such as toner (transfer residual toner) remaining on the photosensitive drum 1 after transfer. In this embodiment, the cleaning device 6 has a cleaning blade 6a as a cleaning member that is arranged to abut against the surface of the photosensitive drum 1. With respect to the rotation direction of the photosensitive drum 1, the position on the photosensitive drum 1 where the cleaning blade 6a removes toner (in this embodiment, the position where the cleaning blade 6a abuts) is the cleaning position Pe.

[0024] Further, at the bottom of the apparatus main body M in the figure, a recording material cassette 7 is arranged to store recording materials (transfer materials, recording media, sheets) P ​​such as paper. Further, arranged in this order from the recording material cassette 7 along the conveyance path of the recording material P are a feed roller 8, a conveyance roller 9, a top sensor 10, a pre-transfer conveyance guide 15, a transfer-fixing conveyance guide 11, a fixing device 12, discharge rollers 13, and a discharge tray 14. Further, the apparatus main body M is provided with a control unit 40 that controls the image forming apparatus 100.

[0025] Next, the image forming operation of the image forming apparatus 100 of this embodiment will be described. The photosensitive drum 1 is driven by a drive source (not shown) to rotate in the direction of arrow Rd (clockwise) at a peripheral speed (process speed) of 320 mm / sec. The surface of the rotating photosensitive drum 1 is uniformly charged by the charging roller 2 to a predetermined potential (dark potential, charging potential) of the same polarity as the normal charging polarity of the toner (negative in this embodiment). During charging, a charging voltage (charging bias), which is a negative DC voltage, is applied to the charging roller 2 from a charging power source (high-voltage power source) 21 via a charging current detection circuit 22. In this embodiment, as an example, a charging voltage of −1100 V is applied to the charging roller 2, and a dark potential of −500 V is formed on the surface of the photosensitive drum 1.

[0026] The surface of the charged photosensitive drum 1 is scanned and exposed by the exposure device 3 with image light L according to image information, and an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. In this embodiment, the charge on the photosensitive drum 1 in the portion exposed by the exposure device 3 is removed, and a bright area potential of -100 V is formed on the surface of the photosensitive drum 1. As a result, an electrostatic latent image is formed on the photosensitive drum 1 with a contrast between the dark area potential and the bright area potential.

[0027] The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) by the developing device 4 supplying toner, and a toner image (toner image, developer image) is formed on the photosensitive drum 1. During development, a developing voltage (developing bias), which is a DC voltage of the same polarity (negative in this embodiment) as the normal charging polarity of the toner, is applied to the developing roller 4a from a developing power supply (high-voltage power supply) 16. In this embodiment, as an example, a developing voltage of −380 V is applied to the developing roller 4a. In this embodiment, toner charged to the same polarity (negative in this embodiment) as the charging polarity of the photosensitive drum 1 adheres to the exposed portion (image portion) of the photosensitive drum 1, which has been subjected to a substantially uniform charging process and then exposed to light, thereby reducing the absolute value of the potential (reverse development method).

[0028] The toner image formed on the photosensitive drum 1 is transferred onto the recording material P at the transfer nip Nt by the action of the transfer roller 5. During transfer, a transfer voltage (transfer bias), which is a DC voltage of opposite polarity (positive in this embodiment) to the normal charging polarity of the toner, is applied to the transfer roller 5 from a transfer power supply (high-voltage power supply) 18 via a transfer current detection circuit 19 serving as a transfer current detection means. This causes the toner image on the photosensitive drum 1 to be electrostatically transferred to a predetermined position on the recording material P. The recording material P is stored in a recording material cassette 7 serving as a recording material storage unit and is fed one sheet at a time by a feed roller 8 serving as a feeding member. The recording material P is transported by a transport roller 9 serving as a transport member and supplied to the transfer nip Nt along a pre-transfer transport guide 15 serving as a guide member. The transport roller 9 supplies the recording material P to the transfer nip Nt in synchronization with the toner image on the photosensitive drum 1, based on the detection result of the leading edge of the recording material P in the transport direction by a top sensor 10 serving as a recording material detection means.

[0029] The recording material P, onto which the toner image has been transferred at the transfer nip Nt, has excess charge on its surface removed by a charge removal needle 20. After passing through the charge removal needle 20, the recording material P is transported along a transfer / fixing transport guide 11, which serves as a guide member, to a fixing device 12, which serves as a fixing means. The fixing device 12 has a fixing roller 12a incorporating a heater, and a pressure roller 12b that presses against the fixing roller 12a. The fixing device 12 applies heat and pressure to the recording material P carrying an unfixed toner image that passes through the nip between these rollers, thereby fixing the toner image onto the recording material P.

[0030] In the case of single-sided image formation, the recording material P, after the toner image has been fixed on one side by the fixing device 12, is discharged (output) by discharge rollers 13 onto a discharge tray 14 formed on the top surface of the apparatus main body M in the drawing. In this embodiment, the image forming apparatus 100 is also capable of double-sided image formation (automatic double-sided printing), and is provided with a double-sided conveying mechanism 30. The double-sided conveying mechanism 30 includes a flapper 31 as a conveying path switching unit, a double-sided conveying path 32, and double-sided conveying rollers 33 as a conveying member. In the case of double-sided image formation, before the trailing end of the recording material P, with the toner image fixed on its first side, passes through the discharge rollers 13, the rotation of the discharge rollers 13 is reversed and the position of the flapper 31 is changed, so that the recording material P is guided to the double-sided conveying path 31. The recording material P is conveyed to the conveying rollers 9 by the double-sided conveying rollers 33 and the like. Then, a toner image is transferred and fixed on the second side of the recording material P in the same manner as on the first side. Thereafter, the recording material P with the toner images fixed on both sides is discharged onto a discharge tray 14 by a discharge roller 13.

[0031] On the other hand, deposits such as toner (transfer residual toner) that are not transferred to the recording material P during transfer and remain on the surface of the photosensitive drum 1 are removed and collected by the cleaning device 6. The cleaning device 6 scrapes off deposits such as transfer residual toner from the surface of the rotating photosensitive drum 1 using a cleaning blade 6a and collects them in a container of the cleaning device 6.

[0032] By repeating the above operation, it is possible to form images one after another. In this embodiment, the image forming apparatus 100 can execute printing at a print speed of 60 sheets per minute.

[0033] In this embodiment, the photosensitive drum 1, the charging roller 2 as a process means acting on the photosensitive drum 1, the developing device 4, and the cleaning device 6 are integrally formed into a process cartridge that is detachably attached to the device main body M of the image forming device 100.

[0034] The control unit 40 has a CPU 41 as an arithmetic and control means, a ROM 41a and a RAM 41b as storage means, an input / output unit (not shown) that controls the exchange of signals between the control unit 40 and each unit outside the control unit 40, and the like. The CPU 41 controls various operations related to image formation by executing various programs stored in the ROM 41a, while using the RAM 41b as a work area. The ROM 41a also stores a data table of various preset control values ​​(operation settings) and information on various preset threshold values, which are used in controlling the image formation conditions described below.

[0035] The image forming apparatus 100 executes a print job (printing operation, print job), which is a series of operations initiated by a single start command to form and output images on one or multiple recording materials P. A print job generally includes an image formation process, a pre-rotation process, an inter-sheet 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 of the image to be actually formed and output on the recording material P is formed, a toner image is formed, and the toner image is transferred. This period is referred to as the image formation time. More specifically, the timing of the image formation time differs depending on the positions where the electrostatic latent image formation, toner image formation, and toner image transfer processes are performed, and corresponds to the period during which the image formation area on the photosensitive drum 1 passes through each of the above positions. The pre-rotation process is a period during which preparatory operations are performed before the image formation process, from when a start command is input until the actual start of image formation. The inter-sheet process (inter-image process, inter-recording material process) is a period corresponding to the interval between recording materials P when image formation is performed continuously on multiple recording materials P (continuous image formation). The post-rotation process is a period during which a tidying operation (preparatory operation) is performed after the image formation process. Non-image formation time refers to periods other than image formation, including the pre-rotation process, the sheet-to-sheet process, the 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 the image forming apparatus 100 returns from a sleep state. More specifically, the timing of non-image formation corresponds to the period during which the non-image formation area on the photosensitive drum 1 passes through each position where the electrostatic latent image formation, toner image formation, and toner image transfer processes are performed. The image formation area on the photosensitive drum 1 or the recording material P is an area where a toner image, which is predetermined according to the size of the recording material P and output from the image forming apparatus 100, can be formed. The non-image formation area is an area other than the image formation area. In this embodiment, margins, which are non-image formation areas, are provided in predetermined areas at the leading and trailing ends of the recording material P in the conveyance direction of the recording material P.

[0036] (2) Longitudinal positional relationship 2 is a schematic diagram for explaining the positional relationship of various parts around the photosensitive drum 1 in a direction substantially perpendicular to the movement direction of the surface of the photosensitive drum 1 (the conveyance direction of the recording material P). This positional relationship varies depending on the size of the recording material P used for image formation (particularly the width in the direction substantially perpendicular to the conveyance direction), but FIG. 2 shows the positional relationship when the recording material P is A4 size. Note that the direction substantially perpendicular to the movement direction of the surface of the photosensitive drum 1 (the conveyance direction of the recording material P) (i.e., the direction substantially parallel to the rotation axis direction of the photosensitive drum 1) is sometimes referred to as the "longitudinal direction."

[0037] In FIG. 2, "photosensitive region A" indicates the region in the longitudinal direction where the photosensitive layer of the photosensitive drum 1 is formed, or the width of that region. Also in FIG. 2, "charging region B" indicates the region in the longitudinal direction where the charging roller 2 can contact the surface of the photosensitive drum 1, or the width of that region. Also in FIG. 2, "transfer region C" indicates the region in the longitudinal direction where the transfer roller 5 can contact the surface of the photosensitive drum 1, or the width of that region. Also in FIG. 2, "paper passing region D" indicates the region in the transfer nip Nt where the recording material P passes, or the width of that region. Also in FIG. 2, "non-paper passing region E" indicates the region in the transfer nip Nt where the recording material P does not pass, or the width of that region (i.e., the region that is the difference between the transfer region C and the paper passing region D, or the width of that region). For convenience, the areas on the photosensitive drum 1 corresponding to the above-mentioned "charged area B," "transfer area C," "paper passing area D," and "non-paper passing area E" will also be referred to as "charged area B," "transfer area C," "paper passing area D," and "non-paper passing area E," respectively.

[0038] In this embodiment, the photosensitive region A, charging region B, transfer region C, and paper passage region D are arranged so that their longitudinal centers are approximately aligned with the longitudinal center of the image formation region (region where a toner image can be formed) (center reference). Of the above regions, those with relatively shorter longitudinal widths are contained within those with relatively longer widths. Note that Figure 2 illustrates the range from the longitudinal center to one end.

[0039] (3) Surface potential difference in the longitudinal direction of the photosensitive drum Next, the transition of the surface potential of the photosensitive drum 1 during image formation will be described using FIG. 3. In FIG. 3, the horizontal axis indicates the position on the photosensitive drum 1 in the longitudinal direction, and illustrates the aforementioned transfer region C, paper passage region D, and non-paper passage region E. Also, in FIG. 3, the vertical axis indicates the surface potential of the photosensitive drum 1, and indicates that the surface potential of the photosensitive drum 1 becomes more negative (i.e., the absolute value of the negative surface potential is larger) toward the top of the figure. Also, the surface potential of the photosensitive drum 1 in paper passage region D here is the surface potential of the non-exposed portion of the photosensitive drum 1. Also, the surface potential of the photosensitive drum 1 shown in FIG. 3, which will be described below, is a value that can change depending on various conditions, such as the environment, the type of recording material P, and the resistance value of the transfer roller 5. Here, an example is shown in which the resistance value of the transfer roller 5 is relatively low. The influence of the resistance value of the transfer roller 5 will be further explained in Section (6) below.

[0040] First, State 1 shows the surface potential of the photosensitive drum 1 after charging (and before transfer). In State 1, the surface of the photosensitive drum 1 is charged approximately uniformly to a predetermined dark potential Vd by the charging roller 2 to which a predetermined charging voltage is applied. In this embodiment, as an example, a charging voltage of −1100 V is applied to the charging roller 2 during charging, and the surface of the photosensitive drum 1 is charged to a dark potential Vd of −500 V.

[0041] Next, State 2 shows the surface potential of the photosensitive drum 1 after transfer (and before charging). When the recording material P passes through the transfer nip Nt, a positive transfer voltage is applied to the transfer roller 5 at the transfer nip Nt. This causes the surface potential of the photosensitive drum 1 in the transfer area C to drop. This causes a potential difference between the surface potential of the photosensitive drum 1 in the paper passage area D and the surface potential of the photosensitive drum 1 in the non-paper passage area E. This is because the recording material P acts as an electrical resistor when the toner image is transferred from the photosensitive drum 1 to the recording material P, so the transfer current tends to flow concentratedly in the non-paper passage area E. In this embodiment, as an example, after transfer, the surface potential of the photosensitive drum 1 in the paper passage area D is −400 V, and the surface potential of the photosensitive drum 1 in the non-paper passage area E is −270 V. In State 2, the potential difference between the surface potential of the photosensitive drum 1 in the paper passage area D and the surface potential of the photosensitive drum 1 in the non-paper passage area E is 130 V.

[0042] Next, State 3 shows the surface potential of the photosensitive drum 1 after recharging (and before transfer). As described above, the surface of the photosensitive drum 1 is charged again by the charging roller 2 in a state where a potential difference occurs between the surface potential in the paper passing area D and the surface potential in the non-paper passing area E. In State 3, a predetermined charging voltage (-1100 V) is applied to the charging roller 2 as described above. After recharging, the surface potential of the photosensitive drum 1 returns to the predetermined dark potential Vd (-500 V) in the paper passing area D as described above, but in the non-paper passing area E it is -430 V and does not return to the predetermined dark potential Vd. In State 3, the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E is 70 V.

[0043] As described above, the surface potential of the photosensitive drum 1 has a potential difference between the paper passing area D and the paper non-passing area E.

[0044] Next, a method for reducing the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E will be described. In this embodiment, the potential difference is reduced by increasing the charging voltage applied to the charging roller 2.

[0045] The surface potential of the photosensitive drum 1 after transfer in State 2 is lower in the non-paper passing area E than in the paper passing area D. Therefore, in the recharging process, the non-paper passing area E, which has a relatively large potential difference with the charging roller 2, is charged more than the paper passing area D, which has a relatively small potential difference with the charging roller 2. This tendency becomes more pronounced when the charging voltage applied to the charging roller 2 is increased. Therefore, by increasing the charging voltage applied to the charging roller 2, the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E can be reduced.

[0046] 4 shows the change in the surface potential of the photosensitive drum 1 when the charging voltage applied to the charging roller 2 is increased. The horizontal and vertical axes in FIG. 4 have the same meanings as those in FIG. 3, respectively.

[0047] First, State 1 shows the surface potential of the photosensitive drum 1 after charging (and before transfer). As an example, a charging voltage of -1150V is applied to the charging roller 2. In other words, the charging voltage is increased by 50V on the negative side compared to the case in Figure 3 (i.e., the absolute value of the negative charging voltage is increased). As a result, the surface potential (dark area potential) of the photosensitive drum 1 after charging in State 1 is -550V.

[0048] Next, State 2 shows the surface potential of the photosensitive drum 1 after transfer (and before charging). As in the case of FIG. 3, because the recording material P passes through the transfer nip portion Nt, a potential difference occurs between the surface potential of the photosensitive drum 1 in the paper passing region D and the surface potential of the photosensitive drum 1 in the non-paper passing region E. However, unlike the case of FIG. 3, the surface potential of the photosensitive drum 1 in the paper passing region D is −435 V, and the surface potential of the photosensitive drum 1 in the non-paper passing region E is −335 V. In this way, because the charging voltage is increased to increase the surface potential of the photosensitive drum 1, the surface potential of the photosensitive drum 1 in the non-paper passing region E is higher than in the case of FIG. 3. In State 2, the potential difference between the surface potential of the photosensitive drum 1 in the paper passing region D and the surface potential of the photosensitive drum 1 in the non-paper passing region E is 100 V.

[0049] Next, State 3 shows the surface potential of the photosensitive drum 1 after recharging (and before transfer). As in the case of Figure 3, the surface potential of the photosensitive drum 1 after recharging returns to the predetermined dark potential Vd in the paper passing region D, but does not return to the predetermined dark potential Vd in the non-paper passing region E. However, because the charging voltage is increased, the surface potential of the photosensitive drum 1 in the non-paper passing region E is higher than in Figure 3. In State 3, the surface potential of the photosensitive drum 1 in the paper passing region D is -550 V, and the surface potential of the photosensitive drum 1 in the non-paper passing region E is -520 V. In State 3, the potential difference between the surface potential of the photosensitive drum 1 in the paper passing region D and the surface potential of the photosensitive drum 1 in the non-paper passing region E is 30 V.

[0050] As described above, when the charging voltage is increased (FIG. 4), compared to when the charging voltage is not increased (FIG. 3), the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E is reduced by 40 V. In this way, increasing the charging voltage is an effective means for reducing the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E.

[0051] (4) Edge stains Next, "edge staining" will be described. When the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area D and the surface potential of the photosensitive drum 1 in the non-paper passing area E increases, Vback, which is the potential difference between the photosensitive drum 1 and the developing roller 4a in the non-paper passing area E, decreases. This Vback is the potential difference between the dark area potential (surface potential of the non-exposed area) on the photosensitive drum 1 and the potential (developing voltage) of the developing roller 4a. When Vback decreases in the non-image area E, a phenomenon called "fogging" may occur, in which toner adheres to the surface of the photosensitive drum 1. Then, if the recording material P is transported at an angle during transfer, the toner that has adhered to the surface of the photosensitive drum 1 in the non-paper passing area E due to "fogging" may adhere to the widthwise edge of the recording material P, causing "edge staining" that soils the recording material P.

[0052] FIG. 5 shows the relationship between Vback and the degree of "fog." Fog on the photosensitive drum 1 was measured as follows: Toner was collected by attaching the adhesive side of a transparent adhesive tape to the photosensitive drum 1. The adhesive tape was then attached to a specific piece of paper, and the density (fog density (%)) of the toner-adhered adhesive tape was measured to quantify the fog. When no fog occurred, the fog density was 0%. A higher fog density indicates a greater degree of fog and a greater amount of toner adhered to the surface of the photosensitive drum 1. As shown in FIG. 5, in this embodiment, the degree of fog was smallest when Vback was around 120 V, with a fog density of 2%. This level of fog is difficult to see on the recording material P and does not pose a problem. On the other hand, when Vback is less than 120 V, the degree of fog (background fog) increases. When the fog density exceeds 6%, the image becomes highly visible when transferred to the recording material P, potentially reducing the quality of the output.

[0053] 3 and 4, the "fogging" after the initial charging (state 1) and after the recharging (state 3) will be described. In Fig. 3 and Fig. 4, the broken line indicates the potential of the developing roller 4a, and the potential difference between the surface potential of the photosensitive drum 1 and the potential of the developing roller 4a is Vback.

[0054] First, referring to FIG. 3, a case where a charging voltage of −1100 V is applied to the charging roller 2 will be described. In State 1 after the initial charging, the surface potential of the photosensitive drum 1 is approximately uniformly −500 V. Also, a developing voltage of −380 V is applied to the developing roller 4a. Therefore, Vback(1) is approximately uniformly 120 V, which is a level at which fogging is not a problem. In State 3 after recharging, the surface potential of the photosensitive drum 1 in the paper passage area D is −500 V, so Vback(2) is 120 V, which is a level at which fogging is not a problem, as in State 1. On the other hand, in State 3 after recharging, the surface potential of the photosensitive drum 1 in the non-paper passage area E is −430 V, so Vback(3) is 50 V, and the fogging concentration exceeds 6% (FIG. 5), which may cause edge contamination.

[0055] Next, referring to FIG. 4, we will explain the case where a charging voltage of −1150 V is applied to the charging roller 2. In State 1 after the initial charging, the surface potential of the photosensitive drum 1 is approximately uniform at −550 V. In this case, a developing voltage of −430 V is applied to the developing roller 4a to set Vback(4) to 120 V. Therefore, as in the case of FIG. 3 before the charging voltage was changed, the fogging level is not a problem. In State 3 after recharging, the surface potential of the photosensitive drum 1 in the paper passing region D is −550 V, and the surface potential of the photosensitive drum 1 in the non-paper passing region E is −520 V. In this case, in State 3, Vback(5) in the paper passing region D is 120 V, and as in State 1, the fogging level is not a problem. Furthermore, in State 3, Vback(6) in the non-paper passing region E is 90 V, and the fogging density does not exceed 6% (FIG. 5), so the fogging level is not a problem.

[0056] In this way, by increasing the voltage applied to the charging roller 2, it is possible to suppress edge contamination.

[0057] In this embodiment, the potential difference between the surface potential of the photosensitive drum 1 in the paper-passing area D and the surface potential of the photosensitive drum 1 in the non-paper-passing area E is reduced by increasing the applied voltage to the charging roller 2, but the present invention is not limited to this configuration. Fog, which causes edge staining, may be reduced by increasing Vback, the potential difference between the photosensitive drum 1 in the non-paper-passing area E and the developing roller 4a, by decreasing the applied voltage to the developing roller 4a. However, increasing Vback may cause toner charged with a polarity opposite to the normal polarity contained in the toner in the developing device 4 to adhere to the non-image area (non-paper-passing area), resulting in fog (reverse fog) (see FIG. 5 ). Therefore, increasing the applied voltage to the charging roller 2 to reduce the potential difference between the surface potential of the photosensitive drum 1 in the paper-passing area D and the surface potential of the photosensitive drum 1 in the non-paper-passing area E is a more preferable approach.

[0058] (5) Image flow Next, we will explain "image deletion." When the photosensitive drum 1 is charged, discharge products such as ozone and NOx are generated. A contact charging method like the one used in this embodiment is characterized by the fact that fewer discharge products are generated than with the non-contact corona charging method. However, with the contact charging method, discharge products are generated in the minute gap between the photosensitive drum 1 and the charging roller 2. Therefore, small amounts of discharge products such as ozone and NOx adhere to the surface of the photosensitive drum 1. When the discharge products adhered to the surface of the photosensitive drum 1 absorb moisture in a high-temperature, high-humidity environment, the charge retention ability of the surface of the photosensitive drum 1 decreases. This prevents the photosensitive drum 1 from being properly charged, resulting in an image defect known as "image deletion," which reduces image density. Furthermore, since the amount of discharge products adhering increases the longer the charging time, the greater the total number of images formed on the photosensitive drum 1, the greater the likelihood of image deletion.

[0059] One method for suppressing image flow is to perform the following image flow prevention sequence. The image flow prevention sequence is an operation in which, before image formation, the photosensitive drum 1 is rotated a relatively large number of times to increase the number of frictions between the photosensitive drum 1 and the cleaning blade 6a or the like in contact with the photosensitive drum 1, thereby removing discharge products from the surface of the photosensitive drum 1. However, if such an image flow prevention sequence is performed and the photosensitive drum 1 is rotated a relatively large number of times, the image formation operation will take longer. Therefore, it is desirable to minimize the frequency of execution of such an image flow prevention sequence.

[0060] As described above, it is conceivable to eliminate the potential difference between the surface potential of the photosensitive drum 1 in the non-paper passing area and the surface potential of the photosensitive drum 1 in the paper passing area, which occurs at the transfer nip Nt, by using a pre-exposure means or the like to neutralize the surface of the photosensitive drum 1. However, in this case, the increase in the amount of discharge to the photosensitive drum 1 during charging is relatively large, making image deletion more likely. Even when the charging voltage is increased to reduce the potential difference as described above, the amount of discharge to the photosensitive drum 1 in the non-paper passing area during charging increases, although to a lesser extent than when the pre-exposure means is used, making image deletion more likely. In recent years, the lifespan of image forming apparatuses 100 has been extended, and the tendency for the amount of discharge to the photosensitive drum 1 to increase with long-term use is also one factor that makes image deletion more likely.

[0061] In this way, there is a trade-off between "edge staining" and "image deletion" from the viewpoint of the amount of discharge, and it is desirable to achieve both of these suppressions.

[0062] (6) Resistance of the transfer roller Next, the influence of the resistance value of the transfer roller 5 on the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area and the surface potential of the photosensitive drum 1 in the paper non-passing area will be described.

[0063] As the resistance value of the transfer roller 5 decreases, the potential difference between the surface potential of the photosensitive drum 1 in the paper-passing area after transfer and the surface potential of the photosensitive drum 1 in the non-paper-passing area increases. This potential difference occurs because, when recording material P is present in the transfer nip Nt, the transfer current concentrates in the non-paper-passing area, avoiding the recording material P, which acts as an electrical resistor. This concentration of the transfer current in the non-paper-passing area is determined by the relationship between the resistance value of the recording material P and the resistance value of the transfer roller 5. There are two paths through which the transfer current flows from the transfer roller 5 to the photosensitive drum 1: one that flows to the photosensitive drum 1 via the recording material P, and one that flows to the photosensitive drum 1 without passing through the recording material P. When the resistance value of the transfer roller 5 is low, the transfer current flowing through the latter path (the path to the photosensitive drum 1 without passing through the recording material P) with a relatively low resistance increases. Therefore, when the resistance value of the transfer roller 5 is low, the amount of decrease in the surface potential of the photosensitive drum 1 in the non-paper-passing area increases.

[0064] Therefore, the magnitude of Vback in the non-paper passing region changes depending on the resistance value of the transfer roller 5, and the degree of edge staining also changes. The resistance value of the transfer roller 5 changes depending on the usage conditions of the transfer roller 5, such as the environment (at least one of the temperature and humidity inside or outside the image forming apparatus 100) and paper passing history (for example, the amount of use of the transfer roller 5). Therefore, by controlling changes in the image formation conditions (charging voltage and development voltage) based on the resistance value of the transfer roller 5, it is possible to appropriately address edge staining. In other words, it is possible to suppress edge staining while suppressing image deletion.

[0065] Here, a method for detecting the resistance value of the transfer roller 5 will be described. In this embodiment, the image forming apparatus 100 detects the resistance value of the transfer roller 5 using ATVC (Auto Transfer Voltage Control). ATVC is a control executed before the recording material P is conveyed to the transfer nip Nt. ATVC is executed as follows. First, the control unit 40 controls the transfer power supply 18 to apply an initial voltage to the transfer roller 5 and waits until the output of the initial voltage stabilizes. The control unit 40 then samples the detection results of the current flowing through the transfer roller 5 by the transfer current detection circuit 19 for a certain period of time and calculates the average current value. The control unit 40 compares this average value with a preset ATVC target current and adjusts the voltage to be applied to the transfer roller 5 so that the difference becomes smaller. By repeating this current detection and voltage adjustment, the voltage applied to the transfer roller 5 is controlled so that the average value of the detection results of the transfer current detection circuit 19 converges to the ATVC target current. By performing ATVC, the control unit 40 can determine the voltage V required to pass a predetermined current (typically, the target current for ATVC) I through the transfer nip Nt. This enables the control unit 40 to determine the resistance value R of the transfer roller 5 based on the current (typically, the target current value for ATVC) I and the voltage V, according to the following equation (1). In this way, in this embodiment, the transfer current detection circuit 19, the control unit 40, etc. constitute an acquisition unit that acquires information regarding the resistance value of the transfer roller 5. R=V / I (1)

[0066] In this embodiment, the initial voltage is set to 500 V, the stabilization waiting time for the initial voltage is set to 100 ms, the current sampling time is set to 50 ms, and the target current of the ATVC is set to 3 μA.

[0067] Furthermore, the control unit 40 functions as a transfer voltage detection means, and can detect (understand) the voltage value applied to the transfer roller 5 based on a voltage output instruction to the transfer power supply 18. However, for example, a transfer voltage detection circuit serving as a transfer voltage detection means may be provided within the transfer power supply 18, and the control unit 40 may acquire the voltage value applied to the transfer roller 5 from the transfer voltage detection circuit.

[0068] Furthermore, a voltage-current characteristic expressed by a straight line or a curve may be obtained based on the applied voltage and the detected current, and the voltage required to pass a predetermined current may be obtained based on this relationship. Furthermore, the resistance value of the transfer roller 5 may be obtained based on this relationship. The number of measurement points to obtain this relationship is typically multiple, but the number of measurement points may be limited to one by using a zero point.

[0069] Furthermore, the control unit 40 may be configured to determine the transfer voltage during transfer by adding a recording material distribution voltage that is predetermined depending on the type of recording material P, to the voltage V required to pass a predetermined current I obtained by the ATVC. Then, during transfer, the control unit 40 can perform constant voltage control of the transfer voltage at the determined value during at least part of the period during which the recording material P passes through the transfer nip Nt.

[0070] Here, the resistance value of the transfer roller 5 gradually increases as the elastic layer of the transfer roller 5 deteriorates due to repeated application of voltage. Therefore, edge staining, which occurs when there is a large potential difference between the surface potential of the photosensitive drum 1 in the paper passing area and the surface potential of the photosensitive drum 1 in the non-paper passing area, is likely to occur when the transfer roller 5 has a low resistance value and is close to a new condition.

[0071] (7) Control of image formation conditions In this embodiment, in consideration of the mechanisms and factors described above, control is performed to change the image forming conditions (charging voltage and developing voltage) based on the resistance value of the transfer roller 5. Figure 6 is a flowchart showing an outline of the control procedure for changing the image forming conditions based on the resistance value of the transfer roller 5 in this embodiment. The process in Figure 6 is started when the image forming apparatus 100 receives information about a print job, and is executed by the control unit 40 (more specifically, the CPU 41).

[0072] First, the control unit 40 receives print job information and executes image formation preparation (S101). More specifically, the control unit 40 drives the motors to drive various rotating members (such as the photosensitive drum 1 and various rollers) inside the image forming apparatus 100, and also supplies power to the heater of the fixing device 12 to preheat the fixing device 12. Next, the control unit 40 sets the count value of the number of images formed on a counter provided in the RAM 41b to an initial value (0 in this embodiment) (S102). In this embodiment, the control unit 40 adds 1 to the count value of the number of images formed on RAM 41b each time an image is formed on one side of the recording material P (each time the recording material P is supplied to the transfer nip Nt), thereby updating and storing the count value. Next, the control unit 40 executes ATVC to detect the resistance value of the transfer roller 5 (S103). Next, the control unit 40 determines whether the count value of the number of images formed on RAM 41b is equal to or greater than 100 (S104). If the control unit 40 determines in S104 that the count value of the number of sheets on which images have been formed is less than 100, the process proceeds to S107. If the control unit 40 determines in S104 that the count value of the number of sheets on which images have been formed is 100 or more, the control unit 40 temporarily stops conveying the recording material P, and proceeds to S107 after passing through S105 and S106. That is, the control unit 40 resets the count value of the number of sheets on which images have been formed to the initial value (0 in this embodiment) (S105), re-executes ATVC (S106), and proceeds to S107. Here, in this embodiment, the reason why the threshold value of the count value of the number of sheets on which images have been formed is set to 100 is as follows: In other words, in the configuration of this embodiment, when 100 or more sheets are continuously formed, the resistance value of the transfer roller 5 may decrease to a level that may cause edge contamination due to the influence of an internal temperature rise or the like, and the resistance value of the transfer roller 5 must be re-measured.

[0073] After starting feeding of the recording material P to the transfer nip Nt (S107), the control unit 40 increments the count value of the number of images formed in the RAM 41b by one (S108). Next, the control unit 40 determines image formation conditions, such as transfer voltage, charging voltage, and developing voltage, based on the resistance value of the transfer roller 5 obtained in S103 or S106 (S109). More specifically, in this embodiment, if the resistance value of the transfer roller 5 is equal to or less than a predetermined threshold, the control unit 40 increases the charging voltage and, accordingly, increases the developing voltage so as to maintain Vback. Specific examples of image formation conditions will be described later. The control unit 40 then performs image formation under the determined image formation conditions (S110). Next, the control unit 40 determines whether output of all images specified in the print job has been completed (S111). If not, the control unit 40 returns to the process of S104. If it is determined that all images have been completed, the control unit 40 ends the print job.

[0074] (8) Effects Next, the effects of this embodiment will be further described using specific examples of changes in image formation conditions. Here, edge staining and image deletion were evaluated using the image forming apparatus 100 configured in this embodiment under a room temperature and humidity environment (23°C / 50% RH). Regarding edge staining, 500 consecutive images were formed, and the degree of staining at the widthwise edge of the recording material P was evaluated. Regarding image deletion, image formation was repeated at intervals, and the degree of image density reduction when a halftone image was formed after 10,000 images were formed was evaluated. The above evaluation was also performed using double-sided image formation, which is prone to edge staining. When double-sided image formation was performed, the recording material P was heated during the fixing process of the first side image and then fed to the transfer nip Nt for the transfer of the second side image. Therefore, when double-sided image formation was performed continuously, the transfer roller 5 was easily heated, and the resistance value of the transfer roller 5 was easily reduced, making edge staining more likely to occur. Furthermore, GF-C081 (product name, manufactured by Canon Inc.) was used as the recording material P. The transfer roller 5 has a resistance of 2.0×10 when left in the above-mentioned normal temperature and humidity environment at the beginning of use (when new). 8A transfer roller 5 with a resistance of Ω was used. Table 1 shows the resistance values ​​of the transfer roller 5 detected when ATVC was performed every 100 sheets.

[0075] [Table 1]

[0076] Table 2 shows the image formation conditions and evaluation results when image formation was performed on the first to 500th sheets for the image forming apparatus 100 of this embodiment and the image forming apparatuses 100 of Comparative Examples 1 and 2. The configuration and operation of the image forming apparatuses 100 of Comparative Examples 1 and 2 are substantially the same as those of the image forming apparatus 100 of this embodiment, except for the different image formation conditions described below. The same reference numerals are used for Comparative Examples 1 and 2 to describe the edge staining. In Table 2, with regard to edge staining, a fog density value in the non-sheet passing area of ​​less than 6% is indicated as "Good," and a fog density value of 6% or greater is indicated as "Poor." Furthermore, with regard to image deletion, a fog density value of no decrease in density is indicated as "Good," and a fog density value of 6% or greater is indicated as "Poor."

[0077] [Table 2]

[0078] Comparative Example 1 is an example in which image formation was performed without changing the image formation conditions based on the resistance value of the transfer roller 5. In this case, when continuous image formation is performed, the resistance value of the transfer roller 5 decreases, and the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area and the surface potential of the photosensitive drum 1 in the non-paper passing area gradually increases. In this case, Vback becomes small in the non-paper passing area, causing edge contamination. On the other hand, in Comparative Example 1, the image formation conditions were not changed to increase the amount of discharge, so image deletion did not occur.

[0079] Comparative Example 2 is an example in which the charging voltage is always high from the initial state during charging processing to suppress edge staining. In this case, even if the resistance value of the transfer roller 5 decreases, the photosensitive drum 1 is charged with a high charging voltage, so the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area and the surface potential of the photosensitive drum 1 in the non-paper passing area can be reduced. Therefore, edge staining does not occur. However, as a result of always keeping the charging voltage high from the initial state during charging processing, a decrease in density due to image deletion occurred. Therefore, in Comparative Example 2, it is necessary to perform the image deletion prevention sequence described above.

[0080] On the other hand, in this embodiment, the resistance value of the transfer roller 5 is detected by the ATVC, and the image forming conditions are changed based on the detection result. After forming images on 200 sheets, the resistance value of the transfer roller 5 is detected by the ATVC to be 1.3×10 8 Ω, the control unit 40 increased the charging voltage and developing voltage from the 201st sheet onward, which is likely to cause edge contamination. In other words, in this embodiment, in the process of S109 in FIG. 6, the control unit 40 sets the resistance value of the transfer roller 5 to a predetermined threshold value of 1.3×10 8 When the resistance was Ω or less, the charging voltage and developing voltage were increased. As a result, edge staining did not occur. In this example, the image forming conditions were not changed in the initial state when the resistance value of the transfer roller 5 was high, so the level of image deletion was better than in Comparative Example 2, and image deletion did not occur.

[0081] In this embodiment, the image forming apparatus 100 does not have a pre-exposure unit that exposes the photosensitive drum 1 downstream of the transfer position Pd and upstream of the charging position Pa in the rotational direction of the photosensitive drum 1. When the surface potential difference of the photosensitive drum 1 is reduced by increasing the charging voltage, the increase in the amount of discharge to the photosensitive drum 1 during charging is smaller than when the surface potential difference of the photosensitive drum 1 is eliminated using the pre-exposure unit, making image deletion less likely to occur. Furthermore, in this embodiment, the change in the charging voltage is controlled based on the resistance value of the transfer roller 5, so the amount of discharge to the photosensitive drum 1 during charging is not unnecessarily increased. Therefore, this embodiment makes it easy to simultaneously suppress both "edge contamination" and "image deletion." Furthermore, this embodiment allows for a simpler and more compact configuration than when the pre-exposure unit is used. Thus, this embodiment can suppress edge contamination while suppressing image deletion with a simple configuration.

[0082] In this embodiment, a threshold value is set for the resistance value of the transfer roller 5, and the image forming conditions are changed when the detected resistance value of the transfer roller 5 is equal to or less than the threshold value, but the present invention is not limited to this configuration. The image forming conditions may be changed in multiple stages based on the detection results of the resistance value of the transfer roller 5. For example, as the resistance value of the transfer roller 5 decreases, the charging voltage may be increased in stages, and the developing voltage may be increased accordingly to maintain Vback.

[0083] In this embodiment, the image forming conditions are changed by changing the charging voltage and the developing voltage in conjunction with each other to prevent fogging from worsening, but the present invention is not limited to this. For example, the charging voltage and the developing voltage may be changed separately. Also, only one of the charging voltage and the developing voltage may be changed.

[0084] Furthermore, in this embodiment, ATVC is performed periodically during continuous image formation to detect the change in the resistance value of the transfer roller 5, but the present invention is not limited to this configuration. For example, the number of images formed on sheets during continuous image formation may be counted, and the current resistance value of the transfer roller 5 may be estimated from the resistance value of the transfer roller 5 detected using ATVC in the pre-rotation process and the number of images formed on sheets. For example, the resistance value of the transfer roller 5 may be estimated based on information regarding the change in the resistance value of the transfer roller 5, as shown in Table 1, which has been obtained in advance.

[0085] Although the present embodiment utilizes the ATVC to determine the resistance value of the transfer roller 5, the present invention is not limited to this configuration. For example, information regarding the resistance value of the transfer roller 5 measured in advance may be stored in a nonvolatile memory serving as a storage means of the image forming apparatus 100, allowing for easier detection of the resistance value of the transfer roller 5. That is, for example, the resistance value of the transfer roller 5 measured in advance according to the amount of usage of the transfer roller 5 (e.g., the amount of usage from when it was new, the amount of usage from the initial state of each continuous image formation), the environment, etc., may be stored in the nonvolatile memory of the image forming apparatus 100. The control unit 40 can then estimate the current resistance value of the transfer roller 5 according to the current amount of usage of the transfer roller 5 and the environment. The amount of usage of the transfer roller 5 may be any index value correlated with the passage of time associated with the use of the transfer roller 5, such as the number of images formed or the number of rotations (rotation time) of the transfer roller 5. In this case, the nonvolatile memory, the control unit, etc. constitute an acquisition unit that acquires information regarding the resistance value of the transfer roller 5.

[0086] In addition, in this embodiment, the resistance value calculated from the current value and voltage value is used as information regarding the resistance value of the transfer roller 5, but values ​​that correlate with the resistance value, such as the current value and voltage value, may also be used.

[0087] As described above, in this embodiment, the image forming apparatus 100 includes a rotatable photosensitive member 1, a charging member 2 that charges the surface of the photosensitive member 1, a charging power source 21 that applies a charging voltage to the charging member 2 during charging, a developing member 4a that supplies toner to the surface of the photosensitive member 1 that has been charged to form a toner image, a transfer member 5 that contacts the surface of the photosensitive member 1 to form a transfer portion Nt and transfers the toner image from the surface of the photosensitive member 1 to a recording material P that passes through the transfer portion Nt, a transfer power source 18 that applies a transfer voltage to the transfer member 5, and an acquiring unit (transfer current detection unit) that acquires information about the electrical resistance value of the transfer member 5. The control unit 40 controls the charging voltage applied in the image forming operation to form a toner image on the recording material P to be a first charging voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquiring unit is a first resistance value, and controls the charging voltage applied in the image forming operation to be a second charging voltage having an absolute value greater than that of the first charging voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquiring unit is a second resistance value lower than the first resistance value. In this embodiment, the acquiring unit acquires information about the electrical resistance value of the transfer member 5 based on at least one of the voltage value applied to the transfer member 5 and the current value flowing through the transfer member 5 when a voltage is applied to the transfer member 5 by the transfer power supply 18 when no recording material P is present at the transfer section Nt. In this embodiment, the image forming apparatus 100 has a development power supply 16 that applies a development voltage to the developing member 4a when forming a toner image, and the control unit 40 controls the development voltage so that when the charging voltage is the first charging voltage, the development voltage is the first development voltage, and when the charging voltage is the second charging voltage, the development voltage is the second development voltage whose absolute value is greater than that of the first development voltage. In this embodiment, the control unit 40 changes the development voltage as the charging voltage is changed so that the difference between the charging voltage and the development voltage is kept approximately constant.The control unit 40 may be configured to control the developing voltage applied in the image forming operation to form a toner image on the recording material P to be a first developing voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit is a first resistance value, and to control the developing voltage applied in the image forming operation to be a second developing voltage having an absolute value smaller than that of the first developing voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value. Also, in this embodiment, the image forming apparatus 100 is not provided with a pre-exposure unit that exposes the surface of the photoconductor 1 downstream of the transfer position Pd where transfer is performed and upstream of the charging position Pa where charging is performed with respect to the rotation direction of the photoconductor 1.

[0088] As described above, in this embodiment, the charging voltage is increased based on the resistance value of the transfer roller 5. This makes it possible to suppress image deletion and edge staining.

[0089] [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, in the image forming apparatus of this embodiment, elements having 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 of embodiment 1, and detailed descriptions thereof will be omitted.

[0090] In the first embodiment, image deletion and edge staining were suppressed by changing the image forming conditions based on the resistance value of the transfer roller 5. In this embodiment, the image forming conditions are further changed based on the resistance value of the recording material P used in image formation, thereby suppressing edge staining and increasing the margin for suppressing image deletion.

[0091] Here, the influence of the resistance value of the recording material P on the potential difference between the surface potential of the photosensitive drum 1 in the paper passing area and the surface potential of the photosensitive drum 1 in the paper non-passing area will be described.

[0092] If the resistance value of the recording material P is high, the potential difference between the surface potential of the photosensitive drum 1 in the paper-passing area after transfer and the surface potential of the photosensitive drum 1 in the non-paper-passing area increases. This potential difference occurs because, when the recording material P is present in the transfer nip Nt, the transfer current flows concentratedly in the non-paper-passing area, avoiding the recording material P, which acts as an electrical resistor. If the resistance value of the recording material P is high, the ratio of the current flowing in the non-paper-passing area to the paper-passing area increases relatively, and therefore the amount of decrease in the surface potential of the photosensitive drum 1 in the non-paper-passing area increases.

[0093] Therefore, similar to the resistance value of the transfer roller 5 described in the first embodiment, the magnitude of Vback in the non-sheet-passing region also changes depending on the resistance value of the recording material P, and therefore the degree of edge staining also changes. The resistance value of the recording material P changes depending on the state of the recording material P, such as the thickness and material of the recording material P, the environment (at least one of the temperature and humidity inside or outside the image forming apparatus 100), and the sheet passing history (for example, whether it is the first or second side in double-sided image formation). Therefore, by controlling the change of the image formation conditions (charging voltage and development voltage) based on the resistance value of the recording material P, it is possible to appropriately address edge staining. In other words, it is possible to suppress edge staining while suppressing image deletion.

[0094] The resistance value of the recording material P can be detected, for example, as follows. That is, the resistance value of the transfer nip Nt, which holds the recording material P, is detected based on the voltage value applied when the recording material P passes through the transfer nip Nt and the current value flowing through the transfer roller 5. Then, the resistance value of the recording material P can be detected based on the difference between this resistance value and the resistance value of the transfer roller 5 detected by the ATVC (the resistance value when the recording material P is not in the transfer nip Nt). If the difference in resistance values ​​is large, it can be said that the recording material P has a high resistance value. Note that, since the resistance value of the recording material P may not be detected correctly depending on the presence or absence of toner on the recording material P, it is preferable to detect the resistance value of the transfer nip Nt, which holds the recording material P, at the margin on the leading edge of the recording material P in the conveyance direction. In this embodiment, the resistance value is 1.0×10 10A recording material P with a resistance value of Ω or more is determined to be a high resistance recording material P. In this manner, in this embodiment, similar to the acquisition unit that acquires information about the resistance value of the transfer roller 5 in the first embodiment, a separate acquisition unit that acquires information about the resistance value of the recording material P is configured by the transfer current detection circuit 19, the control unit 40, etc.

[0095] 7 is a flowchart showing an outline of a control procedure in this embodiment for changing image forming conditions based on the resistance value of the transfer roller 5 and the resistance value of the recording material P. The processing in FIG. 7 is started when the image forming apparatus 100 receives information about a print job, and is executed by the control unit 40 (more specifically, the CPU 41).

[0096] The processes of S201 to S208 in FIG. 7 are the same as S101 to S108 in FIG. 6 in the first embodiment. In this embodiment, the control unit 40 acquires information on the resistance value of the recording material P in S209. As described above, in this embodiment, the control unit 40 determines whether the resistance value of the recording material P is high or not based on the detection result of the resistance value when the margin on the leading edge side in the conveyance direction of the recording material P passes through the transfer nip portion Nt and the detection result of the resistance value of the transfer roller 5 by the ATVC in S203 or S206. More specifically ... as described above, the control unit 40 determines whether the resistance value of the recording material P is high based on the resistance value of the recording material P when the margin on the leading edge side in the conveyance direction of the recording material P passes through the transfer nip portion Nt. 10 If the resistance value is Ω or more, it is determined that the resistance value of the recording material P is high. Note that the image forming conditions may be changed based on the detection results from the image formation on the recording material P for which information on the resistance value of the recording material P has been obtained. However, if the control of the image forming conditions cannot be completed in time, the image forming conditions for the image formation from the image formation on the next recording material P until the next detection of the resistance value of the recording material P may be changed based on the detection results, for example.

[0097] Next, the control unit 40 determines image forming conditions, such as transfer voltage, charging voltage, and developing voltage, based on the information about the resistance value of the transfer roller 5 obtained in S203 or S206 and the information about the resistance value of the recording material P obtained in S209 (S210). More specifically, in this embodiment, when the resistance value of the transfer roller 5 is equal to or less than a predetermined threshold, the control unit 40 increases the charging voltage and accordingly increases the developing voltage so as to maintain Vback. At this time, in this embodiment, the setting of the charging voltage differs depending on whether the recording material P has a high resistance value. Specific examples of image forming conditions will be described later. Then, the control unit 40 performs image forming operations under the determined image forming conditions (S211). Next, the control unit 40 determines whether the output of all images specified in the print job has been completed (S212). If it determines that the output has not been completed, the control unit 40 returns to the processing of S204. If it determines that the output has been completed, the control unit 40 ends the print job.

[0098] Next, the effects of this embodiment will be further described using specific examples of changes in image forming conditions. Here, edge staining was evaluated in a room temperature and humidity environment (23°C / 50%RH) using the image forming apparatus 100 configured in this embodiment. To confirm the effect of different resistance values ​​of the recording material P, images were formed on 500 sheets of recording material P, using thin (low resistance) recording material P and thick (high resistance) recording material P. CS-068 (product name, manufactured by Canon Inc.) was used as the thin recording material P, and GF-C081 (product name, manufactured by Canon Inc.) was used as the thick recording material P. The evaluation was performed on the image forming apparatus 100 of Example 1 and the image forming apparatus 100 of this embodiment.

[0099] In Example 1, image formation is performed without changing the image formation conditions based on the resistance value of the recording material P. In this case, as described in Example 1, the charging voltage applied to the charging roller 2 was set to -1200 V from the 201st sheet onwards, when the resistance value of the transfer roller 5 becomes low. By increasing the charging voltage when the resistance value of the transfer roller 5 becomes low, edge staining did not occur.

[0100] Furthermore, in this embodiment, when the recording material P is thin and has a low resistance (when the resistance of the recording material P is less than a predetermined threshold), the charging voltage is lower than in Example 1. In this embodiment, when the recording material P is thin and has a low resistance, the charging voltage applied to the charging roller 2 is set to −1150 V, lower than in Example 1, from the 201st sheet onward, when the resistance of the transfer roller 5 becomes low. In this case, although the resistance of the transfer roller 5 is low, the resistance of the recording material P is low, so edge staining does not occur. Furthermore, in this embodiment, the charging voltage is lower when the recording material P is thin and has a low resistance than in Example 1. Therefore, when forming an image on such a recording material P, the total amount of discharge from the photosensitive drum 1 is reduced. As a result, when forming an image on such a recording material P, it is easier to suppress image deletion. In other words, the margin for image deletion is increased.

[0101] In this embodiment, a threshold value is set for the resistance value of the recording material P, and the image forming conditions are changed when the detected resistance value of the recording material P is less than the threshold value, but the present invention is not limited to this embodiment. The image forming conditions may be changed in multiple stages based on the detection results of the resistance value of the recording material P. For example, as the resistance value of the recording material P increases, the charging voltage may be increased in stages, and the developing voltage may be increased accordingly to maintain Vback.

[0102] Furthermore, in this embodiment, the image formation conditions are changed based on the detection results of the resistance value of the recording material P, but the present invention is not limited to this configuration. The image formation conditions may also be changed based on the resistance value of the recording material P estimated from information about the state of the recording material P, such as information about the thickness of the recording material P, information about the environment, and information about the state of the recording material P. In this case, the image formation conditions can be changed for each sheet of recording material P conveyed to the transfer nip Nt based on the above information. Note that the information about the thickness of the recording material P is an example of information about the type of recording material P. The information about the recording material P includes any information that can distinguish the recording material P, such as attributes based on general characteristics (so-called paper type categories) such as plain paper, thick paper, thin paper, numerical values ​​or numerical ranges for basis weight, thickness, size, stiffness, or brand (including manufacturer, product name, part number, etc.). Each recording material P distinguished by the information about the recording material P can be considered to constitute a type of recording material P. The control unit 40 can acquire information regarding the type of recording material P from print job information input to the image forming apparatus 100 from an external device or information input from an operation unit provided in the image forming apparatus 100. The information regarding the type of recording material P may be information that directly indicates the type of recording material P, or may be included in or substituted by information that specifies the operation settings of the image forming apparatus 100, such as "plain paper mode" or "cardboard mode."

[0103] Information about the thickness and resistance of the recording material P may be obtained by a media sensor provided on the recording material P transport path from the recording material P feed section to the transfer nip Nt. Media sensors that use light or ultrasound are known and can be used to detect or estimate the basis weight, surface properties, and moisture content of the recording material P, which are correlated with the thickness of the recording material P. Furthermore, a mechanism (such as a pair of conductive rollers and a power supply) that can detect the resistance of the recording material P from the current and voltage values ​​when a voltage is applied, similar to the ATVC, may be provided on the recording material P transport path from the recording material P feed section to the transfer nip Nt. Based on the information obtained by the media sensor, changes to the image formation conditions may be controlled for each sheet of recording material P transported to the transfer nip Nt.

[0104] Furthermore, when double-sided image formation is performed, the resistance value of the recording material P when the image on the second side is formed is higher than the resistance value of the recording material P when the image on the first side is formed. This is because the moisture in the recording material P evaporates due to the fixing operation during image formation on the first side. Using this, the resistance value of the recording material P can be determined depending on whether the image is being formed on the first or second side, and the image formation conditions can be changed. For example, when forming an image on the second side, the charging voltage can be made higher than when forming an image on the first side, and the developing voltage can also be made higher to maintain Vback.

[0105] Furthermore, as described in the first embodiment, fogging that causes edge staining may be reduced by increasing Vback, which is the potential difference between the photosensitive drum 1 in the non-sheet passing area E and the developing roller 4a, by lowering the applied voltage to the developing roller 4a. For example, in the above specific example of this embodiment, when the resistance value of the transfer roller 5 decreases, instead of increasing the charging voltage, the developing voltage can be lowered. Also, when the resistance value of the recording material P is low, instead of decreasing the charging voltage, the developing voltage can be increased.

[0106] Thus, in this embodiment, as in the first embodiment, the control unit 40 controls the charging voltage to be a first charging voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit (transfer current detection circuit 19, control unit 40) is a first resistance value, and to be a second charging voltage whose absolute value is greater than that of the first charging voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value. In this embodiment, the image forming apparatus 100 has another acquisition unit (transfer current detection circuit 19, control unit 40) that acquires information regarding the electrical resistance value of the recording material P, and the control unit 40 controls the charging voltage so that when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit is the second resistance value, and when the electrical resistance value of the recording material P indicated by the information acquired by the other acquisition unit is a third resistance value, the charging voltage is set to the second charging voltage, and when the electrical resistance value of the recording material P indicated by the information acquired by the other acquisition unit is a fourth resistance value lower than the third resistance value, the charging voltage is set to the third charging voltage whose absolute value is smaller than the absolute value of the second charging voltage. In addition, the control unit 40 may be configured to control the developing voltage to be the second developing voltage when the electrical resistance value of the transfer member 5 indicated by the information acquired by the acquisition unit is the second resistance value, and the developing voltage to be the third developing voltage when the electrical resistance value of the recording material P indicated by the information acquired by the other acquisition unit is a fourth resistance value lower than the third resistance value, and the developing voltage to be the third developing voltage whose absolute value is greater than that of the second developing voltage when the electrical resistance value of the recording material P indicated by the information acquired by the other acquisition unit 19, 40 is a fourth resistance value lower than the third resistance value.

[0107] In this embodiment, the separate acquisition unit acquires information about the electrical resistance of the recording material P based on at least one of the voltage value applied to the transfer member 5 and the current value flowing through the transfer member 5 when a voltage is applied to the transfer member 5 by the transfer power supply 18 when no recording material P is present at the transfer portion Nt, and at least one of the voltage value applied to the transfer member 5 and the current value flowing through the transfer member 5 when a voltage is applied to the transfer member 5 by the transfer power supply 18 when a recording material P is present at the transfer portion Nt. However, the separate acquisition unit may also acquire information about the electrical resistance of the recording material P based on information about the type of recording material P. In this case, the information about the type of recording material P may include information about the thickness of the recording material P. The separate acquisition unit may also acquire information about the electrical resistance of the recording material P based on information about whether an image is being formed on the first side or the second side in double-sided image formation.

[0108] As described above, in this embodiment, in addition to changing the image forming conditions based on the resistance value of the transfer roller 5, the image forming conditions are also changed based on the resistance value of the recording material P. This makes it possible to suppress edge staining and increase the margin for suppressing image deletion.

[0109] [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.

[0110] In the above-described embodiment, the transfer member is a transfer roller, but the transfer member is not limited to a transfer roller. The transfer member may be, for example, a rotatable endless belt that contacts the photosensitive member. A voltage application member (roller, brush, sheet, etc.) that supplies a transfer voltage to the transfer section via the transfer belt may be disposed on the inner peripheral surface of the transfer belt at a position facing the photosensitive member.

[0111] In the above embodiment, the photosensitive member is a photosensitive drum, but the photosensitive member is not limited to a photosensitive drum. The photosensitive member may be a photosensitive belt configured as an endless belt.

[0112] In the above-described embodiment, the image forming apparatus is not provided with a pre-exposure means, but the present invention is not limited to such a configuration. The present invention can also be applied to an image forming apparatus provided with a pre-exposure means. For example, by reducing the amount of exposure by the pre-exposure means and reducing the amount of discharge to the photosensitive member during charging, it is possible to obtain the effect of easily suppressing image deletion and suppressing edge contamination. [Explanation of symbols]

[0113] 1 Photosensitive drum 2 Charging roller 3 Exposure equipment 4. Developing device 5 Transfer roller 6 Cleaning Device 30 Double-sided transport mechanism 40 Control Unit

Claims

1. a rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; a charging power source that applies a charging voltage to the charging member during the charging process; a developing member that supplies toner to the surface of the photoreceptor that has been charged to form a toner image; a transfer member that contacts the surface of the photoreceptor to form a transfer portion and transfers the toner image from the surface of the photoreceptor to a recording material that passes through the transfer portion; a transfer power supply that applies a transfer voltage to the transfer member; an acquisition unit that acquires information about the electrical resistance value of the transfer member; another acquiring unit that acquires information about the electrical resistance value of the recording material; a control unit capable of controlling the charging power source; and The control unit control the charging voltage to be applied in the image forming operation for forming the toner image on the recording material to be a first charging voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a first resistance value, and to be applied in the image forming operation to be a second charging voltage having an absolute value greater than that of the first charging voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value; an image forming apparatus configured to control the charging voltage to the second charging voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is the second resistance value and when the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a third resistance value, and to control the charging voltage to the third charging voltage whose absolute value is smaller than that of the second charging voltage when the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a fourth resistance value lower than the third resistance value.

2. The image forming apparatus according to claim 1, characterized in that the acquisition unit acquires information regarding the electrical resistance value of the transfer member based on at least one of the voltage value applied to the transfer member and the current value flowing through the transfer member when a voltage is applied to the transfer member by the transfer power supply when the recording material is not present in the transfer unit.

3. a development power source that applies a development voltage to the developing member when the toner image is formed; 3. The image forming apparatus according to claim 1, wherein the control unit controls the developing voltage to be a first developing voltage when the charging voltage is the first charging voltage, and controls the developing voltage to be a second developing voltage having an absolute value greater than that of the first developing voltage when the charging voltage is the second charging voltage.

4. 4. The image forming apparatus according to claim 3, wherein the control section changes the developing voltage in accordance with the change in the charging voltage so as to keep the difference between the charging voltage and the developing voltage substantially constant.

5. a rotatable photoreceptor; a charging member for charging the surface of the photoreceptor; a developing member that supplies toner to the surface of the photoreceptor that has been charged to form a toner image; a development power source that applies a development voltage to the developing member when the toner image is formed; a transfer member that contacts the surface of the photoreceptor to form a transfer portion and transfers the toner image from the surface of the photoreceptor to a recording material that passes through the transfer portion; a transfer power supply that applies a transfer voltage to the transfer member; an acquisition unit that acquires information about the electrical resistance value of the transfer member; a control unit capable of controlling the developing power source; and The control unit controls the developing voltage applied in the image forming operation to form the toner image on the recording material to be a first developing voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a first resistance value, and controls the developing voltage applied in the image forming operation to be a second developing voltage whose absolute value is smaller than that of the first developing voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is a second resistance value lower than the first resistance value.

6. The image forming apparatus according to claim 5, characterized in that the acquisition unit acquires information regarding the electrical resistance value of the transfer member based on at least one of the voltage value applied to the transfer member and the current value flowing through the transfer member when a voltage is applied to the transfer member by the transfer power supply when the recording material is not present in the transfer unit.

7. a separate acquisition unit for acquiring information about the electrical resistance value of the recording material; The image forming apparatus of claim 5 or 6, characterized in that the control unit controls the developing voltage to be the second developing voltage when the electrical resistance value of the transfer member indicated by the information acquired by the acquisition unit is the second resistance value, and when the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a third resistance value, and controls the developing voltage to be the third developing voltage whose absolute value is greater than that of the second developing voltage when the electrical resistance value of the recording material indicated by the information acquired by the other acquisition unit is a fourth resistance value lower than the third resistance value.

8. The image forming apparatus of any one of claims 1, 2, 3, 4 and 7, characterized in that the other acquisition unit acquires information regarding the electrical resistance value of the recording material based on at least one of the voltage value applied to the transfer member and the current value flowing through the transfer member when a voltage is applied to the transfer member by the transfer power source when the recording material is not present in the transfer section, and at least one of the voltage value applied to the transfer member and the current value flowing through the transfer member when a voltage is applied to the transfer member by the transfer power source when the recording material is present in the transfer section.

9. 8. The image forming apparatus according to claim 1, wherein the other acquisition unit acquires information about the electrical resistance value of the recording material based on information about the type of the recording material.

10. 10. The image forming apparatus according to claim 9, wherein the information about the type of recording material includes information about the thickness of the recording material.

11. The image forming apparatus according to any one of claims 1, 2, 3, 4 and 7, characterized in that the other acquisition unit acquires information regarding the electrical resistance value of the recording material based on information regarding whether an image is being formed on the first side or the second side in double-sided image formation.

12. 12. An image forming apparatus according to claim 1, wherein a pre-exposure means for exposing the surface of the photosensitive member is not provided downstream of a transfer position where the transfer is performed and upstream of a charging position where the charging process is performed in relation to the rotation direction of the photosensitive member.

Images