JP2020042148A - Image formation apparatus - Google Patents

Abstract

To provide an image formation apparatus which can suppress the contamination on an end of a recording material while suppressing the reversal fogging in accordance with the size of a recording material used in image formation.SOLUTION: Control means 150 is configured to, when a region where a photoreceptor 1 and a transfer member 5 contact with each other is defined as a first region R1, a prescribed region preset for each size of a recording material P for a region where the recording material P and the photoreceptor 1 may contact with each other is defined as a second region R2, and a region where a non-image part of the surface of the photoreceptor 1 is weakly exposed by exposure means 3 is defined as a weakly-exposed region Vg, change the weakly-exposed region Vg in accordance with the size of the recording material P used in the image formation so that an absolute value Va of the surface potential of the photoreceptor 1 in a region inside the first region R1 and outside the second region R2 on the downstream side with respect to the exposure position b and on the upstream side with respect to the transfer position d is larger than an absolute value Vb of the surface potential of the non-image part of the surface of the photoreceptor 1 in the second region R2.SELECTED DRAWING: Figure 3

Description

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

  In an electrophotographic image forming apparatus, the surface of a photoreceptor is charged, exposed according to image information to form an electrostatic image, and the electrostatic image is developed with toner to form a toner image. The transfer of the toner image to the transfer object is performed. In particular, in the “direct transfer type” image forming apparatus, the toner image formed on the photoconductor is directly transferred from the photoconductor to a recording material such as recording paper as a transfer target. The transfer of the toner image from the photoconductor to the recording material is performed by applying a voltage to a transfer member such as a transfer roller that contacts the photoconductor to form a transfer unit and supplying a transfer current to the transfer unit. There are many.

  At the time of transfer, the transfer portion has a region through which the recording material passes and a region through which the recording material does not pass in a direction substantially orthogonal to the moving direction of the surface of the photoconductor (the conveyance direction of the recording material). Here, the area of the transfer section through which the recording material passes (and the corresponding area on the surface of the photoconductor) is also referred to as a “paper passing area”. The area of the transfer section through which the recording material does not pass (and the corresponding area on the surface of the photoconductor) is also referred to as a “non-sheet passing area”. The non-sheet passing area generally occurs outside the sheet passing area adjacent to both ends of the sheet passing area in a direction substantially orthogonal to the moving direction of the surface of the photoconductor. In a direct transfer type image forming apparatus, when a toner image is transferred from a photoreceptor to a recording material, the recording material acts as an electrical resistor, and thus the transfer current flows intensively in the non-sheet passing area. Therefore, after the transfer, the surface potential of the photoconductor in the non-sheet passing area becomes relatively lower than the surface potential of the photoconductor in the sheet passing area. Here, the magnitude (high or low) of the potential such as the surface potential of the photoreceptor or the transfer voltage refers to the magnitude (high or low) when compared by an absolute value.

  The relative potential difference of the surface potential of the photoconductor between the non-paper passing area and the paper passing area after the transfer as described above is corrected when the surface of the photoconductor is charged thereafter. However, when the relative potential difference is large, such as when the electrical resistance of the recording material is high, the relative potential difference between the non-sheet passing area and the sheet passing area remains even after charging. For this reason, during development, in a non-paper passing area, Vback, which is the difference between the surface potential of the photoconductor and the development potential, becomes small, and a phenomenon called “ground fog” in which toner adheres to the surface of the photoconductor occurs. There is. When the recording material is skewed and conveyed at the time of transfer, the toner adhering to the surface of the photoreceptor in the non-sheet passing area due to “ground fogging” is rotated after the photoreceptor makes one rotation. In some cases, the image may be transferred to an end in a direction substantially perpendicular to the transport direction. As a result, a phenomenon called “edge stain” in which the edge of the recording material is stained with toner may occur.

  The “edge fouling” due to the “ground fog” is suppressed by increasing the charging voltage to increase the surface potential of the charged photoconductor, lowering the developing voltage, or performing both of them, and increasing Vback. be able to. In other words, by increasing Vback in this way, even if the surface potential of the photoconductor in the non-sheet passing area becomes relatively lower than the surface potential of the photoconductor in the sheet passing area after charging, the non-sheet passing area In this case, Vback in which the toner does not adhere to the surface of the photoconductor can be secured. As a result, it is possible to suppress the occurrence of “ground fogging” in the non-sheet passing area and suppress the “edge stain”.

  On the other hand, if the above countermeasures are taken against "edge stains", the value of Vback in the sheet passing area becomes larger than the optimum value. In this case, in the paper passing area, a phenomenon called “reversal fog” occurs in which toner charged to a polarity opposite to the normal charging polarity (here, also referred to as “reverse polarity toner”) adheres to the surface of the photoconductor. May be. Reverse polarity toner adhered to the surface of the photoreceptor due to "reversal fog" is difficult to be transferred onto the recording material during transfer, and is not likely to cause image defects, but the problem of increased toner consumption is a problem. There is.

  Here, in Japanese Patent Application Laid-Open No. H11-157, the following configuration is used to prevent “reverse fog” in an image area and prevent “ground fog” due to toner in an area where the supply roller at the end of the developing roller does not contact. Is disclosed. That is, a configuration is disclosed in which weak exposure is performed on a non-printing portion of an image area other than the non-image area including an area extending from an end of the supply roller to an end of the opening of the developing device.

  Further, in Patent Document 2, in a configuration in which a laser emits a small amount of light to a non-image portion of an image region in order to prevent image thinning, “a print-inhibited region due to the fact that the region that emits a small amount of light is the same as a printable region” The following configuration is disclosed in order to prevent “reversal fog”. In other words, a configuration is disclosed in which the width of the area that emits minute light is larger than the width of the area corresponding to the recording material on which an image is formed and smaller than the width of the charged area of the photoconductor.

JP 2011-28086 A JP 2013-214047 A

  However, in the inventions described in Patent Literatures 1 and 2, “edge contamination” is caused by the fact that the surface potential of the photoconductor in the non-sheet passing area becomes relatively lower than the surface potential of the photoconductor in the paper passing area after charging. Is not considered at all. In the invention described in Japanese Patent Application Laid-Open No. H11-163, particularly when an image is formed on a recording material having a width smaller than the maximum width on which an image can be formed in an image forming apparatus, “edge stain” tends to be remarkable. Further, according to the invention described in Patent Document 2, “edge stains” are likely to occur regardless of the size of the recording material.

  SUMMARY OF THE INVENTION Accordingly, an object of the present invention is to provide an image forming apparatus capable of suppressing edge fouling of a recording material while suppressing reverse fogging according to the size of the recording material used for image formation.

  The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides a rotatable photoreceptor, a charging unit for charging the surface of the photoreceptor, and exposing an image portion of the charged surface of the photoreceptor at a first exposure intensity. Exposure means for forming an electrostatic image on the photoreceptor, developing means for forming a toner image by attaching toner to an image portion of the electrostatic image on the photoreceptor, contactable with the photoreceptor, By applying a voltage, a transfer member that transfers a toner image from the photoconductor to a recording material sandwiched between the photoconductor, and a control unit that controls the exposure unit, an image forming apparatus having: The control unit may include a first region in which the photosensitive member contacts the transfer member in a direction substantially perpendicular to a moving direction of the surface of the photosensitive member, and a region in which a recording material can contact the photosensitive member. Is set in advance for each recording material size When the non-image area on the surface of the photoreceptor is exposed with the second exposure intensity lower than the first exposure intensity by the exposure unit, The absolute value of the surface potential of the photoconductor in the first region and in the region outside the second region downstream of the exposure position where the exposure is performed and upstream of the transfer position where the transfer is performed in the body rotation direction. The low-exposure area according to the size of the recording material used for image formation so that the value Va is larger than the absolute value Vb of the surface potential of the non-image portion on the surface of the photoconductor in the second area. In the image forming apparatus.

  ADVANTAGE OF THE INVENTION According to this invention, the edge fouling of a recording material can be suppressed according to the size of the recording material used for image formation, suppressing reversal fog.

FIG. 2 is a schematic sectional view of the image forming apparatus. FIG. 2 is a schematic block diagram illustrating a control mode of a main part of the image forming apparatus. FIG. 3 is a schematic diagram illustrating a positional relationship in a longitudinal direction of each part and a surface potential setting of a photosensitive drum in the first embodiment. FIG. 4 is a schematic diagram for explaining a change in the surface potential of the photosensitive drum. It is a graph which shows the relationship between Vback and fog density. It is a flowchart figure which shows the outline of the procedure of the control which sets a weak exposure area. FIG. 9 is a schematic diagram illustrating a positional relationship in a longitudinal direction of each part and a surface potential setting of a photosensitive drum in a second embodiment.

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

[Example 1]
1. Overall Configuration and Operation of Image Forming Apparatus FIG. 1 is a schematic cross-sectional view of an image forming apparatus 100 according to the present embodiment. The image forming apparatus 100 according to the present embodiment is a laser printer using an electrophotographic method.

  The image forming apparatus 100 has a photosensitive drum 1 that is a rotatable drum type (cylindrical) photosensitive member (electrophotographic photosensitive member) as an image bearing member that carries a toner image. The photosensitive drum 1 is configured such that a photosensitive layer formed of a photosensitive material such as OPC (organic optical semiconductor), amorphous selenium, or amorphous silicon is provided on a cylindrical drum base formed of aluminum, nickel, or the like. is there. The photosensitive drum 1 is rotatably supported by the apparatus main body 110, and is rotated at a predetermined peripheral speed (process speed) in a direction indicated by an arrow Rd in the figure by a drive motor (not shown) as a drive unit (drive source). Driven. In this embodiment, the photosensitive drum 1 is driven to rotate at a peripheral speed of 230 mm / sec. In this embodiment, the outer diameter of the photosensitive drum 1 is 24 mm.

  The surface of the rotating photosensitive drum 1 is charged to a predetermined potential of a predetermined polarity (negative in this embodiment) by a charging roller 2 which is a roller-shaped charging member as charging means. The charging roller 2 is arranged in contact with the surface of the photosensitive drum 1, and rotates following the rotation of the photosensitive drum 1. At the time of charging, a predetermined charging voltage (charging bias) is applied to the charging roller 2 by a charging power supply (high-voltage power supply) E1. The charging roller 2 charges the surface of the photosensitive drum 1 at a charging position (charging unit) a in the rotation direction (the moving direction of the surface) of the photosensitive drum 1. The charging roller 2 has a small gap between the charging roller 2 and the photosensitive drum 1 formed on the upstream and downstream sides of the contact portion between the charging roller 2 and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. The surface of the photosensitive drum 1 is charged by the action of discharge in at least one of the above. However, in order to understand the present invention, the contact portion between the charging roller 2 and the photosensitive drum 1 may be simulated as the charging position a. The surface of the charged photosensitive drum 1 is scanned and exposed according to image information by an exposure device (laser scanner) 3 as an exposure unit, and an electrostatic image (electrostatic latent image) is formed on the photosensitive drum 1. . The exposure device 3 irradiates the surface of the photosensitive drum 1 with light at an exposure position (exposed portion) b in the rotation direction of the photosensitive drum 1.

  The electrostatic image formed on the photosensitive drum 1 is developed (visualized) by supplying a toner as a developer by a developing device 4 as a developing unit, and a toner image (developer image) is formed on the photosensitive drum 1. Is done. The developing device 4 includes a developing container 4b that stores toner, and a developing roller 4a as a rotatable developing member (developer carrier) supported by the developing container 4b. The developing roller 4a is rotationally driven by a driving motor (not shown) as driving means (driving source). The developing roller 4a carries the toner on the surface and conveys the toner to a portion facing the photosensitive drum 1 (a contact portion in this embodiment). The developing roller 4a supplies toner to the surface of the photosensitive drum 1 at a developing position (developing unit) c in the rotation direction of the photosensitive drum 1. In this embodiment, a contact portion between the photosensitive drum 1 and the developing roller 4a in the rotation direction of the photosensitive drum 1 is the developing position c. During development, a predetermined development voltage (development bias) is applied to the development roller 4a by a development power supply (high-voltage power supply) E2. In the present embodiment, the charge polarity of the photosensitive drum 1 and the exposure portion (image portion) on the photosensitive drum 1 whose absolute value of the potential has been reduced by being exposed according to the image information after being uniformly charged are determined. Toner charged to the same polarity (negative in this embodiment) adheres. That is, in the present embodiment, the normal charge polarity of the toner, which is the charge polarity of the toner during development, is negative. In this embodiment, the developing device 4 uses a non-magnetic one-component developer (non-magnetic toner) as a developer. However, the developing device 4 may use a magnetic one-component developer (magnetic toner) or a two-component developer including a non-magnetic toner and a magnetic carrier.

  A transfer roller 5, which is a transfer member in the form of a roller, is provided as a transfer unit so as to face the photosensitive drum 1. The transfer roller 5 is urged toward the photosensitive drum 1 and can contact the surface of the photosensitive drum 1 directly or via the recording material P. The toner image formed on the photosensitive drum 1 as described above is transferred at a transfer position d onto a recording material P such as recording paper conveyed while being sandwiched between the photosensitive drum 1 and the transfer roller 5. In this embodiment, a contact portion between the photosensitive drum 1 and the transfer roller 5 in the rotation direction of the photosensitive drum 1 is a transfer position (transfer portion, transfer nip) d. The transfer roller 5 rotates following the rotation of the photosensitive drum 1, sandwiching and transporting the recording material P with the photosensitive drum 1. At the time of transfer, a transfer voltage (transfer bias), which is a DC voltage having a polarity opposite to the normal charge polarity of the toner (positive in this embodiment), is applied to the transfer roller 5 by a transfer power supply (high voltage power supply) E3. You. In this embodiment, the outer diameter of the transfer roller 5 is 12.5 mm. The recording material P is stored in a recording material cassette 7 and is fed one by one by a feeding roller 8. The recording material P is supplied to the transfer position d by the conveyance roller 9 or the like at the same timing as the toner image on the photosensitive drum 1. At this time, the top sensor 10 detects the leading end of the recording material P. The recording material P conveyed to the transfer position d is guided by the pre-transfer guide 17.

  The recording material P to which the toner image has been transferred is guided by a conveyance guide 11 and is conveyed to a fixing device 12 as fixing means. The fixing device 12 heats and pressurizes the recording material P carrying the unfixed toner image to fix (melt and fix) the toner image on the recording material P. The recording material P on which the toner image has been fixed is discharged (output) by a conveying roller 14 and a discharge roller 15 onto a discharge tray 16 provided outside the apparatus main body 110 of the image forming apparatus 100. At this time, the trailing end of the recording material P is detected by the discharge sensor 13 to check for the presence or absence of a jam (paper jam). In this embodiment, the recording material P is discharged at a print speed of 45 sheets per minute during continuous printing.

  Further, toner (transfer residual toner) remaining on the photosensitive drum 1 without being transferred to the recording material P at the time of transfer is removed from the photosensitive drum 1 and collected by a cleaning device 6 as a cleaning unit. The cleaning device 6 uses the cleaning blade 6a to scrape off the transfer residual toner from the surface of the rotating photosensitive drum 1, and stores it in the cleaning container 6b. In this embodiment, the contact portion between the cleaning blade 6a and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 is a cleaning position (cleaning section) e where the transfer residual toner is removed from the photosensitive drum 1.

2. Control Mode FIG. 2 is a schematic block diagram illustrating a control mode of a main part of the image forming apparatus 100 according to the present embodiment. The control unit 150 as a control unit is configured to include a CPU 151 as an arithmetic control unit, which is a central element for performing arithmetic processing, and a memory (storage medium) 152 such as a RAM and a ROM as storage units. The RAM, which is a rewritable memory, stores information input to the control unit 150, detected information, calculation results, and the like, and the ROM stores a control program, a data table obtained in advance, and the like. The CPU 151 and the memory 152 can transfer and read data from each other.

  An external device 200 such as a personal computer is connected to the control unit 150. An operation unit (operation panel) 120 provided in the image forming apparatus 100 is connected to the control unit 150. The operation unit 120 includes a display unit that displays various information to an operator such as a user or a service person under the control of the control unit 150, and an input unit that allows the operator to input various settings related to image formation to the control unit 150. And is configured. The control unit 150 is connected to a charging power supply E1, an exposure device 3, a developing power supply E2, a transfer power supply E3, and the like. The control unit 150 controls each unit of the image forming apparatus 100 based on setting information (control command) such as the type of the recording material P input from the operation unit 120 or the external device 200 and image data input from the external device 200. To perform a print job (image forming operation). In particular, in the present embodiment, the control unit 150 performs control to change an area where the exposure apparatus 3 performs weak exposure, as described later in detail. Exposure device 3 controls the light emission level of a laser diode (light emitting element) as a light source in accordance with an instruction from control unit 150 to change the region where light exposure is performed.

  Note that a print job refers to a series of operations for forming and outputting an image on one or a plurality of recording materials P started by one start instruction. In addition, the type of the recording material P is defined as an attribute based on general characteristics such as plain paper, cardboard, thin paper, glossy paper, and coated paper, a maker, a brand, a product number, a basis weight, a thickness, a size, and the like. It contains any discriminable information.

3. FIGS. 3A and 3B show longitudinal positions of respective portions for each size (particularly, width in a direction substantially orthogonal to the transport direction) of the recording material P used for image formation. FIG. 4 is a diagram for explaining the relationship and setting of the surface potential of the photosensitive drum 1. 3A shows the positional relationship and the surface potential setting when the recording material P has the LTR size, and FIG. 3B shows when the recording material P has the A5 size. Here, a direction substantially perpendicular to the direction of movement of the surface of the photosensitive drum 1 (the direction in which the recording material P is conveyed) (that is, a direction substantially parallel to the direction of the rotation axis of the photosensitive drum 1) is also referred to as the “longitudinal direction”.

  3A and 3B, “A” is the width of the region of the photosensitive drum 1 in which the photosensitive layer is formed in the longitudinal direction or the region thereof (also referred to as “photoconductor region” here). “B” is the width of the area of the charging roller 2 that can contact the surface of the photosensitive drum 1 in the longitudinal direction or the area thereof (also referred to as “charging area” here). “C” is the width of the area of the transfer roller 5 that can contact the surface of the photosensitive drum 1 in the longitudinal direction or the area thereof (also referred to herein as the “transfer area”). Further, “E” indicates that the recording material P is nipped and conveyed between the transfer roller 5 and the photosensitive drum 1, and conveys such as skew of the recording material P is negligible or negligible. Is the width of the area through which the recording material P can pass in the longitudinal direction of the transfer position d or the area thereof (also referred to herein as a “paper passing area”). “Vg” is the width of a region in which light exposure is performed in the longitudinal direction or the region thereof (hereinafter, also referred to as “weak exposure region”). Here, the "weak exposure" means that the surface of the photosensitive drum 1 is exposed at a second exposure intensity lower than the first exposure intensity for forming an image portion of an electrostatic image, as described later in detail. Say that. Note that the exposure intensity (exposure amount, light emission level) of the exposure device 3 indicates the amount of light irradiated per unit time per unit area of the surface of the photosensitive drum 1, specifically, the magnitude of the current supplied to the light emitting element. Alternatively, it can be represented by the level of a signal for causing the light emitting element to emit light, or the like.

  The transfer region C corresponds to a region (here, also referred to as a “first region”) R1 where the photosensitive drum 1 and the transfer roller 5 are in contact in the longitudinal direction. Further, in the sheet passing area E, the recording material P and the photosensitive drum 1 in the longitudinal direction of the transfer position d are in contact with each other when the conveyance variation such as the skew of the recording material P is negligibly small or absent. The obtained area (also referred to as “second area” here) R2.

  Further, in the present embodiment, the photoconductor region A, the charging region B, the transfer region C, the sheet passing region E, and the weakly exposed region Vg have their respective centers in the longitudinal direction substantially equal to the image forming region (the toner image is formed in the longitudinal direction). (A region that can be formed) so as to coincide with the center (center reference). A region having a relatively short width in the longitudinal direction among the above-described regions is included inside a region having a relatively long width.

  As shown in FIGS. 3A and 3B, the paper passing area E and the weak exposure width Vg depend on the size of the recording material P used for image formation (particularly, the width in a direction substantially orthogonal to the transport direction). Can change. Further, in this embodiment, since the image forming apparatus 100 has the configuration based on the center as described above, the center position of the image forming area in the longitudinal direction does not change regardless of the size of the recording material P. In this embodiment, as shown in FIGS. 3A and 3B, in the longitudinal direction, the photoconductor area A, the charging area B, the transfer area C (first area R1), and the paper passing area E (first area). Regarding the magnitude relationship (positional relationship) of the widths of the two regions R2) and the weak exposure region Vg, the relationship is A> B> C> E ≧ Vg regardless of the size of the recording material P. In particular, in the present embodiment, the relationship is E ≒ Vg.

4. Next, the setting of the surface potential of the photosensitive drum will be described with reference to FIGS. The surface potential of the photosensitive drum 1 shown in FIGS. 3A and 3B is a surface potential downstream of the exposure position b and upstream of the transfer position d in the rotation direction of the photosensitive drum 1. Note that an image portion potential Vl described later is not shown in order to avoid complication.

  The surface of the photosensitive drum 1 in the charging area B is substantially uniformly charged to a predetermined charging potential (dark portion potential) Vd (about −420 V in this embodiment) by the charging roller 2. At this time, a predetermined charging voltage (a DC voltage of about -900 V in this embodiment) is applied to the charging roller 2 from a charging power source E1. Note that, as the charging voltage, an alternating voltage in which a DC voltage and an AC voltage are superimposed may be applied.

  Further, an image portion (a portion to which toner is to be adhered) on the surface of the charged photosensitive drum 1 is exposed by the exposure device 3 at a first exposure intensity. As a result, the charge of the portion (image portion) of the surface of the photosensitive drum 1 exposed at the first exposure intensity is removed, and the exposed portion potential (bright portion potential) Vl (about -70 V in this embodiment) is formed. You. The area on the surface of the photosensitive drum 1 that can be exposed at the first exposure intensity corresponds to an image forming area set for each size of the recording material P. In this embodiment, the width of the image forming area in the longitudinal direction is equal to or less than the width of the weakly exposed area Vg in the longitudinal direction.

  Further, the non-image portion (portion where toner is not adhered) in the weakly exposed region Vg on the surface of the charged photosensitive drum 1 is exposed by the exposure device 3 at a second exposure intensity lower than the first exposure intensity. (Weak exposure). As a result, the charge on the portion of the surface of the photosensitive drum 1 exposed at the second exposure intensity (weakly exposed portion) is removed, and a weakly exposed portion potential Vbg (about -365 V in this embodiment) is formed.

  On the other hand, an area F (here, also referred to as an “end area”) within the transfer area C and outside the weakly exposed area Vg on the surface of the charged photosensitive drum 1 is exposed at the first exposure intensity. The exposure with the second exposure intensity (weak exposure) is not performed. That is, since the relationship of E ≧ Vg (especially E ≒ Vg in the present embodiment) is satisfied, the region inside the transfer region C and outside the paper passing region E is exposed to the first exposure intensity and the second exposure region. Exposure by intensity (weak exposure) is not performed. Therefore, the surface potential of the photosensitive drum 1 in the end region F is substantially the same as that after charging, and is maintained at about -420V. As described above, a difference occurs between the end region F and the sheet passing region E in the surface potential of the photosensitive drum 1 downstream of the exposure position b and upstream of the transfer position d in the rotation direction of the photosensitive drum 1. In this embodiment, the potential difference ΔX is about 55V. In this embodiment, since the relationship of E ≒ Vg is satisfied as described above, the end region F is substantially all outside the paper passing region E (hereinafter, “end portion in this embodiment”). An area corresponding to the “area F” may be referred to as a “non-sheet passing area F”.)

  The developing device 4 supplies toner to the exposed portion potential Vl formed on the photosensitive drum 1 and develops it as a toner image. At this time, a predetermined developing voltage (a DC voltage of about -240 V in this embodiment) is applied to the developing roller 4a from the developing power source E2. As a result, a developing electric field is formed between the photosensitive drum 1 and the developing roller 4a. That is, in the portion of the exposure portion potential Vl, a developing electric field is generated in which the toner can transfer from the developing roller 4a to the surface of the photosensitive drum 1. Therefore, the toner is transferred and adheres to the portion of the exposed portion potential Vl. On the other hand, the developing electric field generated at the portion of the weakly exposed portion potential Vbg is less than the electric field at which the toner can transfer from the developing roller 4a to the surface of the photosensitive drum 1. Therefore, the toner does not transfer onto the photosensitive drum 1 from the developing roller 4a at the portion of the weak exposure portion potential Vbg. Note that, as the developing voltage, an alternating voltage in which a DC voltage and an AC voltage are superimposed may be applied.

  The toner image formed on the photosensitive drum 1 is transferred to the recording material P at a transfer position d. At this time, a transfer voltage (a DC voltage of about +1500 V in this embodiment) having a polarity opposite to the normal charge polarity of the toner is applied to the transfer roller 5 from the transfer power source E3. The toner image on the photosensitive drum 1 is transferred onto the recording material P by the transfer electric field generated by the transfer voltage.

5. Transition of Surface Potential of Photosensitive Drum Next, with reference to FIG. 4, transition of the surface potential of the photosensitive drum 1 during image formation in this embodiment and Comparative Examples 1 and 2 will be described. Comparative Examples 1 and 2 are different from the present embodiment in that weak exposure is not performed. Comparative Example 2 differs from Comparative Example 2 in that the charging voltage is relatively high. The configurations of Comparative Examples 1 and 2 are substantially the same as the configuration of the present embodiment, except for the above points.

  FIG. 4 shows the transition of the surface potential of the photosensitive drum 1 for each of the present embodiment and Comparative Examples 1 and 2, (1) after charging, (2) after exposure, (3) after transfer, and (4). It shows the surface potential of the photosensitive drum 1 after re-charging and (5) after re-exposure. The left columns (a), (b), (c), (d), and (e) in FIG. 4 are Comparative Example 1, the central columns (f), (g), (h), (i), and (j). ) Shows the results of Comparative Example 2, and right columns (k), (l), (m), (n), and (о) show changes in the surface potential of the photosensitive drum 1 in this embodiment. As the surface potential of the weakly exposed region Vg, the surface potential of the non-image portion is illustrated, and the surface potential of the image portion is not illustrated to avoid complication. Further, the surface potential of the photosensitive drum 1 is the surface potential of the photosensitive drum 1 in a region corresponding to the weakly exposed region Vg (paper passing region E) in this embodiment, and the end region (non-paper passing region) in this embodiment. And the surface potential of the photosensitive drum 1 in a region corresponding to F.

The surface potential of the photosensitive drum 1 shown in FIG. 4 was measured by using a surface potentiometer TREK JAPAN (Model 344) when the image forming operation was performed under the following conditions. Value. The measured value of the surface potential can vary depending on various conditions such as the environment, the type of recording material P (paper type), the electric resistance of the transfer roller 5, the thickness of the photosensitive layer of the photosensitive drum 1, and the like. Here, the temperature / humidity environment is a temperature of 25 ° C., a humidity of 50%, the paper type is made by Canon (GF600 / A4 size), the electric resistance of the transfer roller 5 is 2.0 × 10 ⁸ Ω, and the photosensitive layer of the photosensitive drum 1 is The film thickness was 12 μm. As a measurement result of the surface potential of the photosensitive drum 1, a representative value such as an average value obtained from a plurality of measurement values can be used.

  Here, FIG. 5 is a graph showing the relationship between Vback and the fog density on the photosensitive drum 1 in this embodiment. The measurement of the fog density on the photosensitive drum 1 was performed as follows. The toner was collected by attaching the adhesive surface of the transparent adhesive tape to the surface of the photosensitive drum 1. Further, the pressure-sensitive adhesive tape was stuck on a predetermined sheet of paper, and the density (optical density) of the pressure-sensitive adhesive tape to which the toner was adhered was measured to quantify the fog density. The fogging density is 0% when no fogging occurs, and the larger the value, the more fogging and the larger amount of toner, indicating that the toner adheres to the surface of the photosensitive drum 1. As shown in FIG. 5, in the configuration of the present embodiment, the fog density is the smallest and becomes 2% when Vback is around 125 V. At this level of fog density, there is no problem. In this embodiment, Vback of the weak exposure area Vg (≒ paper passing area E) is set to be about 125V. When Vback is smaller than the above value, “ground fog” is more likely to occur, and the fog density increases. For example, when Vback becomes 40 V, the fog density becomes 11%. Since the toner that has caused “ground fogging” is charged to the normal charging polarity, it is transferred to the recording material P at the time of transfer, causing image defects. Conversely, when Vback is set to a value larger than the above value, “inversion fog” is more likely to occur, so that the fog density increases. For example, when Vback becomes 180 V, the fog density becomes 6%. Since the toner that has undergone “reverse fog” is a toner of opposite polarity, it is difficult to be transferred to the recording material P during transfer, but a problem may arise in that the toner consumption increases.

<Comparative Example 1>
The transition of the surface potential of the photosensitive drum 1 in Comparative Example 1 ((a), (b), (c), (d), (e) in the left column of FIG. 4) will be described.

  First, the surface potential of the photosensitive drum 1 after charging shown in FIG. In Comparative Example 1, the charging voltage is set to about -845 V, and the surface potential of the photosensitive drum 1 in the charged area B after charging is set to about -365 V.

  Next, the surface potential of the photosensitive drum 1 after the exposure shown in FIG. In Comparative Example 1, since the weak exposure is not performed, the surface potential of the photosensitive drum 1 after the exposure is substantially the same as the surface potential of the photosensitive drum 1 after the charging, and is maintained at about −365 V in the charging region B.

  Next, the surface potential of the photosensitive drum 1 after the transfer shown in FIG. When the transfer is performed, the recording material P acts as an electrical resistor, so that the transfer current flowing through the transfer roller 5 flows intensively in the non-sheet passing area F. Therefore, the surface potential of the photosensitive drum 1 in the non-sheet passing area F is lower than the surface potential of the photosensitive drum 1 in the sheet passing area E. Specifically, after the transfer, the surface potential of the photosensitive drum 1 decreases to about −280 V in the paper passing area E and decreases to about −150 V in the non-paper passing area F.

  Next, the surface potential of the photosensitive drum 1 after the recharging shown in FIG. In Comparative Example 1, after recharging, the surface potential of the photosensitive drum 1 returns to about −365 V in the paper passing area E, but only rises to about −280 V in the non-paper passing area F.

  Next, the surface potential of the photosensitive drum 1 after the re-exposure shown in FIG. In Comparative Example 1, since the weak exposure is not performed, the surface potential of the photosensitive drum 1 is maintained at about −365 V in the paper passing area E and about −280 V in the non-paper passing area F after the re-exposure. As a result, at the time of development, in the paper passing area E, Vback, which is the difference between the surface potential (approximately -365 V) of the photosensitive drum 1 and the development potential (approximately -240 V), is approximately 125 V, which is an optimal value for suppressing fogging. Become. Therefore, in the paper passing area E, the phenomenon that the toner adheres to the surface of the photosensitive drum 1 other than the portion of the exposure portion potential Vl during the development does not occur. On the other hand, in the non-sheet passing area F, Vback, which is the difference between the surface potential (about -280 V) of the photosensitive drum 1 and the development potential (about -240 V), is about 40 V, which is an optimal value (around 125 V) for suppressing fog. Is small. For this reason, in the non-sheet passing area F, a “ground fog” occurs in which toner adheres to the surface of the photosensitive drum 1 during development. In particular, "ground fogging" occurs frequently in a width of about 5 mm near the end of the recording material P, and "ground fogging" improves as the distance from the end of the recording material P to the outside of the sheet passing area E increases. Tend. As a result, for example, when the recording material P is skewed and conveyed during transfer, the toner adhering to the surface of the photosensitive drum 1 in the non-sheet passing area F due to “ground fogging” causes the photosensitive drum 1 to make one revolution. Later, the image is transferred onto the recording material P, and "edge stains" may occur.

<Comparative Example 2>
The transition of the surface potential of the photosensitive drum 1 in Comparative Example 2 (center rows (f), (g), (h), (i), and (j) in FIG. 4) will be described.

  First, the surface potential of the photosensitive drum 1 after charging shown in FIG. In Comparative Example 2, by setting the charging voltage to about −900 V (about −845 V in Comparative Example 1), the surface potential of the photosensitive drum 1 in the charged region B after charging was about −420 V (about −365 V in Comparative Example 1). ).

  Next, the surface potential of the photosensitive drum 1 after the exposure shown in FIG. In Comparative Example 2, since the weak exposure was not performed as in Comparative Example 1, the surface potential of the photosensitive drum 1 after the exposure was substantially the same as the surface potential of the photosensitive drum 1 after the charging, and substantially the entire area of the charged area B. At about -420V.

  Next, the surface potential of the photosensitive drum 1 after the transfer shown in FIG. In Comparative Example 2, similarly to Comparative Example 1, the transfer current flowing through the transfer roller 5 during the transfer is concentrated in the non-sheet passing area F. At this time, in Comparative Example 2, the surface potential of the photosensitive drum 1 in the non-sheet passing area F before transfer is higher at about −420 V than in Comparative Example 1. Therefore, the surface potential of the photosensitive drum 1 in the non-sheet-passing area F after the transfer is higher than that in Comparative Example 1, and is about -240V. Further, in Comparative Example 2, the surface potential of the photosensitive drum 1 before the transfer was as high as about −420 V even in the sheet passing area E, so that the surface potential of the photosensitive drum 1 after the transfer was higher than that in Comparative Example 1. It is about -320V.

  Next, the surface potential of the photosensitive drum 1 after the recharging shown in FIG. In Comparative Example 2, after recharging, the surface potential of the photosensitive drum 1 is made substantially uniform at about −420 V in substantially the entire charging area B. This is because, in Comparative Example 2, the charging voltage is about -900 V (Comparative Example 1 is about -845 V). Is high.

  Next, the surface potential of the photosensitive drum 1 after the re-exposure shown in FIG. In Comparative Example 2, since the weak exposure is not performed as in Comparative Example 1, the surface potential of the photosensitive drum 1 is maintained at about −420 V in substantially the entire charged region B after re-exposure. As a result, at the time of development, Vback, which is the difference between the surface potential (approximately −420 V) of the photosensitive drum 1 and the development potential (approximately −240 V), is approximately 180 V over substantially the entire charging region B, which is optimal for suppressing fogging. This value is large with respect to the value (around 125 V). Therefore, in the comparative example 2, the occurrence of “ground fogging” in the non-sheet passing area F which is a problem in the comparative example 1 during development can be suppressed, but the non-image area on the surface of the photosensitive drum 1 is almost entirely covered with the charging area B. "Reverse fog" occurs in which the reverse polarity toner adheres to the toner. Since the opposite polarity toner is not easily transferred onto the recording material P, it is unlikely to cause an image defect, but there is a case where an increase in toner consumption may cause a problem.

<Example>
The transition of the surface potential of the photosensitive drum 1 (right column (k), (l), (m), (n), (о) in FIG. 4) in this embodiment will be described.

  First, the surface potential of the photosensitive drum 1 after charging shown in FIG. In this embodiment, the surface voltage of the photosensitive drum 1 in the charged area B after charging is set to about -420 V by setting the charging voltage to about -900 V as in Comparative Example 2.

  Next, the surface potential of the photosensitive drum 1 after the exposure shown in FIG. In this embodiment, since the weak exposure is performed, the surface potential of the photosensitive drum 1 after the exposure is about -365 V (weak exposure portion potential Vbg) in the weak exposure region Vg (Ｖ paper passing region E). On the other hand, since the weak exposure is not performed in the non-sheet passing area F, the surface potential of the photosensitive drum 1 after the exposure is maintained at about -420V.

  Next, the surface potential of the photosensitive drum 1 after the transfer shown in FIG. Also in this embodiment, similarly to Comparative Examples 1 and 2, the transfer current flowing through the transfer roller 5 when performing the transfer flows intensively in the non-sheet passing area F. At this time, in the present embodiment, as in Comparative Example 2, the surface potential of the photosensitive drum 1 in the non-sheet passing area F before the transfer is higher at about −420 V than in Comparative Example 1. Therefore, the surface potential of the photosensitive drum 1 in the non-sheet-passing area F after the transfer is higher than that in Comparative Example 1, and is about -240V. On the other hand, in the present embodiment, in the weakly exposed area Vg (≒ paper passing area E), as in Comparative Example 1, the surface potential of the photosensitive drum 1 before transfer is as low as about −365 V, so that the photosensitive drum 1 after transfer is Has a surface potential of about -280 V as in Comparative Example 1.

  Next, the surface potential of the photosensitive drum 1 after the recharging shown in FIG. In the present embodiment, as in Comparative Example 2, after recharging, the surface potential of the photosensitive drum 1 is made substantially uniform at about -420 V in substantially the entire charging area B (the paper passing area E and the non-paper passing area F). Is done. This is because, in the present embodiment, similarly to Comparative Example 2, the charging voltage is as high as about -900 V, and the ability to uniform the unevenness of the surface potential of the photosensitive drum 1 in the longitudinal direction is high.

  Next, the surface potential of the photosensitive drum 1 after the re-exposure shown in FIG. In this embodiment, since the weak exposure is performed, the surface potential of the photosensitive drum 1 after the re-exposure is about -365 V (weakly exposed portion potential Vbg) in the weakly exposed region Vg (≒ paper passing region E). On the other hand, since the weak exposure is not performed in the non-sheet passing area F, the surface potential of the photosensitive drum 1 after the re-exposure is maintained at about -420V. As a result, in the present embodiment, at the time of development, in the non-sheet passing area F, Vback, which is the difference between the surface potential (about −420 V) of the photosensitive drum 1 and the development potential (about −240 V), becomes about 180 V, and This is a large value with respect to the optimal value (around 125 V) for suppression. Therefore, in the present embodiment, the occurrence of “ground fog” in the non-sheet passing area F, which is a problem in Comparative Example 1, can be suppressed during development. Therefore, in this embodiment, even when the recording material is skewed and conveyed at the time of transfer, "edge stain" can be suppressed. On the other hand, in the present embodiment, in the non-sheet passing area F, as described above, Vback is a value larger than the optimum value (around 125 V) for suppressing fog, and thus “reverse fog” occurs. However, the reverse polarity toner having caused the “reverse fog” is difficult to be transferred onto the recording material P, and therefore does not cause an image defect. Further, compared with Comparative Example 2, the toner consumption can be significantly reduced. This is because, in Comparative Example 2, “reversal fog” occurs in substantially the entire area of the charging area B, whereas in the present embodiment, the area in which “reversal fog” occurs is a non-paper passing area which is a part of the charging area B. This is because it is limited to F only. Further, in this embodiment, at the time of development, in the weakly exposed region Vg (≒ paper passing region E), Vback, which is the difference between the surface potential (about −365 V) of the photosensitive drum 1 and the development potential (about −240 V), is about 125 V, which is an optimal value for suppressing fogging. Therefore, in the weakly exposed area Vg (≒ paper passing area E), the phenomenon that the toner adheres to the surface of the photosensitive drum 1 other than the portion of the exposed portion potential V1 during development does not occur, and the “ground fog” is also “Fog”. Inversion fog "is not a problem.

  Table 1 summarizes the problems in Comparative Examples 1 and 2 and the effects of the present embodiment.

  According to the present embodiment, it is possible to suppress the "end fouling" by suppressing the "ground fog" in the non-sheet passing area while suppressing the generation amount of the "reverse fog".

  FIG. 6 is a flowchart illustrating an outline of a control procedure for setting the weak exposure area Vg when a print job is executed. When a print job start instruction is input from the operation unit 120 or the external device 200, the control unit 150 controls the recording material P used for image formation based on the control instruction input from the operation unit 120 or the external device 200. Information about the type is acquired (S1). At this time, the control unit 150 particularly acquires information on the size of the recording material P (particularly, the width in a direction substantially orthogonal to the transport direction). When the type (size) of the recording material P is not specified, the control unit 150 may determine that the specific type (size) of the recording material P is used for image formation. Further, the type (size) of the recording material P is not limited to being specified by the operator in the operation unit 120 or the external device 200, and the control unit 150 can automatically recognize the recording material P based on the detection result of the detection unit. It may be as follows. Next, the control unit 150 acquires information on the weak exposure area Vg preset for each type (size) of the recording material P and stored in the memory 152 as a data table or the like. A weak exposure area Vg is set (S2). For example, when the size of the recording material P used for image formation is the LTR size, the weak exposure area Vg is set as shown in FIG. Further, for example, when the size of the recording material P used for image formation is A5 size, the weak exposure area Vg is set as shown in FIG. As described above, in this embodiment, the weak exposure area Vg is changed according to the size of the recording material P used for image formation. Thereafter, the control unit 150 starts the image formation (S3).

  In the present embodiment, the relationship of E ≒ Vg is set, but the relationship of E> Vg may be set. However, as the difference between the sheet passing area E and the weakly exposed area Vg increases, that is, as the weakly exposed area Vg becomes narrower than the sheet passing area E, the amount of “reversal fog” tends to increase. On the other hand, if the relationship of E <Vg is satisfied, the effect of suppressing the occurrence of “ground fogging” and the “end portion contamination” cannot be obtained. Therefore, ideally, it is desirable to satisfy the relationship of E = Vg. In this embodiment, E ≒ Vg is set in consideration of the tolerance of the exposure position and the like so that E <Vg is not satisfied. Here, E ≒ Vg, that is, the substantially coincidence between the paper passing area E and the weakly exposed area Vg means not only the case where they completely coincide with each other but also the allowable effect in view of the effect of suppressing the edge stain and the effect of reducing the toner consumption. It may include the case where the error is shifted to the extent possible (for example, ± 3 mm). In order to prevent E <Vg from being satisfied, it is only necessary to prevent Vg from being outside of E by exceeding such an error.

  Further, in the present embodiment, the case where the image forming apparatus 100 has the configuration based on the center has been described as an example. However, the present invention is also applicable to the configuration based on the one-sided reference. Obtainable. Note that the configuration based on the offset standard is a configuration in which recording materials P of different sizes are conveyed such that the position of one end in a direction substantially orthogonal to the conveyance direction coincides. Also in the offset standard configuration, similarly to the present embodiment, the weak exposure area Vg is set according to the size of the recording material P, and the surface potential of the photosensitive drum 1 in the first area R1 and the second area R2 is reduced. Just set it.

  Further, the invention is not limited to changing the weak exposure area Vg between all sizes (especially, width in a direction substantially orthogonal to the transport direction) that can be used for image formation in the image forming apparatus 100. The weak exposure area Vg may be changed between some sizes. In this case, the size of the recording material P whose light exposure area Vg is to be changed may be set in advance or may be arbitrarily specified by the operator. The size of the recording material P for which the weak exposure region Vg is to be changed is not limited to two types as illustrated in FIGS. 3A and 3B, but is three or more types. Is also good.

  As described above, in the present embodiment, the image forming apparatus 100 includes the rotatable photosensitive member 1, the charging unit 2 for charging the surface of the photosensitive member 1, and the image portion on the surface of the charged photosensitive member 1. Exposing means 3 for exposing at a first exposure intensity to form an electrostatic image on the photoreceptor. Further, the image forming apparatus 100 includes a developing unit 4 that forms a toner image by attaching toner to an image portion of the electrostatic image on the photoconductor 1. Further, the image forming apparatus 100 is capable of contacting with the photoreceptor 1 and, when a voltage is applied thereto, transfers a toner image from the photoreceptor 1 to the recording material P sandwiched between the photoreceptor 1 and the transfer member 5. Having. Further, the image forming apparatus 100 includes a control unit 150 that controls the exposure unit 3. Here, a region where the photoconductor 1 contacts the transfer member 5 in a direction substantially orthogonal to the moving direction of the surface of the photoconductor 1 is referred to as a first region R1. Further, in the same direction, a predetermined area preset for each size of the recording material P with respect to an area where the recording material P can come into contact with the photoconductor 1 is referred to as a second area (sheet passing area) R2. In the same direction, a region where the non-image portion on the surface of the photoreceptor 1 is exposed by the exposure unit 3 at a second exposure intensity lower than the first exposure intensity is defined as a weak exposure region Vg. At this time, the control unit 150 controls the position in the first area and the area outside the second area downstream from the exposure position b where the exposure is performed in the rotation direction of the photoconductor 1 and upstream from the transfer position d where the transfer is performed. The absolute value Va of the surface potential of the photoconductor 1 is larger than the absolute value Vb of the surface potential of the non-image portion on the surface of the photoconductor 1 in the second region. The weak exposure area Vg is changed according to the size. In this embodiment, the second region R2 is a region where the recording material P and the photosensitive member 1 can come into contact with each other when the variation in the conveyance of the recording material P is negligibly small or absent. Further, in the present embodiment, the weakly exposed region Vg is a region substantially coinciding with the second region R2 in a direction substantially orthogonal to the moving direction of the surface of the photoconductor 1.

  As described above, according to the present embodiment, edge fouling of the recording material P can be suppressed while suppressing reverse fog according to the size of the recording material P used for image formation.

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

  According to the first embodiment, the amount of “reverse fog” can be suppressed, but “reverse fog” occurs in the non-sheet passing area F, and the toner consumption slightly increases. In this embodiment, this toner consumption is further suppressed.

  FIG. 7 is a view for explaining the positional relationship in the longitudinal direction of each part and the setting of the surface potential of the photosensitive drum 1 in this embodiment. The surface potential of the photosensitive drum 1 shown in FIG. 7 is a surface potential downstream of the exposure position b and upstream of the transfer position d in the rotation direction of the photosensitive drum 1. Note that the image portion potential Vl is not shown in order to avoid complication. Although FIG. 7 shows a case where the recording material P has the LTR size, the weak exposure area is changed according to the size of the recording material P in this embodiment as in the first embodiment.

  In FIG. 7, “A”, “B”, “C”, “E”, and “F” indicate the same regions as those described in the first embodiment. The paper passing area E described in the first embodiment is such that when the recording material P is nipped and conveyed between the transfer roller 5 and the photosensitive drum 1, conveyance variations such as skew of the recording material P are negligible. In the case where the recording material P does not exist, the recording material P in the longitudinal direction of the transfer position d can pass through. On the other hand, in FIG. 7, “D” indicates the transfer when the recording material P is conveyed while being pinched and conveyed between the transfer roller 5 and the photosensitive drum 1, in consideration of the conveyance variation such as the skew of the recording material P. This is the width of the largest area through which the recording material P can pass in the longitudinal direction of the position d, or the area thereof (here, also referred to as the “maximum sheet passing area”).

  The maximum paper passing area D has a width of the area where the recording material P and the photosensitive drum 1 can come into contact with each other in the longitudinal direction of the transfer position d when there is a variation in conveyance such as skew of the recording material P. This corresponds to the largest region (also referred to as “third region” here) R3.

  Then, in the present embodiment, the weak exposure is performed in the first weak exposure region Vg1 and the second weak exposure region Vg2 shown in FIG. 7 as the weak exposure region Vg. The first weakly exposed region Vg1 is a region corresponding to the weakly exposed region Vg in the first embodiment. In the present embodiment, as in the first embodiment, the relationship is Vg1 ≒ E. The second weakly exposed region Vg2 will be further described below.

  In the first embodiment, as shown in FIG. 3, the surface potential of the photosensitive drum 1 is increased in the end region F. On the other hand, the present embodiment differs from the first embodiment in that the surface potential of the photosensitive drum 1 is increased only in the region W as shown in FIG. Here, the area W is an area inside the maximum paper passing area D and outside the paper passing area E.

  That is, in Example 1, as shown in FIG. 3, at the time of exposure, only the non-image portion in the weakly exposed region Vg was subjected to the weak exposure. On the other hand, in the present embodiment, as shown in FIG. 7, in addition to the non-image portion in the first weak exposure region Vg1 corresponding to the weak exposure region Vg in the first embodiment, the transfer region C and the maximum Light exposure is also performed on the second light exposure area Vg2, which is an area outside the paper area D.

  In the present embodiment, similarly to the first embodiment, even if the transfer current is concentrated in the non-sheet passing portion, particularly in the vicinity of both ends of the recording material P, the surface potential of the photosensitive drum 1 in the vicinity of both ends of the recording material P is reduced. It is possible to suppress the decrease. Therefore, it is possible to suppress the occurrence of “ground fog” and to suppress the “dirt at the edge”. Further, in the present embodiment, the area where “reverse fogging” occurs can be limited only to the area W that is narrower than the end area F in the first embodiment, so that the toner consumption can be further reduced compared to the first embodiment. it can.

  According to the present embodiment, it is possible to further suppress the occurrence of “reverse fog” and suppress the “land fog” in the non-sheet-passing area, thereby suppressing the “dirt at the edge”. Becomes

  In this embodiment, the exposure intensity in the second weakly exposed region Vg2 is substantially the same as the exposure intensity in the first weakly exposed region Vg1, but may be different. The exposure intensity can be set to a potential optimized for each of the first and second weakly exposed regions.

  As described above, in the direction substantially perpendicular to the direction of movement of the surface of the photoconductor 1, the second area ( A predetermined area wider than the sheet passing area R2 is defined as a third area (maximum sheet passing area) R3. At this time, in the present embodiment, the control unit 150 changes the weak exposure area Vg (first weak exposure area Vg1) as in the first embodiment, and at the downstream of the exposure position b in the rotation direction of the photoconductor 1. In addition, the absolute value Vc of the surface potential of the photoconductor in the third area and outside the second area upstream of the transfer position d is the surface of the non-image portion of the surface of the photoconductor 1 in the second area. The absolute value Vb of the potential and the absolute value Vd of the surface potential of the photoconductor 1 in the first area and the area outside the third area are larger than the absolute value Vd of the recording material P used for image formation. To change the weak exposure area Vg (second weak exposure area Vg2). In the present embodiment, the second region R2 is a region where the recording material P and the photoconductor 1 can come into contact with each other when the variation in the conveyance of the recording material P is negligibly small or absent. In the present embodiment, the third region R3 is the region having the largest width among the regions where the recording material P and the photoconductor 1 can come into contact with each other when the conveyance of the recording material P varies. Further, in the present embodiment, in the direction substantially perpendicular to the moving direction of the surface of the photoconductor 1, the weakly exposed region Vg is substantially the same as the second region R2 (the first weakly exposed region Vg1) and the weakly exposed region Vg. 3 is a region (second weakly exposed region Vg2) from the end of the region R3 to the end of the second region R2.

  As described above, according to this embodiment, the same effects as those of the first embodiment can be obtained, and the amount of toner consumption can be further reduced.

REFERENCE SIGNS LIST 1 photosensitive drum 2 charging roller 3 exposing device 4 developing device 5 transfer roller 100 image forming device 120 operating unit 150 control unit 200 external device

Claims (6)

A rotatable photoconductor,
Charging means for charging the surface of the photoreceptor,
Exposure means for exposing an image portion on the surface of the photoconductor subjected to the charging treatment with a first exposure intensity to form an electrostatic image on the photoconductor,
Developing means for forming a toner image by attaching toner to an image portion of the electrostatic image on the photoconductor,
A transfer member that can be brought into contact with the photoconductor, and that is applied with a voltage to transfer a toner image from the photoconductor to a recording material sandwiched between the photoconductor;
Control means for controlling the exposure means,
In the image forming apparatus having
The control unit may include a first region in which the photosensitive member contacts the transfer member in a direction substantially perpendicular to a moving direction of the surface of the photosensitive member, and a region in which a recording material can contact the photosensitive member. A second region is exposed to a predetermined region preset for each size of the recording material, and a non-image portion on the surface of the photoreceptor is exposed by the exposure unit at a second exposure intensity lower than the first exposure intensity. When the region to be exposed is a weakly exposed region, the first region and the second region are located downstream from the exposure position where the exposure is performed and upstream from the transfer position where the transfer is performed in the rotation direction of the photoconductor. The absolute value Va of the surface potential of the photoconductor in the outer region is larger than the absolute value Vb of the surface potential of the non-image portion of the surface of the photoconductor in the second region. Of recording materials Image forming apparatus and changes the weak exposure region in accordance with's.   2. The image according to claim 1, wherein the second area is an area where the recording material and the photoconductor can come into contact with each other when the variation in the conveyance of the recording material is negligible or negligible. 3. Forming equipment.   The image forming apparatus according to claim 1, wherein the weakly exposed region is a region substantially coincident with the second region in a direction substantially perpendicular to a moving direction of a surface of the photoconductor.   The control unit is configured to set, in a direction substantially perpendicular to the moving direction of the surface of the photoconductor, a second area, which is set in advance for each area of the recording material with respect to an area where the recording material can contact the photoconductor. When a predetermined area having a large width is set as a third area, the third area and the second area are located downstream of the exposure position and upstream of the transfer position in the rotation direction of the photoconductor. The absolute value Vc of the surface potential of the photoconductor in the area is the absolute value Vb of the surface potential of the non-image portion on the surface of the photoconductor in the second area, and the absolute value Vb of the surface potential in the first area and the third area. 2. The light exposure area according to claim 1, wherein the light exposure area is changed according to the size of a recording material used for image formation so as to be larger than an absolute value Vd of a surface potential of the photoconductor in an area outside the area. Image forming apparatus as described   The second area is an area where the recording material and the photosensitive member can come into contact with each other when the variation in the transport of the recording material is negligibly small or absent, and the third area is the transport area of the recording material. 5. The image forming apparatus according to claim 4, wherein a width of the recording material and the photosensitive member in a case where there is a variation is largest.   In a direction substantially orthogonal to the direction of movement of the surface of the photoconductor, the weakly exposed region is a region substantially coinciding with the second region, and an end of the third region to an end of the second region. The image forming apparatus according to claim 4, wherein the image forming apparatus is an area up to the area.

Images