JP6410559B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic system.

  2. Description of the Related Art Conventionally, in an image forming apparatus using an electrophotographic system, an electrophotographic photosensitive member (photosensitive member) whose surface is uniformly charged by a charging unit such as a charging roller is exposed in accordance with image information by an exposure unit. Thus, an electrostatic latent image is formed on the photoreceptor. Thereafter, the electrostatic latent image on the photosensitive member is developed as a toner image with toner, and the toner image is recorded as a transfer target in a transfer portion formed by contact between the photosensitive member and a transfer means such as a transfer roller. It is transferred to a recording material such as paper.

  As a developing method for developing the electrostatic latent image on the photoreceptor, a reversal developing method is often used. In the reversal development method, a toner image is formed by adhering toner charged to the same polarity as the charged polarity of the photosensitive member to a portion where the absolute value of the potential has decreased due to exposure of the photosensitive member.

  In recent years, in order to reduce the size of such an image forming apparatus, there is an image forming apparatus that employs a cleanerless system that does not have a special cleaning device that collects transfer residual toner remaining on a photoreceptor after transfer. (Patent Documents 1 and 2). In the cleanerless system, the transfer residual toner is collected by the developing means (simultaneous development collection). The cleanerless system is effective from the viewpoint of environmental protection because it is effective for downsizing the apparatus and does not emit waste toner.

JP 2003-330252 A JP 2004-191766 A

  By the way, in an image forming apparatus that employs a cleanerless system, if transfer residual toner remains on the photosensitive member after passing through the developing unit, it is transferred to the transfer target at the transfer unit and becomes a transfer residual ghost. Appears.

  Thus, it can be considered that the untransferred toner on the photosensitive member is collected more favorably in the developing unit as follows. That is, by increasing the back contrast potential Vback, which is the potential difference between the non-image portion potential Vd and the potential Vdc of the developing roller provided in the developing means, the transfer residual toner on the non-image portion (Vd portion) is developed. Increase the pulling force on the roller side. Note that here, the magnitude relationship among the potential, voltage, and current is that when compared in terms of absolute values.

  As a method of increasing Vback, it is conceivable to increase Vd or decrease Vdc. However, when Vdc is reduced, the development contrast potential Vcont, which is the potential difference between Vdc and the image portion potential Vl, is reduced, so that the toner on the developing roller does not easily move to the image portion (Vl portion) on the photoreceptor. turn into. Therefore, it is conceivable to set the potential so that Vcont is not reduced while Vback is increased by increasing Vd.

  However, if the potential setting is such that the difference between the non-image portion potential Vd and the image portion potential Vl is large, the transfer current flows into the non-image portion at the transfer portion, so that the toner on the image portion is transferred onto the transfer target. May become insufficient.

  Accordingly, an object of the present invention is to provide an image forming apparatus that employs a cleanerless system, in which the transfer residual toner can be recovered well in the developing unit, and the toner image can be transferred onto the recording material in the transfer unit. A forming apparatus is provided.

The above object is achieved by the image forming apparatus according to the present invention. In summary, the present invention relates to a rotatable photoreceptor, charging means for charging the photoreceptor to a predetermined polarity, and image exposure for exposing the charged photoreceptor to form an electrostatic image on the photoreceptor. And a developing unit for supplying a toner charged with the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photosensitive member at the developing unit, and a toner image on the photosensitive member at the transfer unit. A transfer means for electrostatically transferring a toner image onto a transfer target; a pre-transfer exposure means for exposing the photoconductor downstream of the developing section and upstream of the transfer section in the rotation direction of the photoconductor; and the transfer Control means for changing the exposure amount by the pre-exposure means, and in the image forming apparatus for collecting the toner remaining on the photoreceptor after the transfer to the developing means in the developing unit, the control means includes: When passing through the transfer section Regarding the potential difference between the non-image portion and the image portion of the electrostatic image on the light body, the potential difference between the image forming apparatus and the usage amount of the image forming apparatus is the first value. In the image forming apparatus, the exposure amount by the pre-transfer exposure unit is changed so that the potential difference approaches when the value correlating with the usage amount is a second value larger than the first value. is there.
According to another aspect of the present invention, a rotatable photoconductor, a charging unit that charges the photoconductor to a predetermined polarity, and an image that exposes the charged photoconductor to form an electrostatic image on the photoconductor. An exposure unit; a developing unit configured to supply a toner charged with the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photoconductor in the developing unit; and a toner image formed on the photoconductor in the transfer unit. Transfer means for electrostatically transferring the toner image to the transfer member, pre-transfer exposure means for exposing the photoconductor downstream of the developing unit and upstream of the transfer unit in the rotation direction of the photoconductor, And an image forming apparatus that collects toner remaining on the photoconductor after the transfer to the developing unit in the developing unit. , When passing through the transfer section Regarding the potential difference between the non-image portion and the image portion of the electrostatic image on the photosensitive member, the potential difference of the image forming apparatus is larger than the potential difference when the value correlated with the usage amount of the image forming apparatus is the first value. An image in which the exposure amount by the pre-transfer exposure unit is changed so that the potential difference becomes smaller when the value correlated with the usage amount is a second value larger than the first value. A forming apparatus is provided.
According to still another aspect of the present invention, a rotatable photoconductor, a charging unit for charging the photoconductor to a predetermined polarity, and exposing the charged photoconductor to form an electrostatic image on the photoconductor. An image exposing unit; a developing unit configured to supply a toner charged with the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photosensitive member at a developing unit; and a photosensitive unit configured to form a toner image. Transfer means for electrostatically transferring the toner image on the transfer body; and pre-transfer exposure means for exposing the photoconductor downstream of the developing unit and upstream of the transfer unit in the rotation direction of the photoconductor; Control means for changing the amount of exposure by the pre-transfer exposure means, and the control means for collecting toner remaining on the photoconductor after the transfer to the developing means in the developing unit. Passes through the developing section On the photoconductor when passing through the developing unit, rather than the exposure amount by the pre-transfer exposure means when the absolute value of the potential of the image portion of the electrostatic image on the photoconductor is a first value. An image forming apparatus characterized in that the exposure amount by the pre-transfer exposure unit is reduced when the absolute value of the potential of the image portion of the electrostatic image is a second value larger than the first value. Provided.

  According to the present invention, in an image forming apparatus employing a cleaner-less system, it is possible to collect the transfer residual toner satisfactorily at the developing portion and to transfer the toner image onto the recording material satisfactorily at the transfer portion.

1 is a schematic cross-sectional configuration diagram of an image forming apparatus of Embodiment 1. FIG. 2 is a schematic control block diagram of a main part of the image forming apparatus according to Embodiment 1. FIG. FIG. 6 is a graph showing the relationship between the driver current value of the pre-transfer exposure apparatus of Example 1 and the surface potential of the photosensitive drum. 5A is a schematic diagram of a surface potential of a photosensitive drum of Comparative Example 1 and FIG. It is a graph which shows the relationship between the transfer voltage and transfer residual density of Example 1 and Comparative Example 1. FIG. 6 is a schematic control block diagram of a main part of an image forming apparatus according to a second embodiment. FIG. 6 is a graph showing the relationship between the driver current value of the pre-transfer exposure apparatus of Example 2 and the surface potential of the photosensitive drum. It is a schematic diagram of the surface potential of the photosensitive drum of Comparative Example 2 in (a) L / L environment and (b) H / H environment. FIG. 6 is a schematic diagram of the surface potential of the photosensitive drum of Example 2 in (a) L / L environment and (b) H / H environment. FIG. 10 is a graph showing the relationship between the driver current value of the pre-transfer exposure apparatus of Example 3 and the surface potential of the photosensitive drum. 5A is a schematic diagram of the surface potential of the photosensitive drum of Comparative Example 3 and FIG. 10 is a schematic control block diagram of a main part of an image forming apparatus according to Embodiment 4. FIG. FIG. 10 is a graph showing the relationship between the driver current value of the pre-transfer exposure apparatus of Example 4 and the surface potential of the photosensitive drum. It is a schematic diagram of the surface potential of the photosensitive drum of Comparative Example 4 after (a) initial stage and (b) repeated use. It is a schematic diagram of the surface potential of the photosensitive drum of Example 4-1 after (a) initial stage and (b) repeated use. It is a schematic diagram of the surface potential of the photosensitive drum of Example 4-2 after (a) initial stage and (b) repeated use. It is a graph which shows the relationship between the transfer voltage after the repeated use of Example 4-1, Example 4-2, and Comparative Example 4, and a transfer residual density.

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

[Example 1]
1. FIG. 1 is a schematic cross-sectional configuration diagram of an image forming apparatus 100 of the present embodiment. The image forming apparatus 100 of this embodiment is an electrophotographic LBP (laser beam printer).

  The image forming apparatus 100 includes a photosensitive drum 1 that is a rotatable drum type (cylindrical) photosensitive member as an image carrier that carries an electrostatic latent image. The photosensitive drum 1 is rotationally driven in the direction of arrow a in the figure by a drive source (not shown). The following devices are arranged around the photosensitive drum 1 along the rotation direction of the photosensitive drum 1. First, a charging roller 5 as a charging member (contact charging member) as a charging unit is disposed. Next, an exposure device (laser scanner) 6 as an image exposure unit is arranged. Next, a developing device 3 as a developing unit is arranged. Next, a transfer roller 4 which is a roller-shaped transfer member (contact transfer member) as a transfer unit is disposed.

  At the time of image formation, the surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined polarity (negative polarity in this embodiment) by the charging roller 5. At this time, a predetermined charging voltage (charging bias) is applied to the charging roller 5 from a charging power supply (high-voltage power supply) E1 (FIG. 2) as a charging voltage application means. The surface of the charged photosensitive drum 1 is scanned and exposed by the exposure device 6 with a laser beam corresponding to the image information. Thereby, an electrostatic latent image (electrostatic image) is formed on the photosensitive drum 1. The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) as a toner image using toner by the developing device 3. At this time, a predetermined developing voltage (developing bias) is applied to the developing roller 2 of the developing device 3 from a developing power supply (high-voltage power supply) E2 (FIG. 2) as a developing voltage applying means. The toner image formed on the photosensitive drum 1 is nipped and conveyed between the photosensitive drum 1 and the transfer roller 4 by the action of the transfer roller 4 in a transfer portion (transfer nip) N where the photosensitive drum 1 and the transfer roller 4 abut. The recording material P to be transferred is transferred. At this time, a predetermined transfer voltage (transfer bias) is applied to the transfer roller 4 from a transfer power supply (high voltage power supply) E3 (FIG. 2) as transfer voltage application means. The transfer residual toner remaining on the photosensitive drum 1 after the transfer is simultaneously collected by the developing device 3. The recording material P onto which the toner image has been transferred is then conveyed to a fixing device 30 as a fixing unit, and the toner image thereon is fixed (fixed) by being heated and pressed in the fixing device 30. Thereafter, the recording material P is transported onto the transport tray 31. This completes a series of image forming processes.

  In the present embodiment, the photosensitive drum 1, the charging roller 5 as a process unit acting on the photosensitive drum 1, and the developing device 3 constitute a process cartridge 7 that is detachable from the apparatus main body 50 of the image forming apparatus 100. However, the photosensitive drum 1 and the process means may be fixedly installed on the apparatus main body 50.

  The exposure device 6 irradiates the photosensitive drum 1 with a laser beam that is input to the image forming apparatus 100 or controlled ON / OFF according to an image signal generated inside the apparatus main body 50 such as a test pattern. . As a result, an electrostatic latent image (digital latent image) is formed on the photosensitive drum 1. The exposure device 6 is not limited to a laser scanner, and an exposure device such as an LED print head method or a liquid crystal shutter array method can also be applied. As a method for obtaining density gradation by modulating an image signal input to the exposure apparatus 6, laser light intensity modulation, area gradation methods such as an error diffusion method and a dither method, and combinations thereof can be used. Further, multi-value recording by area gradation of one pixel can be performed using a PWM (pulse width modulation) method. The image signal can be changed at 256 gradation levels from 00h (white) to FF (black). In this embodiment, the PWM method is used.

  The developing device 3 has a developer carrying member for carrying the developer and transporting it to the photosensitive drum 1, and forms a developing portion (developing region) G by bringing the developer carrying member into contact with the photosensitive drum 1. In this developing portion G, the developer is electrically attached to the electrostatic latent image on the photosensitive drum 1. As a result, the electrostatic latent image is visualized to form a toner image. In this embodiment, the developing device 3 adopts a reversal developing method, and supplies toner charged to the same polarity as the charged polarity of the photosensitive member by the developing unit G to the image portion of the electrostatic image on the photosensitive member. A toner image is formed. The developing device 3 includes a developing container 11 that contains a nonmagnetic one-component developer (nonmagnetic toner, toner) 10 as a developer. The developing container 11 is provided with a developing roller 2 as a developer carrying member, a developing blade 9 as a developer regulating member, a toner supply roller 8 as a developer supplying member, and a stirring blade 12 as a stirring member. Yes.

  The developing roller 2 has a multilayer structure in which an elastic layer having a base layer and a surface layer thereon is provided around a cylindrical body of metal such as aluminum, an alloy thereof, or stainless steel. The base layer of the elastic layer is formed using rubber such as butadiene acrylonitrile rubber (NBR), ethylene-propylene-diene polyethylene (EPDM), silicone rubber, urethane rubber, and the surface layer is formed using ether urethane or nylon. . However, the material of the elastic layer is not limited to these, and a foamed material such as sponge may be used for the base layer, and a rubber elastic layer may be formed on the surface layer. Further, the elastic layer may have a single-layer configuration including only a rubber elastic layer such as NBR, EPDM, or urethane rubber. The developing roller 2 is disposed in contact with the photosensitive drum 1. The developing roller 2 is rotationally driven by a driving source (not shown) in the direction of arrow b in the drawing so that the moving directions of the photosensitive drum 1 and the developing roller 2 are the same in the contact portion.

  The developing blade 9 is provided above the developing roller 2 so as to be supported by the holding plate 13 and to be in contact with the outer peripheral surface of the developing roller 2 in the surface contact state near the free end. The abutting direction of the developing blade 9 is a so-called counter direction in which the tip end side is located upstream of the abutting portion in the rotation direction of the developing roller 2. In this embodiment, the developing blade 9 is formed by bonding a polyamide elastomer as the elastic member 9b to a phosphor bronze plate having spring elasticity as the metal thin plate 9a or by injection molding. The elastic member 9b is in contact with the surface of the developing roller 2 with a predetermined linear pressure. When the metal blade 9a maintains the pressure contact force of the developing blade 9 against the developing roller 2 and the toner is a negatively chargeable toner, the polyamide elastomer of the elastic member 9b gives a charge to the toner. The thin metal plate 9a is not particularly limited as long as it maintains the pressure contact force of the developing blade 9, and the elastic member 9b can be selected in consideration of the charging property of the toner. Further, it is not necessary to provide a toner charging member such as the elastic member 9b, and a metal thin plate 9a having spring elasticity such as a stainless thin plate or a phosphor bronze thin plate may be brought into contact with the developing roller 2.

  The toner supply roller 8 preferably has a sponge structure or a fur brush structure in which fibers such as rayon or nylon are planted on a core metal from the viewpoint of supplying toner to the developing roller 2 and peeling off the toner from the developing roller 2. . In this embodiment, the toner supply roller 8 is an elastic roller provided with urethane foam on a core metal. The toner supply roller 8 is disposed in contact with the developing roller 2 and rotates in the direction of the arrow c in the figure so that the moving direction of the developing roller 2 and the toner supply roller 2 is opposite (counter direction) at the contact portion. Driven.

  In this embodiment, the toner regulated on the developing blade 9 and carried on the developing roller 2 is a non-magnetic one-component developer. For this reason, as the force for constraining the toner on the developing roller 2, only the reflection force due to the charge of the toner and a slight van der Waals force act. As a result, when the toner layer on the developing roller 2 becomes thick, the mirroring force with respect to the toner in the upper layer portion of the toner layer becomes weak, so that the toner cannot be carried on the developing roller 2 and the toner is scattered. Therefore, it is necessary to regulate the toner layer on the developing roller 2 thinly. As a result, it may be difficult to obtain a sufficient image density. In such a case, it is possible to obtain a sufficient image density by setting the peripheral speed of the developing roller 2 faster than the peripheral speed of the photosensitive drum 1. As the peripheral speed ratio, a range in which the peripheral speed of the developing roller 2 is 1.1 to 3 times the peripheral speed of the photosensitive drum 1 is preferable. In this embodiment, the peripheral speed ratio is 1.3 times.

  In the image forming apparatus 100 of this embodiment, the transfer residual toner remaining on the photosensitive drum 1 after the transfer is charged when passing through the charging portion of the photosensitive drum 1 by the charging roller 5, and then in the developing portion G, the photosensitive drum 1. The developer is collected in the developing container 11 by the developing roller 2 in contact with the developer. This achieves a cleanerless system.

  The transfer roller 4 that is a contact transfer member contacts the surface of the photosensitive drum 1 with a predetermined pressing force (1 kgf in this embodiment). The transfer roller 4 rotates following the rotation of the photosensitive drum 1. A transfer voltage Vtr, which is a DC voltage, is applied to the transfer roller 4 from the transfer power source E 3, so that the toner image on the photosensitive drum 1 is covered at the transfer nip N between the photosensitive drum 1 and the transfer roller 4. It is electrostatically transferred onto the recording material P as a transfer body. The transfer roller 4 is arranged so that the longitudinal direction (rotation axis direction) of the photosensitive drum 1 and the longitudinal direction (rotation axis direction) of the transfer roller 4 are substantially parallel.

In this embodiment, the transfer roller 4 is configured by providing an elastic layer made of conductive rubber around the cored bar. The electric resistance value of the transfer roller 4 is preferably adjusted to 10 ⁶ to 10 ¹⁰ Ω. In the transfer roller 4 of the present embodiment, the diameter of the elastic layer is φ14 mm, the diameter of the cored bar is φ6 mm, and the thickness of the elastic layer is 4 mm over the entire area in the longitudinal direction. In this embodiment, as the rubber of the elastic layer of the transfer roller 4, foam type NBR rubber is used. Further, the electrical resistance value of the transfer roller 4 of this embodiment is 23 ° C./50% R.D. H. 1.0 × 10 ⁸ Ω in the environment.

  Here, in the image forming apparatus 100 employing the cleanerless system, when a large amount of untransferred toner is present after the transfer, it cannot be sufficiently collected by the developing unit G, and is transferred to the recording material P after one rotation of the photosensitive drum 1. “Transfer residual ghost” may occur. That is, the image forming apparatus 100 employing the cleanerless system is required to have a higher transfer efficiency than the image forming apparatus having the cleaning device.

  In the image forming apparatus 100 of the present embodiment, the following conditions are suitable as conditions for the electrostatic latent image formed on the photosensitive drum 1. When a negatively chargeable toner is used, the non-image area potential (Vd) is preferably in the range of −500 to −1000 V, and the image area potential (Vl) at which the maximum toner image density can be obtained is in the range of −50 to −200 V. Is preferred. When positively charged toner is used, the non-image portion potential (Vd) is preferably in the range of +500 to +1000 V, and the image portion potential (Vl) at which the maximum toner image density is obtained is preferably in the range of +50 to +200 V. It is. In this embodiment, negatively chargeable toner is used.

  When developing the electrostatic latent image formed on the photosensitive drum 1 with toner, a developing voltage, which is a DC voltage, is applied to the developing roller 2 from the developing power source E2. In this embodiment, when the electrostatic latent image is formed under the above conditions, the following conditions are suitable as the development voltage conditions. The development contrast potential | Vl−Vdc | (Vcont), which is the potential difference between the development voltage (Vdc) and the image portion potential (Vl) at which the maximum toner image density is obtained, is preferably in the range of 50 to 400V. On the other hand, in this embodiment, it is necessary to increase the back contrast potential | Vd−Vdc | (Vback), which is a potential difference between the development voltage (Vdc) and the non-image portion potential (Vd). This is to improve the recoverability of the transfer residual toner while taking Vcont sufficiently to put the toner on the image portion. By increasing Vback, the potential difference between the non-image portion potential on the photosensitive drum 1 and the potential of the developing roller 2 increases, and an electric force that pulls back the transfer residual toner after passing through the charging portion to the developing roller 2 is generated. As a result, the transfer residual toner is better collected on the developing roller 2. Therefore, in this embodiment, Vd = −900V, Vl = −100V, Vdc = −300V, | Vcont | = 200V, and | Vback | = 600V.

  FIG. 2 is a block diagram illustrating a schematic control mode of a main part of the image forming apparatus 100 according to the present exemplary embodiment. The image forming apparatus 100 includes an engine controller 18 that performs overall control of operations of each unit of the image forming apparatus 100. The engine controller 18 includes a CPU that is a central element that performs arithmetic processing, a ROM that is a storage element (storage unit), a memory such as a RAM, and the like. The RAM stores sensor detection results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance, and the like. In particular, in relation to the present embodiment, the engine controller 18 includes a charging power source E1, a developing power source E2, a transfer power source E3, and a pre-transfer exposure drive circuit (hereinafter referred to as “driver”) that drives a pre-transfer exposure device 14 described later. 16) and the like are connected. In this embodiment, the engine controller 18 turns ON / OFF the output of each power source E1, E2, E3, the output value, ON / OFF the supply of current from the driver 16 to the pre-transfer exposure device 14, and the supplied current. The value (hereinafter also referred to as “driver current value”) is controlled.

2. Next, the pre-transfer exposure apparatus 14 as the pre-transfer exposure means of this embodiment will be described. The pre-transfer exposure device 14 is disposed downstream of the developing unit G and upstream of the transfer nip N in the rotation direction of the photosensitive drum 1. Then, the pre-transfer exposure device 14 passes the developing portion G between the developing portion G and the transfer nip N in the rotation direction of the photosensitive drum 1 and then scans the surface of the photosensitive drum 1 before passing the transfer nip N. Exposure is performed to remove the non-image portion potential on the photosensitive drum 1. As a result, the non-image portion potential on the photosensitive drum 1 when passing through the transfer nip N (hereinafter also referred to as “transfer passage”) is set to pass through the developing portion G (hereinafter referred to as “development passage”). The non-image portion potential on the photosensitive drum 1 is changed.

  As the pre-transfer exposure device 14, an exposure device such as a laser scanner device, an LED print head method, or a liquid crystal shutter array method can be used. Alternatively, an LED lamp may be provided on the side surface of the apparatus main body 50 of the image forming apparatus 100, and the surface of the photosensitive drum 1 may be irradiated through a light guide or the like. In this embodiment, a laser scanner is used as the pre-transfer exposure device 14. In this embodiment, the transfer exposure device 14 uniformly exposes the entire image forming area (area where a toner image can be formed) in the longitudinal direction of the photosensitive drum 1. Alternatively, the pre-transfer exposure device 14 may selectively expose a non-image portion on the photosensitive drum 1. In this case, the electrostatic latent image formed by the exposure device 6 on the charged photosensitive drum 1 is different from the electrostatic latent image formed by the pre-transfer exposure device 14.

  When the pre-transfer exposure device 14 is not provided, the surface potential of the photosensitive drum 1 after passing through the developing portion G enters the transfer nip N with a large difference between the non-image portion potential Vd and the image portion potential Vl. To do. If Vd and Vl on the photosensitive drum 1 are simultaneously present in the transfer nip N, the difference between Vd and Vtr (| Vd−Vtr |) is the difference between Vl and Vtr (| Vl−Vtr |). Therefore, a large amount of transfer current flows into the non-image area. In this case, since the amount of transfer current to the image portion becomes smaller than an appropriate value, the transfer of the toner on the image portion onto the recording material P becomes insufficient, resulting in a transfer failure. In particular, in the case of an image pattern such as a thin line, Vd and Vl on the photosensitive drum 1 tend to exist simultaneously in the transfer nip N, so that transfer defects tend to become remarkable.

  In the case where the pre-transfer exposure device 14 is provided, the non-image portion potential Vd on the photosensitive drum 1 when passing through the transfer is reduced by the charge removal by the pre-transfer exposure, so the difference between Vd and Vl is reduced. . If it enters the transfer nip N in this state, even if Vd and Vl on the photosensitive drum 1 exist at the same time in the transfer nip N, the difference (| Vd−Vtr |) between Vd and Vtr is Vl. It is not large compared to the difference from Vtr (| Vl−Vtr |). Therefore, a large amount of transfer current is suppressed from flowing into the non-image area, and the transfer current sufficiently flows into the image area, so that even an image such as a fine line can be transferred satisfactorily.

3. Evaluation experiment 3-1. Contents of Evaluation Experiment In the image forming apparatus 100 of this embodiment, the level of residual transfer ghost on the image, the level of penetration, and the level of residual transfer toner were confirmed. The transfer residual ghost is a phenomenon in which the transfer residual toner is not collected by the developing unit G after passing through the charging unit, but is transferred to the recording material P when it reaches the transfer nip N and appears on the image. . The punch-through is an image defect caused by a discharge phenomenon that occurs between the recording material P and the photosensitive drum 1 immediately before the transfer nip N (upstream in the conveyance direction of the recording material P) when a transfer voltage is applied. That is, discharge occurs in a small air gap immediately before the transfer nip N, and the charging polarity of the toner on the photosensitive drum 1 is locally reversed, resulting in transfer failure at that portion.

  Further, as Comparative Example 1, the same evaluation experiment as in this example was performed in the case where the pre-transfer exposure was not performed. The image forming apparatus used in the evaluation experiment of Comparative Example 1 is substantially the same as the image forming apparatus used in the evaluation experiment of the present embodiment except for the above points.

3-2. Experimental conditions ・ Environment:
The evaluation experiment was performed in an environment of a temperature of 15 ° C. and a humidity of 10% (hereinafter referred to as “L / L”).

・ Image forming device body:
As an apparatus main body 50 of the image forming apparatus 100, an HP Laserjet 1020 was used with a print speed of LTR 25 ppm and a pre-transfer exposure apparatus 14 installed.

・ Recording material:
As the recording material P, the second surface of Xx4200 75 g / m ² paper was used.

・ Image pattern:
As an image pattern, development was carried out using the length of one circumference of the developing roller 2 with 10 dots 20 spaces vertical lines and solid black as half a page.

-Pre-transfer exposure conditions:
FIG. 3 shows the relationship between the driver current value correlated with the exposure amount and the surface potential of the photosensitive drum 1 in this evaluation experiment. This figure shows the relationship when the non-image portion potential at the time of development passage is Vd = −900V.

In this embodiment, the pre-transfer exposure device 14 is turned on with a driver current value I = 5.0 × 10 ⁻³ A, and the non-image portion potential when passing through the transfer is set to Vd = −400V.

  In Comparative Example 1, the pre-transfer exposure device 14 was not turned on and the non-image portion potential at the time of passing the transfer was kept at Vd = −900V.

・ Potential setting:
In Comparative Example 1 and this example, each potential setting during development passage and transfer passage is as shown in FIGS. 4A and 4B, respectively.

・ Transfer voltage:
The same evaluation experiment was performed by changing the transfer voltage Vtr to 0.5 kV, 1.5 kV, and 3 kV.

・ Transfer residual density measurement:
The density of the transfer residual toner (transfer residual density) on the photosensitive drum 1 after passing through the transfer nip N was measured as follows. After the transfer residual toner was adhered to Mylar tape, the tape was affixed on paper and measured using a fog reflection densitometer made by TOKYO DENSHOKU. The transfer residual density value was calculated as a value obtained by subtracting the density value of the Mylar tape from which the transfer residual toner was collected, using the Mylar tape not attached to the photosensitive drum 1 as the reference density.

3-3. Evaluation Results Table 1 shows the evaluation results of transfer residual ghosts and punch-throughs of this example and Comparative Example 1. The evaluation results are shown as “O” (OK level) when the transfer residual ghost and punch-through do not occur or when there is no problem even if they occur, and “×” (NG level) when the problem occurs at the problematic level. ). Table 2 shows the ratio between the difference between Vtr and Vd during transfer and the difference between Vtr and Vl under each condition. FIG. 5 shows the evaluation results of the residual transfer density in this example and Comparative Example 1.

  In Comparative Example 1, the residual transfer ghost is at the NG level when the transfer voltage Vtr is 0.5 kV and 1.5 kV (Table 1). This is considered to be due to the following reason. That is, since the pre-transfer exposure is not performed, the non-image portion potential at the time of passing the transfer remains Vd = −900V. As a result, the ratio between the difference between Vtr and Vd and the difference between Vtr and Vl becomes larger (Table 2), so that the transfer current is generated in the non-image area than in the image area where the toner is placed on the photosensitive drum 1. A large amount flows. As a result, since a transfer current hardly flows into the image portion, the toner cannot be attracted onto the recording material P, and a large amount of untransferred toner remains on the photosensitive drum 1. Further, in Comparative Example 1, the transfer residual density is also high at 10% or more when the transfer voltage Vtr is 0.5 kV and 1.5 kV, and it can be seen that there is a large amount of transfer residual toner on the photosensitive drum 1 (FIG. 5). . On the other hand, in Comparative Example 1, the transfer residual ghost is at the OK level when the transfer voltage Vtr is 3.0 kV (Table 1). This is considered to be due to the following reason. That is, since the pre-transfer exposure is not performed, the non-image portion potential at the time of passing the transfer remains Vd = −900V. However, since the transfer voltage becomes as large as 3.0 kV, the ratio between the difference between Vtr and Vd and the difference between Vtr and Vl is considerably higher than that when the transfer voltage Vtr is 0.5 kV and 1.5 kV. (Table 2). Therefore, compared with the transfer voltages Vtr of 0.5 kV and 1.5 kV, a larger amount of transfer current flows into the image portion, so that the toner can be sufficiently attracted onto the recording material P, and the toner is transferred onto the photosensitive drum 1. Does not remain. In Comparative Example 1, the residual transfer density is also about 5% when the transfer voltage Vtr is 3.0 kV, indicating that the residual toner on the photosensitive drum 1 is small (FIG. 5).

  In Comparative Example 1, the penetration is at the OK level when the transfer voltage Vtr is 0.5 kV and 1.5 kV (Table 1). This is presumably because the transfer voltage Vtr is not so high, no discharge occurs immediately before the transfer nip N, and no penetration occurs. On the other hand, in Comparative Example 1, the penetration is at the NG level when the transfer voltage Vtr is 3.0 kV (Table 1). This is presumably because the transfer voltage Vtr is high and discharge occurs immediately before the transfer nip N, thereby causing a punch-through.

  Next, in this embodiment, the residual transfer ghost is at the NG level when the transfer voltage Vtr is 0.5 kV (Table 1). This is considered to be due to the following reason. In other words, pre-transfer exposure is performed, but since the ratio between the difference between Vtr and Vd and the difference between Vtr and Vl is large, a large amount of transfer current flows into the non-image area, and the transfer current flowing into the image area is small. Become. Therefore, the toner cannot be attracted onto the recording material P, and the toner remains on the photosensitive drum 1. Further, in this embodiment, the residual transfer density is 10% when the transfer voltage Vtr is 0.5 kV, and it can be seen that the residual toner on the photosensitive drum 1 is large (FIG. 5). On the other hand, in this embodiment, the transfer residual ghost is at the OK level when the transfer voltage Vtr is 1.5 kV and 3.0 kV (Table 1). This is considered to be due to the following reason. That is, by reducing the non-image portion potential at the time of passing through the transfer pre-exposure to Vd = −400 V, the ratio between the difference between Vtr and Vd and the difference between Vtr and Vl becomes small. As a result, a large amount of transfer current does not flow only into the non-image area, and the transfer current also flows into the image area, so that the toner can be sufficiently attracted onto the recording material P, and the toner remains on the photosensitive drum 1. Absent. In this embodiment, the residual transfer density is also about 5% when the transfer voltage Vtr is 1.5 kV and 3.0 kV, indicating that the residual toner on the photosensitive drum 1 is small (FIG. 5).

  In this embodiment, the penetration is at the OK level when the transfer voltage Vtr is 0.5 kV and 1.5 kV (Table 1). This is presumably because the transfer voltage Vtr is not so high, no discharge occurs immediately before the transfer nip N, and no penetration occurs. On the other hand, in this embodiment, the penetration is at the NG level when the transfer voltage Vtr is 3.0 kV (Table 1). This is presumably because the transfer voltage Vtr is high and discharge occurs immediately before the transfer nip N, thereby causing a punch-through.

  In summary, in Comparative Example 1, it is difficult to suppress both transfer residual ghost and punch-through. On the other hand, in this embodiment, when the transfer voltage Vtr is 1.5 kV, it is possible to suppress both the transfer residual ghost and the penetration.

  As described above, in this embodiment, the potential difference between the image portion and the non-image portion on the photosensitive drum 1 is reduced by performing the pre-transfer exposure. As a result, even when a cleaner-less system is used and an image in which the image portion and the non-image portion exist simultaneously in the transfer nip N such as a fine line is formed, a large amount of transfer current flows into the non-image portion. Insufficient transfer current can be suppressed. As a result, the toner on the photosensitive drum 1 can be satisfactorily transferred, and image defects such as transfer residual ghost can be suppressed.

  As described above, according to the present exemplary embodiment, the image forming apparatus 100 uses the absolute value of the potential of the non-image portion of the electrostatic image on the photosensitive drum 1 when passing through the transfer to the electrostatic image on the photosensitive drum 1 when passing through development. It has a pre-transfer exposure device 14 that makes it smaller than the absolute value of the potential of the non-image area. Then, the non-image portion potential on the photosensitive drum 1 is lowered after passing through the developing portion G and before entering the transfer nip N. Accordingly, a large amount of transfer current flowing into the non-image area at the transfer nip N can be suppressed, and the transfer current can be sufficiently supplied to the image area. In other words, when developing passes, the non-image portion potential is kept sufficiently high to make Vback sufficiently large, and when the transfer passes, the non-image portion potential is sufficiently lowered to suppress a large amount of transfer current flowing into the non-image portion. To do. As a result, even in the image forming apparatus 100 employing the cleanerless system, the transfer residual toner in the developing unit G can be collected more favorably, and the transfer of the toner image to the recording material P in the transfer nip N can be performed better. And a good image can be formed.

[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 first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

1. Outline of the present embodiment In this embodiment, the setting of the non-image portion potential Vd at the time of development passage is changed according to the use environment of the image forming apparatus 100 (process cartridge 7), and the non-image portion at the time of development passage is changed. The difference from the first embodiment is that the exposure amount of the pre-transfer exposure device 14 is changed according to the change of the potential Vd.

  When the image forming apparatus 100 is used in a high-temperature and high-humidity environment, the charge amount of the toner decreases, or the amount of so-called reversal toner charged to a reverse polarity increases. In this case, so-called “reversal fogging” in which the toner moves from the developing roller 2 to the non-image portion on the photosensitive drum 1 may increase. As a countermeasure, it is effective to reduce the back contrast potential Vback according to the use environment of the image forming apparatus 100. In order to reduce Vback, the non-image portion potential Vd is reduced. However, if the exposure amount of the pre-transfer exposure device 14 remains in the state before changing Vback, the non-image portion potential Vd after the pre-transfer exposure becomes too small and approaches the image portion potential Vl. Therefore, “scattering” in which the toner in the image portion on the photosensitive drum 1 is scattered in the non-image portion may occur.

  Therefore, in this embodiment, an environment sensor (temperature / humidity sensor) 20 as an environment detection unit is provided in the apparatus main body 50 of the image forming apparatus 100 (FIG. 6). When the engine controller 18 determines that the environment is a high-temperature and high-humidity environment based on the detection result of the environment sensor 20 when performing image formation, it changes the Vd at the time of development passing as a countermeasure against reversal fogging and transfers the image. The exposure amount of the pre-exposure device 14 is changed. Then, the non-image portion potential Vd after the pre-transfer exposure is maintained at the level before the change of Vd when the development passes.

2. Evaluation experiment 2-1. Contents of Evaluation Experiment In the image forming apparatus 100 of this example, the level of scattering when the non-image portion potential Vd and the exposure amount of the pre-transfer exposure device 14 were changed when a high temperature and high humidity environment was detected was confirmed.

  As Comparative Example 2, the same evaluation experiment as in this example was performed in the case where the exposure amount of the pre-transfer exposure apparatus 14 was not changed even after the non-image area potential Vd was changed. The image forming apparatus used in the evaluation experiment of Comparative Example 2 is substantially the same as the image forming apparatus used in the evaluation experiment of this embodiment except for the above points.

2-2. Experimental conditions ・ Environment:
The evaluation experiment was performed in an environment of L / L, a temperature of 35 ° C., and a humidity of 80% (hereinafter referred to as “H / H”). An evaluation experiment was performed in the order of outputting an image with the environment of the apparatus main body 50 of the image forming apparatus 100 set to L / L and then outputting the image with H / H.

・ Image pattern:
The image pattern was a character pattern.

-Pre-transfer exposure conditions:
FIG. 7 shows the relationship between the driver current value correlated with the exposure amount and the surface potential of the photosensitive drum 1 in this evaluation experiment. This figure shows the relationship when the non-image portion potential at the time of development passage is Vd = −900V or Vd = −800V.

In Comparative Example 2, since the non-image portion potential at the time of developing at L / L is Vd = −900 V, the pre-transfer exposure device 14 is turned on with a driver current value I = 5.0 × 10 ⁻³ A, The non-image portion potential at the time of passing through the transfer was Vd = −400V. In Comparative Example 2, the exposure amount of the pre-transfer exposure device 14 is not changed even in H / H, and the driver flow value I = 5.0 × 10 ⁻³ A is turned on. Whereas Vd = −800V, the non-image portion potential when passing through the transfer was Vd = −200V.

In this embodiment, since the non-image portion potential at the time of development development at L / L is Vd = −900 V, the pre-transfer exposure device 14 is lit at a driver current value I = 5.0 × 10 ⁻³ A, The non-image portion potential at the time of passing through the transfer was Vd = −400V. Further, in this embodiment, at H / H, since the non-image portion potential at the time of development passage is Vd = −800 V, the pre-transfer exposure device 14 is turned on with a driver current value I = 3.5 × 10 ⁻³ A. The non-image area potential during the transfer was maintained at Vd = −400V.

・ Potential setting:
In both of the present example and the comparative example 2, the potential Vd at the time of development passage is set to be lowered in the H / H environment in order to prevent reversal fog.

  Each potential setting at the time of development passage and transfer passage at L / L and H / H in Comparative Example 2 is as shown in FIGS. 8A and 8B, respectively. In addition, each potential setting at the time of development passage and transfer passage at L / L and H / H in this embodiment is as shown in FIGS. 9A and 9B, respectively.

  Note that the experimental conditions of the image forming apparatus main body and the recording material are the same as in the evaluation experiment of Example 1. Further, the transfer voltage Vtr was set to 1.5 kV in Example 1 in which good results were obtained from the viewpoint of transfer residual ghost and penetration.

2-3. Evaluation results Table 3 shows the evaluation results of the scattering of this example and comparative example 2. Regarding the scattering, a case where the level does not occur or does not cause a problem is indicated as ◯ (OK level), and a case where the level occurs at a problematic level is indicated as x (NG level) (the same applies hereinafter).

  In Comparative Example 2, the scattering is at the OK level in L / L. This is presumably because the toner in the image area does not scatter to the non-image area because the difference between Vd and Vl after the pre-transfer exposure is large. On the other hand, in Comparative Example 2, the scattering is at the NG level in H / H. This is because the exposure amount of the pre-transfer exposure device 14 was not changed in spite of the reduction of Vd as a countermeasure against reversal fogging, so that Vd was too low and approached Vl, and the toner in the image area moved to the non-image area. It is thought to be scattered.

  Next, in this embodiment, the scattering is at the OK level in L / L. This is presumably because the toner in the image area does not scatter to the non-image area because the difference between Vd and Vl after the pre-transfer exposure is large. In the present embodiment, the scattering is at the OK level even in H / H. This is to reduce Vd as a countermeasure against reversal fog and also reduce the exposure amount of the pre-transfer exposure device 14, and by setting Vd after exposure before transfer to the same setting as L / L, from the image portion to the non-image portion. This is considered to be because the scattering of toner can be suppressed.

  In both of the present example and the comparative example 2, the transfer residual ghost and penetration were at the OK level.

  As described above, according to the present embodiment, the same effects as those of the first embodiment can be obtained, and scattering can be suppressed even when the non-image portion potential at the time of development passage is changed in a high-temperature and high-humidity environment.

  In this embodiment, the exposure amount of the pre-transfer exposure apparatus 14 is changed between the L / L environment and the H / H environment. However, the present invention is not limited to this. When the non-image portion potential at the time of development passage is changed for each of more environmental divisions, the exposure amount of the pre-transfer exposure device 14 is changed accordingly. May be. Further, the non-image portion potential at the time of development passage is not limited to being changed according to the environmental temperature and humidity, but is changed according to at least one of the environmental temperature and humidity. Good.

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

1. Outline of the present embodiment In this embodiment, the setting of the image portion potential Vl at the time of development passage is changed according to the use environment of the image forming apparatus 100 (process cartridge 7), and the image portion potential Vl at the time of development passage is changed. This is different from the first embodiment in that the exposure amount of the pre-transfer exposure device 14 is changed in accordance with the change.

  When the image forming apparatus 100 is used in a high-temperature and high-humidity environment, the toner deteriorates in the developing unit G and the toner charge amount decreases. In that case, a large amount of toner adheres to the image portion on the photosensitive drum 1 from the developing roller 2, and in particular, a so-called “line image thickening” may occur in which a line image or the like becomes thick. As a countermeasure, it is effective to reduce the amount of toner adhering to the image portion by reducing the development contrast potential Vcont according to the use environment of the image forming apparatus 100. In order to reduce Vcont, the image portion potential Vl is increased. However, if the exposure amount of the pre-transfer exposure device 14 remains in the state before changing Vcont, the non-image portion potential Vd after the pre-transfer exposure approaches the image portion potential Vl. “Splattering” may occur in the image area.

  Therefore, in this embodiment, an environment sensor (temperature / humidity sensor) 20 as an environment detection unit is provided in the apparatus main body 50 of the image forming apparatus 100 (FIG. 6). When the engine controller 18 determines that the environment is a high-temperature and high-humidity environment based on the detection result of the environment sensor 20 when performing image formation, the Vl is changed as a countermeasure for the line image thickening, and the pre-transfer exposure apparatus 14 exposure amount is changed. Then, the difference between Vd and Vl after the pre-transfer exposure is maintained at the difference before the change of Vl.

2. Evaluation experiment 2-1. Contents of Evaluation Experiment In the image forming apparatus 100 of this example, the level of scattering when the image portion potential Vl and the exposure amount of the pre-transfer exposure device 14 were changed when a high temperature and high humidity environment was detected was confirmed.

  As Comparative Example 3, the same evaluation experiment as in the present example was performed in the case where the exposure amount of the pre-transfer exposure device 14 was not changed even after the image portion potential Vl was changed. The image forming apparatus used in the evaluation experiment of Comparative Example 3 is substantially the same as the image forming apparatus used in the evaluation experiment of the present embodiment except for the above points.

2-2. Experimental conditions ・ Environment:
The evaluation experiment was performed in an H / H environment.

-Pre-transfer exposure conditions:
FIG. 10 shows the relationship between the driver current value correlated with the exposure amount and the surface potential of the photosensitive drum 1 in this evaluation experiment. This figure shows the relationship when the non-image portion potential at the time of development passage is Vd = −900V.

In Comparative Example 3, the pre-transfer exposure device 14 was turned on with a driver current value I = 5.0 × 10 ⁻³ A, and the non-image portion potential when passing through the transfer was Vd = −400V.

In this embodiment, the pre-transfer exposure device 14 is turned on with a driver flow value I = 4.2 × 10 ⁻³ A, and the non-image portion potential when passing through the transfer is set to Vd = −500V.

・ Potential setting:
In both of the present example and the comparative example 3, Vl is set to be large for the countermeasure against the thickening of the line image.

  In the comparative example 3 and the present embodiment, each potential setting at the time of development passage and transfer passage is as shown in FIGS. 11 (a) and 11 (b), respectively.

  The experimental conditions for the image forming apparatus main body, the recording material, the image pattern, and the transfer voltage are the same as in the evaluation experiment of Example 2.

2-3. Evaluation Results Table 4 shows the evaluation results of the scattering of the present example and comparative example 3.

  In Comparative Example 3, the scattering is at the NG level. This is because although the exposure amount of the pre-transfer exposure device 14 was not changed in spite of increasing Vl as a countermeasure against the thickening of the line image, Vd decreased too much and approached Vl (potential difference 200V), and the toner in the image area This is considered to be scattered in the non-image area.

  Next, in this embodiment, the scattering level is OK. This is considered to be due to the following reason. In other words, Vl was increased as a countermeasure against the thickening of the line image, and the exposure amount of the pre-transfer exposure device 14 was also reduced, so that the difference between Vd and Vl was set to the same setting (potential difference 300 V) as before Vl was changed. This is considered to be because the scattering of the toner from the image portion to the non-image portion can be suppressed.

  In both of the present example and the comparative example 3, the transfer residual ghost and the punch-through were at the OK level.

  As described above, according to the present embodiment, the same effects as those of the first embodiment can be obtained, and scattering can be suppressed even when the image portion potential at the time of development passage is changed in a high-temperature and high-humidity environment.

  As in the case of the third embodiment, the exposure amount of the pre-transfer exposure device 14 may be changed according to the image portion potential at the time of development passing, which is changed for each of the more environmental divisions. The image portion potential may be changed according to at least one of the environmental temperature and humidity.

[Example 4]
Next, another embodiment of the present invention will be described. The basic configuration and operation of the image forming apparatus of this embodiment are the same as those of the first embodiment. Therefore, elements having the same or corresponding functions or configurations as those of the image forming apparatus of the first embodiment are denoted by the same reference numerals, and detailed description thereof is omitted.

1. Outline of the present embodiment The present embodiment is different from the first embodiment in that the exposure amount of the pre-transfer exposure device 14 is changed in accordance with the repetitive use amount (number of images formed) of the image forming apparatus 100, particularly the process cartridge 7. Different.

  As the amount of repeated use of the process cartridge 7 increases, the surface layer of the photosensitive drum 1 is scraped by contact and rubbing at the charging unit, the developing unit, the transfer unit, and the like, and the surface layer becomes thin. In that case, since the electrostatic capacitance of the surface layer of the photosensitive drum 1 changes, the amount of charge differs when the potential on the photosensitive drum 1 is the same. Therefore, when the same Vd and Vl as the initial stage are formed on the photosensitive drum 1 after repeated use, and the same amount of toner is to be transferred from the image portion to the recording material P before and after repeated use, a necessary transfer current is required. Will be different. In addition, in an image pattern in which Vl and Vd on the photosensitive drum 1 are simultaneously present in the transfer nip N such as a thin line, the charged charge amount of the non-image portion changes after repeated use, and therefore transfer that flows into the non-image portion. The current is also different. In that case, the ratio of the transfer current flowing into Vd and Vl after repeated use differs from the ratio of the transfer current flowing into Vd and Vl in the initial stage.

  Therefore, in this embodiment, the image forming apparatus 100 is provided with a counter 21 that counts the number of images formed using the process cartridge 7 as a usage amount detecting unit (FIG. 12). When the engine controller 18 determines that the usage amount of the process cartridge 7 has reached a predetermined threshold value (5000 sheets in this embodiment), the exposure amount of the pre-transfer exposure device 14 is changed. Thus, the exposure amount of the pre-transfer exposure device 14 is changed before and after repeated use, and the difference between Vd and Vl when passing through the transfer is changed, so that the toner transferability is maintained even after repeated use.

2. Evaluation experiment 2-1. Outline In the image forming apparatus 100 of the present embodiment, the level of residual transfer ghost, the level of punch-through, and the level of residual transfer toner in the initial stage of the process cartridge 7 and when 5000 images are formed (after repeated use). And confirmed. As this example, evaluation experiments were performed on Example 4-1 and Example 4-2 in which the difference between Vd and Vl after repeated use was different.

  Further, as Comparative Example 4, the same evaluation experiment as in this example was performed in the case where the exposure amount of the pre-transfer exposure apparatus 14 was not changed even after repeated use. The image forming apparatus used in the evaluation experiment of Comparative Example 4 is substantially the same as the image forming apparatus used in the evaluation experiment of this embodiment except for the above points.

2-2. Experimental conditions • Pre-transfer exposure conditions:
FIG. 13 shows the relationship between the driver current value correlated with the exposure amount and the surface potential of the photosensitive drum 1 in this evaluation experiment. This figure shows the relationship between the initial time and the time of 5000 sheets (after repeated use) when the non-image portion potential when passing through the development is Vd = −900V.

In Comparative Example 4, initially, the pre-transfer exposure device 14 was turned on with a driver current value I = 5.0 × 10 ⁻³ A, and the non-image portion potential when passing through the transfer was set to Vd = −400V. Further, even after repeated use, the driver current value I = 5.0 × 10 ⁻³ A was lit without changing the exposure amount of the pre-transfer exposure device 14, and the surface potential at the time of passing the transfer was Vd = −650 V.

In Example 4-1, initially, the pre-transfer exposure device 14 was turned on with a driver current value I = 5.0 × 10 ⁻³ A, and the non-image area potential when passing through the transfer was Vd = −400V. Further, after repeated use, the exposure amount of the pre-transfer exposure device 14 is changed to light at a driver current value I = 6.2 × 10 ⁻³ A, and the non-image portion potential when passing through the transfer is set to Vd = −400V. .

In Example 4-2, initially, the pre-transfer exposure device 14 was lit at a driver current value I = 5.0 × 10 ⁻³ A, and the non-image portion potential at the time of transfer passing was set to Vd = −400V. Further, after repeated use, the exposure amount of the pre-transfer exposure device 14 is changed to light at a driver current value I = 8.0 × 10 ⁻³ A, and the non-image portion potential at the time of transfer passing is set to Vd = −300V. .

・ Potential setting:
Each potential setting at the time of development passage and transfer passage after the initial use and repeated use of Comparative Example 4 is as shown in FIGS. 14A and 14B, respectively. In addition, each potential setting at the time of development passage and transfer passage after the initial use and repeated use of Example 4-1 is as shown in FIGS. 15A and 15B, respectively. In addition, each potential setting at the time of development passage and transfer passage after the initial use and repeated use of Example 4-2 is as shown in FIGS. 16A and 16B, respectively.

・ Transfer voltage:
The same evaluation experiment was performed by changing the transfer voltage Vtr to 0.5 kV, 1.5 kV, 2.3 kV, and 3 kV.

  The experimental conditions for the environment, the image forming apparatus main body, the recording material, the image pattern, and the residual transfer density measurement are the same as in the evaluation experiment of Example 1.

2-3. Evaluation Results Table 5 shows the evaluation results of transfer residual ghosts of Example 4-1, Example 4-2, and Comparative Example 4. Table 6 shows the ratio between the difference between Vtr and Vd during transfer and the difference between Vtr and Vl under each condition. Table 7 shows the evaluation results of the penetrations of Example 4-1, Example 4-2, and Comparative Example 4. FIG. 17 shows the evaluation results of the residual transfer density of Example 4-1, Example 4-2, and Comparative Example 4 after repeated use.

  In the evaluation result of the transfer residual ghost shown in Table 5, the reason for the OK level and the reason for the NG level can be considered in the same manner as described in the first embodiment.

  In particular, in Comparative Example 4, the residual transfer ghost becomes NG level when the transfer voltage Vtr is 0.5 kV, 1.5 kV, and 2.3 kV after repeated use. This is because the exposure amount of the pre-transfer exposure apparatus 14 was not changed even after repeated use, so the non-image portion potential decreased only to Vd = −650 V, and the ratio of the difference between Vtr and Vd and the difference between Vtr and Vl Is considered to be because of the large.

  In Example 4-1, the residual transfer ghost becomes NG level when the transfer voltage Vtr is 0.5 kV and 1.5 kV after repeated use. This is considered to be due to the following reason. That is, the exposure amount of the pre-transfer exposure device 14 was changed after repeated use, and the non-image portion potential at the time of passing through the transfer was lowered to the same Vd = −400 V as the initial value. However, since the electrostatic capacity has changed from the initial value due to the shaving of the photosensitive drum 1, even if the ratio of the difference between Vtr and Vd, which is the same as the initial value, and the difference between Vtr and Vl is the same, the transferability is higher than the initial value. It has deteriorated slightly. On the other hand, when the transfer voltage Vtr is 2.3 kV or 3.0 kV, the level is OK. This is presumably because the ratio between the difference between Vtr and Vd and the difference between Vtr and Vl is reduced by increasing the transfer voltage. As described above, in Example 4-1, the exposure amount of the pre-transfer exposure device 14 was changed to reduce the non-image portion potential when passing through the transfer to the same level as the initial value. It is wider than Example 4-1.

  In Example 4-2, the residual transfer ghost becomes an OK level when the transfer voltage Vtr is 1.5 kV, 2.3 kV, and 3.0 kV after repeated use. This is considered to be due to the following reason. That is, the exposure amount of the pre-transfer exposure device 14 was increased from that after repeated use of Example 4-1. As a result, the non-image portion potential when passing through the transfer is lowered to Vd = −300 V, and the difference between Vd and Vl becomes smaller than the initial value. As a result, the OK level region becomes wider than after repeated use of Example 4-1. As described above, even if the electrostatic capacity changes due to the surface layer of the photosensitive drum 1 being reduced, the ratio of the difference between Vtr and Vd and the difference between Vtr and Vl is made smaller than the initial value, thereby moving to the non-image portion. Thus, a large amount of the transfer current is suppressed, and the transfer current sufficiently flows into the image portion. Therefore, the toner can be sufficiently attracted onto the recording material P.

  Also, the evaluation result of the residual transfer density shown in FIG. 17 supports the reason why the residual transfer ghost was at the OK level and the reason that it was at the NG level.

  By the way, in Example 4-2, the difference between Vd and Vl at the time of transfer passing after repeated use was 200 V, but the scattering was at a level with no problem. This is considered to be due to the following reason. That is, since the electrostatic capacitance of the photosensitive drum 1 is changed, the potential including the toner in the image portion is also changed. Therefore, the potential of the image portion including the toner after repeated use becomes smaller than the potential of the image portion including the initial toner, and the potential difference from Vd is increased, so that the toner is not scattered in the non-image portion. .

  Moreover, in the evaluation result of the penetration shown in Table 7, the reason for the OK level and the reason for the NG level can be considered in the same manner as described above for the first embodiment.

  In summary, in Comparative Example 4, it is difficult to suppress both the transfer residual ghost and the penetration after repeated use. On the other hand, in Example 4-1, the exposure amount of the pre-transfer exposure device 14 is changed after repeated use so that the difference between Vd and Vl approaches the initial value (substantially the same in this embodiment). I did it.) As a result, even after repeated use, when the transfer voltage Vtr is 2.3 kV, both the transfer residual ghost and the punch-through can be suppressed. Further, in Example 4-2, the exposure amount of the pre-transfer exposure device 14 was changed after repeated use to make the difference between Vd and Vl smaller than the initial value. As a result, even after repeated use, when the transfer voltage Vtr is 1.5 kV and 2.3 kV, it is possible to suppress both the transfer residual ghost and the penetration. That is, in Example 4-2, the range in which both the transfer residual ghost and the punch-through can be suppressed is further widened than in Example 4-1. As described above, by reducing the difference between the image portion potential and the non-image portion potential from the initial state after repeated use, the toner on the photosensitive drum 1 can be transferred more satisfactorily by passing a sufficient transfer current to the image portion. This makes it possible to increase the effect of suppressing image defects such as transfer residual ghosts.

  In the present embodiment, the exposure amount of the pre-transfer exposure device 14 is changed between the initial use and the case where the number of formed images reaches 5000. However, the present invention is not limited to this, and the exposure amount of the pre-transfer exposure device 14 may be changed for every smaller number of image formations (or for every larger number of image formations) as the number of image formations increases. Good. In this case, the difference between Vd and Vl can be reduced as the number of formed images increases.

[Others]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  For example, each potential setting or the like is not limited to that in each of the above-described embodiments, and can be appropriately set in an apparatus that implements the present invention.

  In the second and third embodiments, the environment sensor 20 capable of detecting the temperature and humidity of the environment is used as the environment detection unit. However, any information may be used as long as at least one of the temperature and humidity of the environment can be obtained. For example, a method of acquiring at least one of environmental temperature and humidity from a transfer current (or transfer voltage) at the time of pre-rotation, which is a preparatory operation before image formation, and feeding back to Vd and Vl may be used. . That is, there is a method in which a constant transfer current is supplied to the transfer unit during the pre-rotation before image formation, and the voltage value of the transfer voltage at the time of image formation is determined based on the generated voltage value at that time. The generated voltage value at this time has a correlation with the electric resistance of the transfer portion, and the electric resistance has a correlation with at least one of the environmental temperature and humidity. Therefore, for example, information on at least one of the environmental temperature and humidity can be obtained from the information on the generated voltage value obtained in the control of the transfer voltage and used for the control of Vd and Vl. In addition, since it is only necessary to obtain information related to the electrical resistance of the transfer unit, a constant voltage may be applied to the transfer unit and a current flowing at that time may be detected.

  In Example 4, the value of the number of images formed using an image forming apparatus, particularly a process cartridge, was used as a value correlated with the amount of use of the image forming apparatus. However, the present invention is not limited to this, and any value that correlates with the amount of use of the image forming apparatus, more specifically, the photoconductor, may be used. The operating time of the image forming apparatus, particularly the process cartridge (the rotation time of the photoconductor, It can be represented by the number of rotations, charging time, etc.).

  Further, the exposure amount of the pre-transfer exposure apparatus is changed in accordance with the change in Vd and Vl when passing the development described in the second and third embodiments, and the amount of use of the image forming apparatus described in the fourth embodiment is correlated. You may implement combining combining changing the exposure amount of a pre-transfer exposure apparatus according to a value.

DESCRIPTION OF SYMBOLS 1 Photosensitive drum 2 Developing roller 3 Developing apparatus 4 Transfer roller 5 Charging roller 6 Laser scanner 14 Pre-transfer exposure apparatus

Claims (8)

A rotatable photoreceptor,
Charging means for charging the photoreceptor to a predetermined polarity;
Image exposing means for exposing the charged photoreceptor to form an electrostatic image on the photoreceptor;
Developing means for forming a toner image by supplying toner charged to the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photosensitive member at a developing portion;
Transfer means for electrostatically transferring the toner image on the photosensitive member to the transfer target at the transfer portion;
Pre-transfer exposure means for exposing the photoconductor downstream of the developing unit and upstream of the transfer unit in the rotation direction of the photoconductor;
Control means for changing the exposure amount by the pre-transfer exposure means;
Have
In the image forming apparatus for collecting the toner remaining on the photoreceptor after the transfer to the developing unit in the developing unit,
The control means has a first value that correlates with the amount of use of the image forming apparatus with respect to the potential difference between the non-image portion and the image portion of the electrostatic image on the photosensitive member when passing through the transfer portion. By the pre-transfer exposure means, the potential difference in the case where the value correlated with the amount of use of the image forming apparatus is a second value larger than the first value approaches the potential difference in a certain case. An image forming apparatus characterized by changing an exposure amount . A rotatable photoreceptor,
Charging means for charging the photoreceptor to a predetermined polarity;
Image exposing means for exposing the charged photoreceptor to form an electrostatic image on the photoreceptor;
Developing means for forming a toner image by supplying toner charged to the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photosensitive member at a developing portion;
Transfer means for electrostatically transferring the toner image on the photosensitive member to the transfer target at the transfer portion;
Pre-transfer exposure means for exposing the photoconductor downstream of the developing unit and upstream of the transfer unit in the rotation direction of the photoconductor;
Control means for changing the exposure amount by the pre-transfer exposure means;
Have
In the image forming apparatus for collecting the toner remaining on the photoreceptor after the transfer to the developing unit in the developing unit,
The control means has a first value that correlates with the amount of use of the image forming apparatus with respect to the potential difference between the non-image portion and the image portion of the electrostatic image on the photosensitive member when passing through the transfer portion. than the potential difference between some cases, as towards the potential difference when the value that correlates with the amount of the image forming apparatus is a second value larger than the first value decreases, the pre-transfer exposure images forming apparatus you and changes the exposure of the unit. Said value correlated with the amount, the image forming apparatus according to claim 1 or 2, characterized in that a number of image formation. It said value correlated with the amount, the image forming apparatus according to claim 1 or 2, characterized in that the operating time. A rotatable photoreceptor,
Charging means for charging the photoreceptor to a predetermined polarity;
Image exposing means for exposing the charged photoreceptor to form an electrostatic image on the photoreceptor;
Developing means for forming a toner image by supplying toner charged to the same polarity as the predetermined polarity to an image portion of the electrostatic image on the photosensitive member at a developing portion;
Transfer means for electrostatically transferring the toner image on the photosensitive member to the transfer target at the transfer portion;
Pre-transfer exposure means for exposing the photoconductor downstream of the developing unit and upstream of the transfer unit in the rotation direction of the photoconductor;
Control means for changing the exposure amount by the pre-transfer exposure means;
Have
In the image forming apparatus for collecting the toner remaining on the photoreceptor after the transfer to the developing unit in the developing unit,
It said control means, than the exposure amount by the pre-transfer exposure means of the case where the absolute value of the image portion potential of the electrostatic image on the photosensitive member is a first value when passing through the pre-Symbol developing unit, The amount of exposure by the pre-transfer exposure means when the absolute value of the potential of the image portion of the electrostatic image on the photoconductor when passing through the developing portion is a second value larger than the first value. reducing towards images forming device you characterized. The pre-transfer exposure means uses the absolute value of the potential of the non-image portion of the electrostatic image on the photoconductor when passing through the transfer portion, and the electrostatic image on the photoconductor as passed through the developing portion. be smaller than the absolute value of the potential of the non-image portion image forming apparatus according to any one of claims 1 to 5, wherein the. The pre-transfer exposure means, said photoconductive image forming apparatus according to any one of claims 1 to 6, wherein exposing the entire area of the image forming area in the longitudinal direction of. The pre-transfer exposure unit, the image forming apparatus according to any one of claims 1 to 6, wherein exposing the non-image portion of the electrostatic image on the photosensitive member.

Images