Image forming device

Abstract

To prevent an image defect in an image forming device of a cleaner-less system and of a contact charging system.SOLUTION: An external additive application operation is performed in such a manner that the surface potential of a photosensitive drum is made lower than a development voltage applied to developing means for developing with a negative-polarity toner containing a positive-polarity external additive, thereby moving the external additive from a developing roller to the photosensitive drum, and a charging voltage is made lower than the surface potential of the photosensitive drum, thereby moving the external additive from the photosensitive drum to a charging roller. A potential difference Vback between the surface potential of the photosensitive drum and the development voltage in a developing unit is made larger in the external additive application operation than in an image forming operation to increase an amount of development with the external additive. In the external additive application operation, a transfer voltage is set higher than the surface potential of the photosensitive drum to hinder transfer of the external additive. The potential difference between the surface potential of the photosensitive drum and the transfer voltage is made smaller than a discharge threshold between a transfer roller and the photosensitive drum, thereby preventing the external additive from becoming strongly positive.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to an image forming apparatus. [Background technology]

[0002] Conventionally, image forming apparatuses that form images using an electrophotographic process, such as copying machines and laser printers, have been known. In a transfer process, this image forming apparatus electrostatically transfers a toner image formed on the surface of a photosensitive drum serving as an image carrier onto a recording material or the like by applying a voltage from a power source to a transfer member disposed opposite the photosensitive drum. Then, an image is formed by fixing the toner image onto the recording material with a fixing device. Patent Document 1 proposes a cleanerless type (simultaneous development and cleaning) image forming apparatus that recovers residual toner remaining on the photosensitive drum from the photosensitive drum with a developing device and reuses the toner as a means for recovering the residual toner remaining on the photosensitive drum. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] JP 2001-183905 A Summary of the Invention [Problem to be solved by the invention]

[0004] In the case of a cleanerless type contact charging type in which charging is performed by contacting the drum, foreign matter may be transferred to the photosensitive drum via the recording material during image formation, and then transferred to the charging roller. If the foreign matter is then carried around to the surface of the charging roller, the foreign matter adhering to the charging roller may scratch the surface of the photosensitive drum, and the scratched area may not be able to achieve the desired potential, resulting in a defective image.

[0005] SUMMARY OF THE PRESENT DISCLOSURE An object of the present invention is to suppress image defects in a cleaner-less, contact-charging image forming apparatus. [Means for solving the problem]

[0006] The present invention includes a rotatable photosensitive drum and a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, The charging voltage Vcg during the external additive application operation, the surface voltage Vcg on the charged surface of the photosensitive drum where Vdg1 is a surface potential, Vdg2 is a surface potential in a developing unit facing the developing means, Vdg3 is a surface potential in a transfer unit facing the transfer means, Vrg is a developing voltage applied to the developing means, Vtg is a transfer voltage applied to the transfer means, Vtth is a threshold value of a potential difference at which discharge occurs between the transfer means and the photosensitive drum, Vback_g is a potential difference between the developing voltage and the surface potential of the photosensitive drum in the developing unit, and Vback_p is a potential difference between the developing voltage in the image forming operation and the surface potential of the photosensitive drum in the developing unit. The control means does not perform exposure by the exposure means during the external additive application operation, and Vrg>Vdg2, Vdg1>Vcg, Vback_g>Vback_p, Vtg>Vdg3, Vtg-Vdg3 <Vtth The image forming apparatus is characterized in that the applied voltage is controlled so as to be

[0007] The present invention also provides a method for manufacturing a photosensitive drum comprising: a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, Let Vcg be the charging voltage in the external additive application operation, Vlg1 be the surface potential on the exposed surface of the photosensitive drum, Vlg2 be the surface potential in the developing section facing the developing means, Vlg3 be the surface potential in the transfer section facing the transfer means, Vrg be the developing voltage applied to the developing means, and Vtg be the transfer voltage applied to the transfer means. The control unit causes the exposure unit to expose the entire area of ​​the photosensitive drum in a direction intersecting the circumferential direction during the external additive application operation, and Vlg2>Vrg, Vlg1>Vcg, Vtg=Vlg3 The image forming apparatus is characterized in that the applied voltage is controlled so as to be Effect of the Invention

[0008] According to the present invention, image defects can be suppressed in a cleaner-less type and contact charging type image forming apparatus. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 is a cross-sectional view of an image forming apparatus according to a first embodiment. [Diagram 2] FIG. 2 is a control block diagram of the first embodiment. [Diagram 3] 5A to 5C are diagrams for explaining an image forming operation according to the first embodiment. [Figure 4] 4A to 4C are diagrams illustrating an external additive application operation according to the first embodiment. [Diagram 5] 11A to 11C are diagrams for explaining a method for verifying image defects. [Figure 6] FIG. 4 is a diagram illustrating a cleaning operation according to the first embodiment. [Figure 7] FIG. 2 is a diagram for explaining a brush according to the first embodiment. [Figure 8] 5A to 5C are diagrams illustrating an image forming operation and an external additive application operation when a brush is present according to the first embodiment. [Figure 9] 6A to 6C are diagrams illustrating an external additive application operation according to the second embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of the components described in the following embodiments may be changed as appropriate depending on the configuration and various conditions of the device to which the present invention is applied. Therefore, unless otherwise specified, it is not intended to limit the scope of the present invention to only those.

[0011] Example 1 1 is a schematic diagram showing an image forming apparatus according to embodiment 1. The image forming apparatus 100 according to embodiment 1 is a monochrome laser beam printer that employs a cleanerless system and a contact charging system.

[0012] In the image forming apparatus 100 of the first embodiment, a cylindrical rotatable electrophotographic photosensitive member (hereinafter, photosensitive drum) 1 is disposed as an image carrier. Around the photosensitive drum 1, a charging roller 2 as a charging means, an exposure device 3 as an exposure means, a developing device 4 as a developing means, a transfer roller 5 as a transfer means, and a pre-charge exposure device 6 as a means for removing charge potential are disposed. In the following description, the short side direction is the direction perpendicular to the rotation axis direction of the photosensitive drum 1, and the long side direction is the direction parallel to the rotation axis direction of the photosensitive drum 1.

[0013] The photosensitive drum 1 is an image carrier that rotates in the direction of the arrow and carries a toner image. A control unit 99 (FIG. 2), such as a controller, receives an image signal from an external device 130 (FIG. 2). This starts an image forming operation, and the photosensitive drum 1 is rotated. During the rotation process, the photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in the first embodiment) by the charging roller 2. The photosensitive drum 1 is exposed to light by the exposure device 3 in accordance with the image signal. This forms an electrostatic latent image. The electrostatic latent image is then developed by the development device 4 at the development position and visualized as a toner image.

[0014] The charging roller 2 is rotated in contact with the photosensitive drum 1. The charging roller 2 is a charging means for charging the surface of the photosensitive drum 1 at a charging portion formed between the charging roller 2 and the photosensitive drum 1. The charging roller 2 may be a driven roller. The charging roller 2 is provided with an elastic layer made of a conductive elastic body and has a surface roughness of Ra 1.5 to 2.5 μm. The charging roller 2 contacts the surface of the photosensitive drum 1 with a predetermined pressure contact force to form a charging portion. The charging roller 2 and the photosensitive drum 1 rotate in the same direction at the charging portion. The peripheral speed of the charging roller 2 is faster than that of the photosensitive drum 1. The charging roller 2 is rotated at a peripheral speed of 105% of that of the photosensitive drum 1 by a charging roller driving device 98 (FIG. 2). A predetermined DC voltage is applied to the rotating shaft of the charging roller 2 from a charging voltage power source 92 (FIG. 2) according to the image forming operation. In response to the image forming operation, a DC voltage of -1350V is applied as a charging voltage Vcp to the rotating shaft of the charging roller 2, and the surface of the photosensitive drum 1 is charged to a predetermined potential of -780V. The surface potential of the photosensitive drum 1 is measured using a Trek Corporation surface potential meter Model 344. The surface potential of the photosensitive drum 1 at this time is -780V. The surface potential is equal to the dark potential Vdp2, and no toner image is developed.

[0015] The exposure device 3 exposes the charged surface of the photosensitive drum 1, which is charged by the charging roller 2. The exposure device 3 is an exposure means that exposes the surface of the photosensitive drum 1 at an exposure section downstream of the charging section in the rotation direction of the photosensitive drum 1. The exposure device 3 is a laser scanner device. The exposure device 3 irradiates laser light based on image information input from an external device 130 such as a host computer. The exposure device 3 is an exposure means that forms an electrostatic latent image on the uniformly charged charged surface of the photosensitive drum 1. In the first embodiment, the amount of exposure is adjusted so that the image formation potential (light area potential Vlp2) of the electrostatic latent image section of the photosensitive drum 1 after exposure by the exposure device 3 is -100V.

[0016] The developing device 4 supplies toner containing an external additive to the exposed surface of the photosensitive drum 1 exposed by the exposure device 3. The developing device 4 is a developing means that supplies toner containing an external additive to the surface of the photosensitive drum 1 in a developing section downstream of the exposure section in the rotation direction of the photosensitive drum 1. The photosensitive drum 1 faces the developing device 4 in the developing section. The developing device 4 is composed of a developing roller 41 as a developer carrier, a toner supply roller 42 as a developer supplying means, a toner storage chamber 43 that stores toner, and a developing blade 44. The toner is supplied from the toner storage chamber 43 to the developing roller 41 by the toner supply roller 42. The toner supplied to the developing roller 41 is charged to a predetermined polarity by passing through a contact portion with the developing blade 44.

[0017] The toner is a non-magnetic toner with a negative normal polarity produced by a suspension polymerization method, has a volume average particle size of 6.0 μm, and is charged to a negative polarity when supported on the developing roller 41. An external additive is added to the surface of the toner. The charging polarity of the external additive is opposite to that of the toner. In the first embodiment, the toner is negative, whereas the external additive uses 200 nm hydrotalcite particles with a positive polarity. By externally adding particles with a polarity opposite to that of the toner, the toner can have a stable polarity. The external additive may be other positive external additives such as particles using metals such as titanium or metal oxides.

[0018] The developing roller 41 contacts the surface of the photosensitive drum 1 with a predetermined pressure to form a developing portion. The developing roller 41 is driven to rotate at a peripheral speed of 140% relative to the photosensitive drum 1 by a developing roller driving device 90 (FIG. 2). The developing roller 41 and the photosensitive drum 1 may be driven by a common driving source. A DC voltage of -380V (developing voltage Vrp) is applied to the rotating shaft of the developing roller 41 from a developing voltage power source 93 (FIG. 2). During image formation, the toner image carried on the developing roller 41 is developed into the image forming potential Vlp2 portion of the photosensitive drum 1 by electrostatic force generated by the potential difference between this developing voltage -380V and the image forming potential Vlp2 of the photosensitive drum 1 = -100V.

[0019] The transfer roller 5 transfers the toner supplied to the photosensitive drum 1 by the developing device 4 onto the recording material P, which is the object to be transferred. The transfer roller 5 is a transfer means that transfers the toner on the surface of the photosensitive drum 1 onto the recording material P, which is the object to be transferred, at a transfer section downstream of the developing section in the rotation direction of the photosensitive drum 1. The photosensitive drum 1 faces the transfer roller 5 at the transfer section. The transfer roller 5 abuts against the surface of the photosensitive drum 1 with a predetermined pressure to form the transfer section. A predetermined voltage is applied to the rotation shaft of the transfer roller 5 at a predetermined timing from a transfer voltage power source 94 (FIG. 2).

[0020] The pre-charge exposure device 6 is a pre-exposure unit that exposes the surface of the photosensitive drum 1 at a pre-charge exposure section that is downstream of the transfer section and upstream of the charging section in the rotation direction of the photosensitive drum 1. By performing exposure using the pre-charge exposure device 6, it is possible to eliminate unevenness in the surface potential of the photosensitive drum 1 after it has passed through the transfer section.

[0021] The recording material P stored in a cassette 10 is fed by a paper feed unit 7 in accordance with the timing when the toner image formed on the photosensitive drum 1 reaches the transfer portion. The toner image formed on the photosensitive drum 1 is transferred onto the recording material P by the transfer roller 5 to which a predetermined transfer voltage is applied by a transfer voltage power source 94.

[0022] Thereafter, the recording material P carrying the toner image is conveyed to a fixing device 9. The recording material P is heated and pressurized in the fixing device 9. As a result, the toner is melted and fixed to the recording material P. Thereafter, the recording material P is discharged outside the apparatus.

[0023] By the above operation, a monochrome print image is formed. The image forming apparatus 100 is a cleanerless type image forming apparatus in which the developing device 4 collects toner that has not been transferred to the recording material P, which is the transfer target, in the transfer section.

[0024] Foreign matter present inside the image forming apparatus 100 or foreign matter that has entered from outside the apparatus may be conveyed to the transfer section via the recording material P, and may be transferred to the photosensitive drum 1 at the transfer section. The foreign matter may be transferred to the charging roller 2 at the charging section, adhere to the charging roller 2, and remain on the charging roller 2. In that case, the foreign matter may damage the surface of the photosensitive drum 1. If the damage becomes deep, the charge retention ability of the surface of the photosensitive drum 1 decreases. Then, in the developing section, toner is developed at the damaged part on the photosensitive drum 1, and a black dot image is generated. In particular, in a high temperature and high humidity environment, the charge around the damaged part of the photosensitive drum 1 is likely to flow to the damaged part with low resistance. Therefore, the visibility of the black dot tends to increase. The foreign matter referred to here includes metal pieces, resin pieces, minerals such as quartz, etc. Such relatively hard foreign matter is more likely to damage the photosensitive drum 1 and become a cause of black dot images. Adhesion of foreign matter to the charging roller 2 is more likely to occur when the surface of the charging roller 2 has high tackiness. Therefore, when the charging roller 2 is in an initial state of use, where there is no toner attached to the surface of the charging roller 2 and the tackiness is high, adhesion of foreign matter to the charging roller 2 is likely to occur.

[0025] In order to solve this problem, the first embodiment is characterized in that an external additive application operation is performed to attach a certain amount of external additive to the charging roller 2 at the beginning of use of the charging roller 2. In the external additive application operation, the external additive or toner containing the external additive is moved from the developing roller 41 to the photosensitive drum 1. Furthermore, the external additive is moved from the photosensitive drum 1 to the charging roller 2. In the external additive application operation, the charging voltage, the developing voltage, the transfer voltage, and the exposure amount are controlled so as to realize the movement of the external additive or the toner containing the external additive. Here, the beginning of use can be exemplified as, for example, when the charging roller 2 is in a new state, when the number of rotations of the charging roller 2 is less than a threshold value, when the image forming operation has not been performed or the number of times it has been performed is less than a threshold value, and the like. In the following description, an example will be described in which the external additive application operation is performed when the charging roller 2 is in a new state. Note that the effect of reducing tackiness can be obtained even if toner, rather than external additive, is attached to the charging roller 2. The effect of reducing tackiness is greater when an external additive is attached.

[0026] The reason why applying an external additive is more effective at reducing tack than applying a toner will be described below. Fig. 3 shows the surface potential formed on the photosensitive drum 1 during image forming operation and the applied voltages of each member. The surface potential immediately before the charging section of the photosensitive drum 1 during image forming operation is Vdp1, the surface potential of the area not exposed by the exposure device 3 immediately before the development section is Vdp2, the surface potential of the exposed area is Vlp2, and the surface potential immediately before the transfer section is Vdp3. The charging voltage, developing voltage, and transfer voltage during image forming operation are Vcp, Vrp, and Vtp, respectively. In Example 1, pre-charge exposure is performed by the pre-charge exposure device 6 (pre-charge exposure ON), and Vdp1=0V. The charging voltage Vcp is -1350V, the dark potential Vdp2 in the developing portion of the photosensitive drum 1 is -780V, the light potential Vlp2 is -100V, the dark potential Vdp3 in the transfer portion is -780V, the developing voltage Vrp is -380V, and the transfer voltage Vtp is +1500V.

[0027] In the first embodiment, the charging roller 2 and the photosensitive drum 1 are rotated with a difference in peripheral speed therebetween. Therefore, the toner on the charging roller 2 is easily charged to negative polarity by friction. During the image forming operation shown in FIG. 3, the charging voltage Vcp is greater on the negative polarity side than the potential Vdp1 of the photosensitive drum 1 immediately before the charging section. In the case of such a potential relationship, the negative toner attached to the charging roller 2 is expelled onto the photosensitive drum 1. On the other hand, the external additive has a charging polarity (positive polarity) opposite to that of the toner. Therefore, even if it is rubbed on the charging roller 2, it is difficult to become negative, and it is difficult to be expelled from the charging roller 2 to the photosensitive drum 1. In addition, the external additive of the first embodiment is 1 / 10 or less in size than the toner. Therefore, it is easier to apply uniformly than the toner, regardless of the unevenness of the surface of the charging roller 2. Therefore, the particles to be attached to the charging roller 2 are preferably external additives, which are less likely to be expelled from the charging roller 2 and are easier to apply uniformly than the toner.

[0028] The external additive application operation will be described. The external additive application operation is an operation sequence in which the external additive is developed on the photosensitive drum 1 in the developing section, and then the external additive is efficiently transferred to the charging roller 2 by not discharging in the transfer section and charging section.

[0029] The external additive application operation is executed by a control unit 99, which is a control means provided in the image forming apparatus 100. FIG. 2 is a block diagram showing the control unit 99 and various components of the image forming apparatus 100 that are the objects of control. The control unit 99 controls the drum driving device 91 to rotate and drive the photosensitive drum 1. The control unit 99 controls the charging voltage power supply 92 to apply a DC voltage, which is a charging voltage, to the charging roller 2, and controls the exposure device 3 to expose the photosensitive drum 1. The control unit 99 controls the charging roller driving device 98 to rotate and drive the charging roller 2. The control unit 99 also controls the developing voltage power supply 93 to apply a DC voltage, which is a developing voltage, to the developing device 4. The control unit 99 also controls the pre-charge exposure device 6 to expose the photosensitive drum 1 at a position upstream of the charging unit. The control unit 99 also controls the transfer voltage power supply 94 to apply a DC voltage, which is a transfer voltage, to the transfer roller 5. The control unit 99 also determines whether the image forming apparatus 100 is new or not. For example, information indicating that the image forming apparatus 100 is in a new state is stored in the memory 97 at the time of shipment, and when the external additive application operation is performed correctly, the information indicating the new state is rewritten to information indicating that the image forming apparatus 100 is in a used state. The control unit 99 also acquires information on the operating environment of the image forming apparatus 100 by the environmental sensor 96. Examples of the environmental sensor 96 include a temperature sensor and a humidity sensor. In a configuration having a brush member 11 described later, the control unit 99 controls a brush voltage power supply 95 to apply a voltage to the brush member 11. In a configuration not having a brush member 11, the brush voltage power supply 95 is not provided. Also, a configuration may be adopted in which the power supply that applies a voltage to the brush member 11 is shared with another power supply (e.g., the development voltage power supply 93) and no separate power supply that applies a voltage to the brush member 11 is provided.

[0030] FIG. 4 shows the surface potential formed on the photosensitive drum 1 in the external additive application operation and the applied voltages of each member. The surface potential immediately before the charging section of the photosensitive drum 1 in the external additive application operation is Vdg1, the surface potential immediately before the developing section is Vdg2, and the surface potential immediately before the transfer section is Vdg3. The charging voltage, the developing voltage, and the transfer voltage in the external additive application operation are Vcg, Vrg, and Vtg, respectively. In the following description, the potential relationship in which an electrostatic force acts from the position of potential V1 to the position of potential V2 on particles having the charging polarity of the external additive is represented as V1>V2. In the first embodiment, the charging polarity of the external additive is positive, so that when, for example, V1=+150V and V2=-340V, the potential relationship V1>V2 is satisfied in the above sense. When the charging polarity of the external additive is negative, for example, when V1=-380V and V2=-100V, the potential relationship V1>V2 is satisfied in the above sense. In the additive application operation, exposure by the exposure device 3 is not performed, and the applied voltages are controlled so that Vrg>Vdg2 and Vdg1>Vcg. This causes the additive to move from the developing device 4 to the photosensitive drum 1, and also causes the additive to move from the photosensitive drum 1 to the charging roller 2. In the first embodiment, exposure by the pre-charge exposure device 6 is not performed, and the charging voltages are Vcg=-900V, Vrg=+150V, and Vdg2=Vdg1=-340V. The charging voltage in the additive application operation is Vcg=-900V, and the charging voltage in the image forming operation is Vcp=-1350V, and the potential relationship is Vcg>Vcp. The dark area potential Vdp2 of the developing section in the image forming operation is Vdp2=-78 With respect to 0V, the dark potential Vdg2 of the developing section during the external additive application operation is −340V.

[0031] Let the potential difference between the developing voltage in the developing unit and the surface potential of the charged surface of the photosensitive drum 1 be Vback, and let the potential difference Vback_p of the developing unit in the image forming operation and the potential difference Vback_g of the developing unit in the external additive coating operation be defined. In Example 1, in the external additive coating operation, the applied voltage is controlled so that Vback_g > Vback_p is satisfied. Vback_g = Vrg - Vdg2 and Vback_p = Vrp - Vdp2. As a result, more positively charged external additive can be developed during the image forming operation and moved from the developing device 4 to the photosensitive drum 1. In Example 1, the potential difference Vback_g of the developing unit in the external additive coating operation is 490 V, and the potential difference Vback_p of the developing unit in the image forming operation is 400 V.

[0032] In the external additive coating operation, the applied voltage is controlled so that the potential relationship in the transfer unit becomes Vtg > Vdg3. Thereby, the movement of the external additive from the photosensitive drum 1 to the transfer roller 5 can be suppressed. In Example 1, the transfer voltage Vtg = +100 V in the external additive coating operation, and the surface potential Vdg3 of the photosensitive drum 1 in the transfer unit is -340 V. Therefore, the adhesion of the positively charged external additive to the transfer roller 5 in the transfer unit can be suppressed.

[0033] When the threshold value Vtth of the potential difference at which discharge occurs between the transfer roller 5 and the photosensitive drum 1 is set, in the external additive coating operation, the applied voltage is controlled so that Vtg - Vdg3 < Vtth. Thereby, no discharge occurs between the photosensitive drum 1 and the transfer roller 5 in the transfer unit. Therefore, an increase in the charge of the external additive can be suppressed. In Example 1, the transfer voltage Vtg = +100 V, the surface potential Vdg3 of the photosensitive drum 1 in the transfer unit is -340 V, and the potential difference between the transfer roller 5 and the photosensitive drum 1 is 440 V. The discharge threshold value Vtth between the transfer roller 5 and the photosensitive drum 1 is 600 V. Therefore, since no discharge occurs in the transfer unit, strong polarization of the external additive due to discharge can be suppressed, and strong adhesion of the external additive to the photosensitive drum 1 can be suppressed.

[0034] When the threshold value Vcth of the potential difference at which discharge occurs between the charging roller 2 and the photosensitive drum 1 is set, in the external additive application operation, the applied voltage is controlled so that Vdg1 - Vcg < Vcth. Thereby, no discharge occurs between the photosensitive drum 1 and the charging roller 2 in the charging section. In Example 1, in the external additive application operation, exposure by the pre-charge exposure device 6 is not performed (pre-charge exposure OFF), or the exposure amount by the pre-charge exposure device 6 is made less than that during the image forming operation, so that Vdg1 - Vcg < Vcth. Thereby, it is possible to suppress the charging polarity of the external additive from becoming the same polarity as the charging polarity of the toner in the charging section. In Example 1, Vcth = 600V, and since pre-charge exposure is not performed, Vdg1 = Vdg3 = -340V and Vcg = -900V, so Vdg1 - Vcg < Vcth. Thereby, the negative charging of the positive external additive on the photosensitive drum 1 is suppressed, and the positive external additive is efficiently electrically transferred from the photosensitive drum 1 to the charging roller 2.

[0035] The external additive application operation is executed for a predetermined time so that more external additive adheres to the charging roller 2 than during the image forming operation, and a sufficient amount of external additive can adhere to the charging roller 2. In Example 1, the execution time of the external additive application operation is 30 seconds. This execution time is approximately 150 revolutions of the charging roller 2.

[0036] Note that the execution time of the external additive application operation may be set according to environmental information such as the temperature and humidity detected by the environmental sensor 96 attached to the image forming apparatus 100. For example, the temperature is detected by the environmental sensor 96, and the execution time of the external additive application operation may be made longer as the temperature is higher. Also, the humidity is detected by the environmental sensor 96, and the execution time of the external additive application operation may be made longer as the humidity is higher. In Example 1, in a high temperature and high humidity environment (temperature 32.5°C, humidity 80%) where the visibility of black dots is high, the execution time of the external additive application operation is about 60 seconds (about 300 revolutions). The memory 97 stores information on a predetermined correspondence between the environmental conditions and the execution time of the external additive application operation. The control unit 99 can acquire the execution time of the external additive application operation based on the detection result by the environmental sensor 96 and the correspondence stored in the memory 97. An example of the correspondence is a correspondence in which the execution time of the external additive application operation is longer as the temperature is higher, and the execution time of the external additive application operation is longer as the humidity is higher. The control of the external additive application operation according to the environmental conditions such as temperature and humidity is not limited to the execution time, and the rotation speed of the photosensitive drum 1 and the charging roller 2, the applied voltage, etc. may be changed. In addition, the control of the external additive application operation according to the environmental conditions may be changed according to the type of external additive.

[0037] Next, the effect of Example 1 will be described. In Example 1, in a brand new image forming apparatus 100, an external additive application operation is performed before an image forming operation is performed. This causes the external additive to adhere to the charging roller 2, and reduces the adhesive force of the surface of the charging roller 2, thereby suppressing adhesion of foreign matter. In order to confirm the effect of Example 1, the number of black spots generated in Example 1 and Comparative Example 1 was verified.

[0038] The details will be explained below.

[0039] The number of black spots was verified by passing one sheet of paper with 500 quartz particles (particle diameter: 100 to 300 μm) sprinkled on it, and then passing 1000 sheets of paper without sprinkling on it, and evaluating the image on the 1000th sheet. A new image forming apparatus was used, and the evaluation was performed in a high-temperature and high-humidity environment with a temperature of 32.5° C. and a humidity of 80%. In Example 1, the above evaluation was performed after applying an external additive to a new image forming apparatus for 60 seconds, and in Comparative Example, the above evaluation was performed without applying an external additive to a new image forming apparatus. The number of black spots here refers to the number of black spots with a pitch of 2 mm in the conveying direction and the perpendicular direction on the surface of the recording material P. For example, the case shown in FIG. 5 is counted as one. The evaluation was performed three times for each of Example 1 and Comparative Example.

[0040] Table 1 shows the number of black spots generated in Example 1 and Comparative Example 1. [Table 1]

[0041] As shown in Table 1, in Example 1, zero black spots occurred, whereas in Comparative Example 1, 4 to 6 black spots occurred, demonstrating the effect of suppressing the occurrence of black spots by performing the external additive application operation.

[0042] As described above, by performing the external additive application operation on a brand new image forming apparatus, it is possible to prevent foreign matter from adhering to the charging roller 2 and prevent the occurrence of black dot images.

[0043] (Cleaning operation) In addition, when the toner contains a large amount of positive toner, the above-mentioned additive application operation may not be sufficient to apply only the additive. In addition, a large amount of toner also transfers to the charging roller 2. Therefore, after the external additive application operation, a cleaning operation may be performed to control the charging voltage, the developing voltage, and the transfer voltage so as to move the toner adhering to the charging roller 2 to the photosensitive drum 1.

[0044] FIG. 6 shows the surface potential formed on the photosensitive drum 1 during the cleaning operation and the applied voltages of each member. The surface potential immediately before the charging section of the photosensitive drum 1 during the cleaning operation is Vdc1, the surface potential immediately before the developing section is Vdc2, the surface potential immediately before the transfer section is Vdc3, the charging voltage is Vcc, the developing voltage is Vrc, and the transfer voltage is Vtc. During the cleaning operation, exposure is performed by the pre-charge exposure device 6, exposure is not performed by the exposure device 3, and the applied voltages are controlled so that Vdc1>Vcc and Vrc>Vdc2. This allows the toner to be moved from the charging roller 2 to the photosensitive drum 1 and the toner to be moved from the photosensitive drum 1 to the developing device 4. In the first embodiment, Vdc1=0V, Vcc=-500V, Vrc=+150V, and Vdc2=0V.

[0045] In the cleaning operation, while applying a negative charging voltage, the charging roller 2 and the photosensitive drum 1 are rotationally driven. As a result, due to the frictional rubbing caused by the peripheral speed difference between the charging roller 2 and the photosensitive drum 1, the toner adhering to the charging roller 2 is charged negatively. The pre-charging exposure device 6 discharges the photosensitive drum 1 to erase the history of the charging potential of the photosensitive drum 1 before the cleaning operation is executed and make the potential of the photosensitive drum 1 approximately 0V. Due to the electrostatic repulsive force between the charging roller 2 and the negatively charged toner, the toner can be transferred from the charging roller 2 onto the photosensitive drum 1.

[0046] When the threshold value Vcth of the potential difference at which discharge occurs between the charging roller 2 and the photosensitive drum 1 is set, in the cleaning operation, the applied voltage may be controlled so that Vdc1 - Vcc < Vcth. Thereby, it is possible to suppress the toner adhering to the charging roller 2 in the charging section from being charged to the reverse polarity (positive polarity). In Example 1, Vcc = -500V, Vdc1 = 0V, and Vcth = 600V. Thereby, it is possible to suppress the toner adhering to the charging roller 2 from being polarized to the positive polarity due to discharge.

[0047] Since the developing voltage Vrc = +150V and the surface potential Vdc2 of the photosensitive drum 1 in the developing section is 0V, the negatively charged toner adhering to the photosensitive drum 1 moves to the developing device 4.

[0048] In the cleaning operation, the applied voltage may be controlled so that Vtc > Vdc3. Thereby, it is possible to suppress the movement of the toner charged to the reverse polarity (positive polarity) from the photosensitive drum 1 to the transfer roller 5. In the cleaning operation, since the potential relationship is Vrc > Vdc2, the positive-polarity toner is developed in the developing section, but by setting Vtc > Vdc3, it is possible to prevent the developed positive-polarity toner from being transferred in the transfer section. In Example 1, in the cleaning operation, Vtc = +100V and Vdc3 = 0V. Thereby, a potential relationship is formed in which the positive-polarity toner does not adhere to the transfer roller 5.

[0049] When the threshold value Vtth of the potential difference at which discharge occurs between the transfer roller 5 and the photosensitive drum 1 is set, in the cleaning operation, the applied voltage may be controlled so that Vtc - Vdc3 < Vtth. Thereby, an increase in the charge of the toner charged to the reverse polarity (positive polarity) in the transfer unit is suppressed. In Example 1, Vtc = +100V, Vdc3 = 0V, and Vtth = 600V. Thereby, it is possible to prevent the toner from being strongly positively polarized by discharge in the transfer unit.

[0050] By driving the above cleaning operation for at least the time corresponding to one rotation of the charging roller 2, the toner on the charging roller 2 can be cleaned. Note that the time of the cleaning operation can be appropriately changed according to the usage environment of the image forming apparatus 100, the type of external additive, and the like.

[0051] (Brush member) Hereinafter, an example in which the present invention is applied to an image forming apparatus having a contact member that contacts the surface of the photosensitive drum 1 downstream of the transfer unit and upstream of the charging unit in the rotation direction of the photosensitive drum 1 will be described. As the contact member, a brush member 11 as shown in FIG. 7 can be exemplified. The brush member 11 is a selective collection member that collects paper dust and allows toner to pass through. The brush member 11 can be exemplified by a base fabric 11a formed of synthetic fiber containing carbon, in which conductive yarn 11b made of conductive 6 nylon is woven. The conductive yarn 11b of the brush member 11 can be, for example, 2 denier in thickness, 240 kF / inch in density 2 , 6.5 mm in length, and 1 mm in penetration amount. The thickness of the conductive yarn 11b is preferably 1 to 10 denier, more preferably 1 to 6 denier. Also, the density of the conductive yarn 11b is preferably 150 kF / inch 2 or more.

[0052] A brush voltage, which is a predetermined DC voltage, is applied to the brush member 11 from a brush voltage power source 95 (FIG. 2). FIG. 8(a) shows the surface potential formed on the photosensitive drum 1 and the applied voltage to each member in the image forming operation, and FIG. 8(b) shows the potential of the photosensitive drum 1 and the applied voltage to each member in the external additive application operation.

[0053] 8(a), when the brush voltage in the image forming operation is Vbp, the applied voltage is controlled so that Vbp>Vdp3 in the image forming operation. As a result, positive toner passes through the brush member 11, so that the toner on the photosensitive drum 1 is not collected by the brush member 11. In the first embodiment, Vbp=-380V, Vdp3=-780V.

[0054] 8(b), when the brush voltage in the external additive application operation is Vbg, the applied voltage is controlled so that Vbg>Vdg3. This prevents the positive toner and external additive from moving from the photosensitive drum 1 to the brush member 11. Since the positive toner and external additive pass through the brush member 11, the toner and external additive on the photosensitive drum 1 are not collected by the brush member 11. In the first embodiment, Vbg=+150V, Vdg3=-340V.

[0055] 8B, in the cleaning operation of the charging roller 2, when the brush voltage in the cleaning operation is Vbc, the applied voltage is controlled so that Vbc>Vdc3. This makes it possible to suppress the movement of toner charged with the opposite polarity (positive polarity) from the photosensitive drum 1 to the brush member 11. The potential relationship other than that of the brush member 11 is as described in the first embodiment.

[0056] Example 2 A second embodiment will be described. In the second embodiment, the same members as those in the first embodiment are given the same reference numerals, and the description will be omitted. The second embodiment is characterized in that, in the additive application operation, the photosensitive drum 1 is exposed by the exposure device 3, so that the absolute value of the potential of the photosensitive drum 1 is made smaller than that before the exposure section, and the toner is developed in the development section. In the development section, basically, negative toner is developed, but the external additive is also developed along with the toner. The amount of the external additive is proportional to the amount of the toner to be developed. Therefore, the amount of the external additive to be developed can be controlled by adjusting the amount of exposure and changing the absolute value of the potential of the photosensitive drum 1. For example, when the image forming apparatus is used in a high-temperature and high-humidity environment, the amount of the external additive to be applied can be increased by adjusting the amount of exposure without extending the execution time of the additive application operation.

[0057] The external additive application operation of the second embodiment is an operational sequence in which the toner is developed onto the photosensitive drum 1 in the developing section, and then the external additive contained in the toner is efficiently transferred to the charging roller 2 by not discharging in the transfer section.

[0058] FIG. 9 shows the surface potential formed on the photosensitive drum 1 and the voltage applied to each member during the external additive application operation in the second embodiment. The surface potential Vlg1 immediately before the charged portion of the photosensitive drum 1 during the external additive application operation, the surface potential (dark area potential) Vdg2 immediately before the exposed portion, and the surface potential Vlg2 immediately before the developed portion The surface potential Vdg2 (light area potential) and the surface potential Vlg3 immediately before the transfer section are defined as the charging voltage Vcg, the developing voltage Vrg, and the transfer voltage Vtg in the external additive application operation. In the external additive application operation, the exposure device 3 performs exposure in the entire area in the direction (rotation axis direction) intersecting the circumferential direction of the photosensitive drum 1, and the applied voltage is controlled so that Vlg2>Vrg, Vlg1>Vcg, and Vtg=Vlg3. As a result, the toner containing the external additive is moved from the developing device 4 to the photosensitive drum 1, the external additive is moved from the photosensitive drum 1 to the charging roller 2, and the movement of the external additive and the toner from the photosensitive drum 1 to the transfer roller 5 is suppressed. In the second embodiment, the charging voltage Vcg=-1350V is applied in the external additive application operation to set the potential Vdg2=-780V of the photosensitive drum 1 downstream of the charging section, and the potential Vlg2=-100V of the photosensitive drum 1 in the exposed section is set by the exposure of the exposure device 3. The developing voltage Vrg is set to -380V and the toner is developed in the developing section.

[0059] At this time, the area exposed by the exposure device 3, in other words the area where the toner is developed, is set to have a short side width of 25 mm, which is one revolution of the charging roller 2, and a long side width that covers the entire image area. When the toner reaches the transfer section, a transfer voltage Vtg = -100V is applied, which is the same as the potential Vlg3 = -100V of the photosensitive drum 1 at the transfer section. This creates a potential relationship where the positive polarity external additive and the negative polarity toner do not electrically transfer to the transfer roller 5.

[0060] At the timing when the toner reaches the pre-charge exposure section, the pre-charge exposure by the pre-charge exposure device 6 is stopped (OFF). As a result, the potential Vlg3=-100V of the photosensitive drum 1 at the transfer section remains unchanged, and the potential Vlg1=-100V of the photosensitive drum 1 at the charging section becomes, and the positive external additive at the charging section moves from the photosensitive drum 1 to the charging roller 2 by electrostatic force. Note that, although an example in which the pre-charge exposure by the pre-charge exposure device 6 is turned off has been described, the pre-charge exposure may be performed with an exposure amount less than the exposure amount by the pre-charge exposure device 6 in the image forming operation.

[0061] For the purpose of confirming the effect of Example 2, the same verification as in Example 1 was performed for Example 2 and Comparative Example 2. For Example 2, an external additive application operation was performed on a brand new image forming apparatus, and then an evaluation similar to that of Example 1 was performed. For Comparative Example 2, an external additive application operation was not performed on a brand new image forming apparatus, and an evaluation similar to that of Example 1 was performed. Table 2 shows the number of black spots generated in Example 2 and Comparative Example 2. [Table 2]

[0062] As shown in Table 2, in Example 2, zero black spots occurred, whereas in Comparative Example 2, five to six black spots occurred, demonstrating the effect of suppressing the occurrence of black spots by performing the external additive application operation.

[0063] As described above, by performing the external additive application operation on a brand new image forming apparatus, it is possible to prevent foreign matter from adhering to the charging roller 2 and prevent the occurrence of black dot images.

[0064] In the external additive application operation, the applied voltage may be controlled such that Vlg1 - Vcg < Vcth by increasing the charging voltage Vcg at the timing when the toner and the external additive reach the charging unit. In this specification, increasing the voltage V1 to the voltage V2 means changing from the voltage V1 to the voltage V2 that satisfies the potential relationship V2 > V1 in the above meaning. Vcth is the threshold value of the potential difference at which discharge occurs between the charging roller 2 and the photosensitive drum 1. Thereby, it is possible to suppress the charging polarity of the external additive from becoming the same polarity as that of the toner in the charging unit. In Example 2, until the toner reaches the charging unit, the charging voltage Vcg is set to -1350 V, and at the timing when the toner reaches the charging unit, the charging voltage Vcg is changed to -600 V. Thereby, Vlg1 - Vcg = 500 V, which is smaller than the discharge threshold value Vcth = 600 V. Thereby, it is possible to suppress the negative charge of the positive external additive contained in the toner in the charging unit.

[0065] Also, the exposure amount by the exposure device 3 in the external additive application operation may be set according to environmental information such as the temperature and humidity detected by the environmental sensor 96. For example, the temperature is detected by the environmental sensor 96, and the exposure amount in the external additive application operation may be increased as the temperature is higher. Also, the humidity is detected by the environmental sensor 96, and the exposure amount in the external additive application operation may be increased as the humidity is higher. In Example 2, for example, the exposure amount may be increased in a high-temperature and high-humidity environment (temperature 32.5 °C, humidity 80%) where the visibility of the black dots is high, and the exposure amount may be decreased in a low-temperature and low-humidity environment where the visibility of the black dots is low.

[0066] Also, in Example 2, when the toner passes through the charging unit, the toner may adhere to the charging roller 2. Therefore, similar to Example 1, a cleaning operation of the charging roller 2 may be performed after the external additive application operation.

[0067] 7 as in the first embodiment. In the case of the configuration having the brush member 11, the brush voltage Vbg in the external additive application operation may be set to the same voltage as the potential Vlg3 of the photosensitive drum 1 at the contact portion between the brush member 11 and the photosensitive drum 1. This makes it possible to suppress the movement of the toner and the external additive from the photosensitive drum 1 to the brush member 11 and to prevent the brush member 11 from collecting the toner and the external additive.

[0068] In the above embodiment, a configuration in which a toner image formed on the photosensitive drum 1 is transferred to a recording material P as a transferee in a transfer section is exemplified, but the image forming apparatus to which the present invention can be applied is not limited to this configuration. For example, the present invention can be applied to an image forming apparatus configured to primarily transfer a toner image to an intermediate transfer belt as a transferee in a transfer section, and secondarily transfer the toner image from the intermediate transfer belt to a recording material P. In addition, in the above embodiment, a configuration in which the image forming apparatus 100 has a photosensitive drum 1, a charging roller 2, and a developing device 4 is exemplified, but the present invention can also be applied to an image forming apparatus of a process cartridge type. In that case, when the charging roller 2 provided in the process cartridge having the photosensitive drum 1 is in a new state, an external additive application operation can be performed on the charging roller 2 before an image forming operation is performed with the process cartridge.

[0069] The disclosure of this embodiment includes the following configuration. (Configuration 1) A rotatable photosensitive drum; a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, where Vcg is the charging voltage during the external additive application operation, Vdg1 is the surface potential on the charged surface of the photosensitive drum, Vdg2 is the surface potential in the development section facing the developing means, Vdg3 is the surface potential in the transfer section facing the transfer means, Vrg is the development voltage applied to the development means, Vtg is the transfer voltage applied to the transfer means, Vtth is the threshold value of the potential difference at which discharge occurs between the transfer means and the photosensitive drum, Vback_g is the potential difference between the development voltage and the surface potential in the development section of the photosensitive drum, and Vback_p is the potential difference between the development voltage during the image forming operation and the surface potential in the development section of the photosensitive drum, The control means does not perform exposure by the exposure means during the external additive application operation, and Vrg>Vdg2, Vdg1>Vcg, Vback_g>Vback_p, Vtg>Vdg3, Vtg-Vdg3 <Vtth The image forming apparatus is characterized in that the applied voltage is controlled so as to (Configuration 2) When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set, The control means, in the external additive application operation, Vdg1-Vcg <Vcth The image forming apparatus according to the first aspect of the present invention is characterized in that the charge polarity of the external additive is prevented from becoming the same polarity as the charge polarity of the toner by the charging means by controlling the applied voltage so as to be equal to the charge polarity of the external additive. (Configuration 3) a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, The image forming apparatus according to configuration 1 or 2, wherein the control means does not perform exposure by the pre-exposure means during the external additive application operation, or the amount of exposure by the pre-exposure means is less than the amount of exposure by the pre-exposure means during the image forming operation. (Configuration 4) a brush member that comes into contact with the surface of the photosensitive drum on which the transfer unit has performed the transfer; When the brush voltage applied to the brush member during the external additive application operation is Vbg, The control means, in the external additive application operation, Vbg>Vdg3 4. The image forming apparatus according to any one of configurations 1 to 3, wherein the applied voltage is controlled so as to suppress movement of the external additive from the photosensitive drum to the brush member. (Configuration 5) A sensor for detecting an environment of the image forming apparatus, 5. The image forming apparatus according to any one of configurations 1 to 4, wherein the control unit sets a duration of the external additive application operation in accordance with environmental information detected by the sensor. (Configuration 6) The sensor detects temperature; 6. The image forming apparatus according to claim 5, wherein the control unit extends a duration of the external additive application operation as the temperature detected by the sensor increases. (Configuration 7) The sensor detects humidity; The image forming apparatus according to configuration 5, wherein the control unit extends a duration of the external additive application operation as the humidity detected by the sensor increases. (Configuration 8) A rotatable photosensitive drum; a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, Let Vcg be the charging voltage in the external additive application operation, Vlg1 be the surface potential on the exposed surface of the photosensitive drum, Vlg2 be the surface potential in the developing section facing the developing means, Vlg3 be the surface potential in the transfer section facing the transfer means, Vrg be the developing voltage applied to the developing means, and Vtg be the transfer voltage applied to the transfer means. The control unit causes the exposure unit to expose the entire area of ​​the photosensitive drum in a direction intersecting the circumferential direction during the external additive application operation, and Vlg2>Vrg, Vlg1>Vcg, Vtg=Vlg3 The image forming apparatus is characterized in that the applied voltage is controlled so as to (Configuration 9) When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set, The control unit increases a charging voltage Vcg at a timing when the toner and the external additive transferred from the developing unit to the photosensitive drum reach a position facing the charging unit during the external additive application operation. Vlg1-Vcg <Vcth The image forming apparatus according to the configuration 8, wherein the charging polarity of the external additive is prevented from becoming the same polarity as the charging polarity of the toner by the charging means by controlling the applied voltage so as to be equal to the polarity of the external additive. (Configuration 10) a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, The control unit is configured to control a time when the toner and the external additive transferred from the developing unit to the photosensitive drum reach a position where the toner and the external additive are exposed by the pre-exposure unit in the external additive application operation. 10. The image forming apparatus according to configuration 8 or 9, wherein exposure by the pre-exposure means is stopped upon stopping the exposure. (Configuration 11) a brush member that comes into contact with the surface of the photosensitive drum on which the transfer unit has performed the transfer; When the brush voltage applied to the brush member during the external additive application operation is Vbg, The control means, in the external additive application operation, Vbg=Vlg3 11. The image forming apparatus according to any one of configurations 8 to 10, wherein the applied voltage is controlled so as to suppress movement of the toner and the external additive from the photosensitive drum to the brush member. (Configuration 12) A sensor for detecting an environment of the image forming apparatus, 12. The image forming apparatus according to any one of configurations 8 to 11, wherein the control means sets an amount of exposure by the exposure means in the external additive application operation in accordance with environmental information detected by the sensor. (Configuration 13) The sensor detects temperature; 13. The image forming apparatus according to claim 12, wherein the control unit increases the amount of exposure by the exposure unit as the temperature detected by the sensor increases. (Configuration 14) The sensor detects humidity; 13. The image forming apparatus according to claim 12, wherein the control unit increases the amount of exposure by the exposure unit as the humidity detected by the sensor increases. (Configuration 15) 15. The image forming apparatus according to any one of Configurations 1 to 14, wherein the charging means and the photosensitive drum are in contact with each other while rotating in the same direction, and the peripheral speed of the charging means is faster than the peripheral speed of the photosensitive drum. (Configuration 16) 16. The image forming apparatus according to any one of configurations 1 to 15, wherein the control unit executes the external additive application operation when the charging unit is in an early stage of use or is brand new. (Configuration 17) 17. The image forming apparatus according to any one of configurations 1 to 16, wherein the control unit executes a cleaning operation for moving the toner adhering to the surface of the charging unit to the photosensitive drum after the external additive application operation. (Configuration 18) In the cleaning operation, the charging voltage is Vcc, the surface potential of the charging surface of the photosensitive drum is Vdc1, the surface potential of the developing unit is Vdc2, and the developing voltage is Vrc, The control means does not perform exposure by the exposure means during the cleaning operation, and Vdc1>Vcc, Vrc>Vdc2 18. The image forming apparatus according to claim 17, wherein the toner is moved from the charging means to the photosensitive drum and from the photosensitive drum to the developing means by controlling an applied voltage so as to satisfy the above condition. (Configuration 19) When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set, The control means, in the cleaning operation, Vdc1-Vcc <Vcth 19. The image forming apparatus according to claim 17 or 18, wherein the toner adhering to the charging means is prevented from being charged to the opposite polarity by controlling the applied voltage so as to be equal to the polarity of the toner. (Configuration 20) When the transfer voltage during the cleaning operation is Vtc and the surface potential of the photosensitive drum at the transfer portion is Vdc3, The control means, in the cleaning operation, Vtc>Vdc3 20. The image forming apparatus according to any one of Configurations 17 to 19, wherein the applied voltage is controlled so as to suppress movement of the toner charged with the opposite polarity from the photosensitive drum to the transfer means. (Configuration 21) When the threshold voltage Vtth of the potential difference at which discharge occurs between the transfer means and the photosensitive drum is set as: The control means, in the cleaning operation, Vtc-Vdc3 <Vtth 21. The image forming apparatus according to claim 20, wherein an increase in charge of the toner charged to the opposite polarity in the transfer portion is suppressed by controlling an applied voltage so as to satisfy the above condition. (Configuration 22) a brush member that comes into contact with the surface of the photosensitive drum on which the transfer unit has performed the transfer; When the brush voltage applied to the brush member in the cleaning operation is Vbc and the surface potential of the photosensitive drum at the transfer portion is Vdc3, The control means, in the cleaning operation, Vbc>Vdc3 22. The image forming apparatus according to any one of Configurations 17 to 21, wherein the applied voltage is controlled so as to suppress movement of the toner charged with the opposite polarity from the photosensitive drum to the brush member. (Configuration 23) a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, 23. The image forming apparatus according to any one of configurations 17 to 22, wherein the control means performs exposure by the pre-exposure means in the cleaning operation. [Explanation of symbols]

[0070] 1: photosensitive drum, 2: charging roller, 3: exposure device, 4: developing device, 5: transfer roller, 100: image forming device

Claims

1. A rotatable photosensitive drum; a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, When the charging voltage in the external additive application operation is Vcg, the surface potential on the charged surface of the photosensitive drum is Vdg1, the surface potential in the development section facing the developing means is Vdg2, the surface potential in the transfer section facing the transfer means is Vdg3, the development voltage Vrg applied to the development means, the transfer voltage Vtg applied to the transfer means, the threshold value Vtth of the potential difference at which discharge occurs between the transfer means and the photosensitive drum, the potential difference Vback_g between the development voltage and the surface potential in the development section of the photosensitive drum, and the potential difference Vback_p between the development voltage in the image forming operation and the surface potential in the development section of the photosensitive drum, The control means does not perform exposure by the exposure means during the external additive application operation, and Vrg>Vdg2, Vdg1>Vcg, Vback_g>Vback_p, Vtg>Vdg3, Vtg-Vdg3<Vtth The image forming apparatus is characterized in that the applied voltage is controlled so as to

2. When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set as The control means, in the external additive application operation, Vdg1-Vcg<Vcth 2. The image forming apparatus according to claim 1, wherein the charging polarity of the external additive is prevented from becoming the same polarity as the charging polarity of the toner by the charging means by controlling the applied voltage so as to be equal to the polarity of the external additive.

3. a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, 3. The image forming apparatus according to claim 1, wherein the control means does not perform exposure by the pre-exposure means during the external additive application operation, or the amount of exposure by the pre-exposure means is less than the amount of exposure by the pre-exposure means during the image forming operation.

4. A brush member is provided to contact the surface of the photosensitive drum on which the transfer is performed by the transfer means. 、 When the brush voltage applied to the brush member in the external additive application operation is Vbg, The control means, in the external additive application operation, Vbg>Vdg3 3. The image forming apparatus according to claim 1, wherein the external additive is prevented from moving from the photosensitive drum to the brush member by controlling an applied voltage so as to satisfy the above condition.

5. A sensor for detecting an environment of the image forming apparatus, 3. The image forming apparatus according to claim 1, wherein the control unit sets a duration of the external additive application operation in accordance with environmental information detected by the sensor.

6. The sensor detects temperature; 6. The image forming apparatus according to claim 5, wherein the control unit extends a duration of the external additive application operation as the temperature detected by the sensor increases.

7. The sensor detects humidity; 6. The image forming apparatus according to claim 5, wherein the control unit extends a duration of the external additive application operation as the humidity detected by the sensor increases.

8. A rotatable photosensitive drum; a charging means for rotating in contact with the photosensitive drum and charging the surface of the photosensitive drum; an exposure unit that exposes the charged surface of the photosensitive drum that is charged by the charging unit; a developing unit that supplies a toner containing an external additive to the exposure surface of the photosensitive drum that is exposed by the exposure unit; a transfer means for transferring the toner supplied to the photosensitive drum by the developing means to a transfer medium; a control unit that controls the charging unit, the developing unit, and the transfer unit; having the charge polarity of the external additive is opposite to that of the toner, an image forming apparatus in which the toner that has not been transferred to the transfer medium by the transfer means is collected by the developing means, the control means is capable of performing an image forming operation for forming an image on a recording material, and an external additive application operation for transferring the external additive or the toner containing the external additive from the developing means to the photosensitive drum, and transferring the external additive from the photosensitive drum to the charging means, The potential relationship in which an electrostatic force acts from a position of potential V1 to a position of potential V2 on the particles having the charge polarity of the external additive is represented as V1>V2, Let Vcg be the charging voltage in the external additive application operation, Vlg1 be the surface potential on the exposed surface of the photosensitive drum, Vlg2 be the surface potential in the developing section facing the developing means, Vlg3 be the surface potential in the transfer section facing the transfer means, Vrg be the development voltage applied to the developing means, and Vtg be the transfer voltage applied to the transfer means. The control unit causes the exposure unit to expose the entire area of ​​the photosensitive drum in a direction intersecting the circumferential direction during the external additive application operation, and Vlg2>Vrg, Vlg1>Vcg, Vtg=Vlg3 The image forming apparatus is characterized in that the applied voltage is controlled so as to

9. When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set as The control unit controls the transfer of the external additive from the developing unit to the photosensitive drum during the external additive application operation. At the timing when the toner and the external additive reach the position facing the charging means, the charging voltage Vcg is increased. Vlg1-Vcg<Vcth 9. The image forming apparatus according to claim 8, wherein the charging polarity of the external additive is prevented from becoming the same polarity as the charging polarity of the toner by the charging means by controlling the applied voltage so as to be equal to the polarity of the external additive.

10. a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, 10. The image forming apparatus according to claim 8, wherein the control means stops exposure by the pre-exposure means at a timing when the toner and the external additive that have moved from the developing means to the photosensitive drum reach a position where they are exposed by the pre-exposure means during the external additive application operation.

11. a brush member that comes into contact with the surface of the photosensitive drum on which the transfer unit has performed the transfer; When the brush voltage applied to the brush member in the external additive application operation is Vbg, The control means, in the external additive application operation, Vbg=Vlg3 10. The image forming apparatus according to claim 8, wherein the applied voltage is controlled so as to suppress movement of the toner and the external additive from the photosensitive drum to the brush member.

12. A sensor for detecting an environment of the image forming apparatus, 10. The image forming apparatus according to claim 8, wherein the control means sets an amount of exposure by the exposure means in the external additive application operation in accordance with environmental information detected by the sensor.

13. The sensor detects temperature; 13. The image forming apparatus according to claim 12, wherein the control unit increases the amount of exposure by the exposure unit as the temperature detected by the sensor increases.

14. The sensor detects humidity; 13. The image forming apparatus according to claim 12, wherein the control unit increases the amount of exposure by the exposure unit as the humidity detected by the sensor increases.

15. 10. An image forming apparatus according to claim 1, wherein said charging means and said photosensitive drum are in contact with each other while rotating in the same direction, and the peripheral speed of said charging means is faster than the peripheral speed of said photosensitive drum.

16. 10. The image forming apparatus according to claim 1, wherein the control unit executes the external additive application operation when the charging unit is in an initial use state or is a brand new state.

17. 10. The image forming apparatus according to claim 1, wherein the control unit executes a cleaning operation for moving the toner adhering to the surface of the charging unit to the photosensitive drum after the external additive application operation.

18. In the cleaning operation, the charging voltage is Vcc, the surface potential of the charging surface of the photosensitive drum is Vdc1, the surface potential of the developing unit is Vdc2, and the developing voltage is Vrc, The control means does not perform exposure by the exposure means during the cleaning operation, and Vdc1>Vcc, Vrc>Vdc2 By controlling the applied voltage so that the toner is charged from the charging means to the photosensitive drum, 18. The image forming apparatus according to claim 17, wherein the toner is transferred from the photosensitive drum to the developing means while transferring the toner to the developing ram.

19. When a threshold value Vcth of the potential difference at which discharge occurs between the charging means and the photosensitive drum is set as The control means, in the cleaning operation, Vdc1-Vcc<Vcth 18. The image forming apparatus according to claim 17, wherein the toner adhering to the charging means is prevented from being charged to the opposite polarity by controlling the applied voltage so as to be equal to the toner adhering to the charging means.

20. When the transfer voltage during the cleaning operation is Vtc and the surface potential of the photosensitive drum at the transfer portion is Vdc3, The control means, in the cleaning operation, Vtc>Vdc3 18. The image forming apparatus according to claim 17, wherein the applied voltage is controlled so as to suppress movement of the toner charged with the opposite polarity from the photosensitive drum to the transfer means.

21. When a threshold value Vtth of the potential difference at which discharge occurs between the transfer means and the photosensitive drum is set as The control means, in the cleaning operation, Vtc-Vdc3<Vtth 21. The image forming apparatus according to claim 20, wherein an increase in charge of the toner charged to the opposite polarity in the transfer portion is suppressed by controlling an applied voltage so as to satisfy the above condition.

22. a brush member that comes into contact with the surface of the photosensitive drum on which the transfer unit has performed the transfer; When a brush voltage Vbc applied to the brush member in the cleaning operation is Vdc3, a surface potential Vdc3 at the transfer portion of the photosensitive drum is The control means, in the cleaning operation, Vbc>Vdc3 18. The image forming apparatus according to claim 17, wherein the applied voltage is controlled so as to suppress movement of the toner charged with the opposite polarity from the photosensitive drum to the brush member.

23. a pre-exposure means for exposing the surface of the photosensitive drum on which the transfer has been performed by the transfer means, 18. The image forming apparatus according to claim 17, wherein the control unit controls the pre-exposure unit to perform exposure during the cleaning operation.

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