JP7140553B2 - image forming device - Google Patents

Description

The present invention relates to image forming apparatuses such as copiers, printers, and facsimiles that form images by electrophotography.

Electrophotographic image forming apparatuses such as copiers and laser beam printers form an electrostatic image (latent image) by irradiating light corresponding to image data onto an electrophotographic photosensitive member (photosensitive drum) uniformly charged by a charging means. ). Then, toner of a developer, which is a recording material, is supplied from a developing device to the electrostatic image to visualize it as a toner image. This toner image is transferred from the photosensitive drum to a recording material such as recording paper by a transfer device. A recorded image is formed by fixing the toner image on the recording material with a fixing device.

A contact-type charging device, which charges a photosensitive drum by bringing a charging member into contact with the photosensitive drum, is widely used because of its advantages such as low ozone and low power consumption. However, when a contact-type charging device is used, the toner remaining on the photosensitive drum without image formation comes into contact with the charging member. Sometimes.

2. Description of the Related Art In recent years, in order to reduce the size of an image forming apparatus, there has been proposed a "cleanerless system" image forming apparatus that does not have a cleaning member for cleaning a photosensitive drum and a waste toner storage section. A cleaner-less system is a system that enables the reuse of toner without requiring a waste toner container by having the developing device recover the toner remaining on the photosensitive drum. When an image is formed using a cleanerless system, the toner remaining on the photosensitive drum without image formation is not cleaned, and therefore tends to adhere particularly to the charging member.

Japanese Unexamined Patent Application Publication No. 2002-100000 discloses a configuration in which the surface of the photosensitive drum is neutralized after transfer and before charging is performed again, thereby suppressing the adhesion of the toner remaining on the photosensitive drum to the charging member. ing. Eliminating the charge on the surface of the photosensitive drum before charging increases the charging contrast, which is the potential difference between the charging voltage applied to the charging member and the surface potential of the photosensitive drum, thereby increasing the amount of discharge in the charging section. Accordingly, by optimizing the charge of the toner remaining on the photosensitive drum, the toner adhesion to the charging member can be reduced, and the occurrence of image defects can be suppressed.

JP 2009-192941

However, when the charging contrast is increased to increase the amount of discharge in the charging portion, the deterioration of the image quality may be caused due to abrasion of the surface of the photosensitive drum due to discharge deterioration and adhesion of discharge products.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to provide an image forming apparatus capable of suppressing image defects caused by toner adhesion to a charging member while suppressing the amount of discharge in a charging section.

To achieve this object, the image forming apparatus of the present invention is an image forming apparatus for forming a toner image on a recording material, comprising a rotatable image carrier and a contact portion that contacts the image carrier. a charging member that charges the image carrier at the contact portion; a first exposure means that exposes the image carrier charged by the charging member to form an electrostatic latent image; a developing member that supplies toner to the electrostatic latent image and develops the toner image on the image carrier; a voltage applying unit that applies a voltage to the charging member; and the image carrier. an intermediate transfer member that forms a transfer portion in contact with a body and carries the toner image formed on the image carrier at the transfer portion; second exposure means for exposing the image carrier downstream of the transfer portion and upstream of the contact portion in the rotational direction of the image carrier; and voltage application. a first image forming mode in which image formation is performed by rotating the image carrier; and the first image forming mode. A second image forming mode in which image formation is performed by rotating the image bearing member at a rotation speed higher than the second image forming mode can be executed, and the control section controls the absolute value of the voltage applied to the voltage applying means and the The difference between the absolute value of the surface potential of the image carrier exposed by the second exposure means and the surface potential of the image carrier is larger when the second image forming mode is performed than when the first image forming mode is performed. , the voltage application means and the second exposure means are controlled.

As described above, according to the present invention, it is possible to suppress image defects due to toner adhesion to the charging member while suppressing the amount of discharge in the charging section.

1 is a diagram illustrating an image forming apparatus according to Embodiment 1; FIG. 4 is a control block diagram according to the first embodiment; FIG. 4A and 4B are diagrams illustrating discharge between a charging roller and a photosensitive drum according to the first embodiment; FIG. 4A and 4B are diagrams for explaining the movement of transfer residual toner in the cleanerless configuration according to the first embodiment; FIG. 4A and 4B are diagrams for explaining movement of re-transferred toner in a cleanerless configuration according to the first embodiment; FIG. 4 is a diagram illustrating a primary transfer configuration of the image forming apparatus according to Embodiment 1; FIG. 5 is a diagram for explaining the state of charging contrast according to Example 1. FIG. 4 is a diagram showing the flow of image forming operation according to the first embodiment; FIG. FIG. 10 is a diagram showing the relationship between the discharge start voltage and the surface potential of the photosensitive drum according to Example 2; FIG. 11 is a diagram illustrating an image forming apparatus according to Example 3; FIG. 10 is a diagram illustrating a primary transfer configuration of an image forming apparatus according to Example 3; FIG. 11 is a diagram illustrating an image forming apparatus according to Example 4;

Preferred embodiments of the present invention will be exemplarily described in detail below with reference to the drawings. However, the dimensions, materials, shapes, and relative positions of components described in the following embodiments should be appropriately changed according to the configuration of the device to which the present invention is applied and various conditions. Accordingly, it is not intended to limit the scope of the present invention solely thereto unless specifically stated.

1. BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention particularly relates to an image forming apparatus using a so-called drum cleanerless system that does not have cleaning means on an image carrier. FIG. 1 is a schematic diagram showing an example of a color image forming apparatus. Using FIG. 1, the configuration and operation of the image forming apparatus according to this embodiment will be described. The image forming apparatus is a tandem type printer provided with image forming stations which are image forming units a to d. The first image forming station a is yellow (Y), the second image forming station b is magenta (M), the third image forming station c is cyan (C), and the fourth image forming station d is black (Bk ) to form an image of each color. Since the configuration of each image forming station is the same except for the color of the toner contained therein, one image forming station will be representatively described in the description of the image forming stations.

Each image forming station includes a rotatable drum-shaped electrophotographic photosensitive member (hereinafter referred to as a photosensitive drum) 1, a charging roller 2 as charging means, an exposure device 3, and a developing device 4. . The photosensitive drum 1 is an image carrier that carries a toner image while being driven to rotate in the direction of the arrow in FIG. An image forming operation is started, and the photosensitive drum 1 is driven to rotate. The surface of the photosensitive drum 1 is uniformly charged to a predetermined potential with a predetermined polarity by a charging roller 2 during rotational driving, and is exposed to light by an exposure device 3 according to an image signal. In this embodiment, the charging polarity is negative. As a result, an electrostatic latent image corresponding to the color component image of the desired color image is formed. Then, the electrostatic latent image is developed by the developing device 4 at the development position and visualized as a toner image.

A charging roller 2 as a charging member is in contact with the surface of the photosensitive drum 1 with a predetermined pressing force, and is driven to rotate relative to the photosensitive drum 1 by friction with the surface of the photosensitive drum 1 . A contact position between the charging roller 2 and the photosensitive drum 1 is defined as a contact portion. A predetermined DC voltage (bias) is applied to the rotating shaft of the charging roller 2 from the charging voltage power supply 20, which is the charging voltage applying means shown in FIG. 2, in accordance with the image forming operation. In the present embodiment, the charging roller 2 is formed on a metal shaft with a diameter of 5.5 (mm) and a conductive roller having a thickness of 1.5 (mm) and a specific volume resistivity of about 1×10 ⁶ (Ωcm). The one provided with the elastic layer which consists of elastic bodies is used. A DC voltage of -1000 (V) is applied from a charging voltage source 20 to the rotating shaft of the charging roller 2 according to the image forming operation, and the surface of the photosensitive drum 1 is discharged between the charging roller 2 and the photosensitive drum 1. is charged. Discharge between the charging roller 2 and the photosensitive drum 1 occurs near the upstream and downstream sides of the contact portion of the photosensitive drum 1 in the rotational direction. As the peripheral speed of the photosensitive drum 1 changes, the balance between the amount of discharge near the upstream side and the amount of discharge near the downstream side changes. The surface potential of 1 becomes almost constant. In this embodiment, the dark portion potential Vd of the surface of the photosensitive drum 1 immediately after the photosensitive drum 1 passes through the contact portion is −500 (V) regardless of the peripheral speed of the photosensitive drum 1 . In order to measure the surface potential of the photosensitive drum 1, a surface potential meter Model 344 manufactured by Trek Corporation was used.

The surface of the charged photosensitive drum 1 is exposed by the exposure device 3 . The exposure device 3 is exposure means for forming an electrostatic latent image on the uniformly charged surface of the photosensitive drum 1 . The exposure device 3 has a laser output unit for outputting a laser beam modulated in accordance with an input time-series electrical digital pixel signal, a rotating polygon mirror, an fθ lens, a reflecting mirror, and the like. , the surface of the photosensitive drum 1 is subjected to main scanning exposure. An electrostatic latent image corresponding to image information is formed by this main scanning exposure and sub-scanning by rotation of the photosensitive drum 1 . In this embodiment, the exposure amount is adjusted so that the light area potential Vl, which is the surface potential of the photosensitive drum 1 after being exposed by the exposure device 3, is -150 (V).

The developing unit 4, which is a developing device, includes a developing roller 41 as a developing member, which is a toner carrier, and a non-magnetic one-component developer (hereinafter referred to as toner), and develops an electrostatic latent image into a toner image. In other words, it is a developing means that develops the photosensitive drum 1 . The developing roller 41 is in contact with the photosensitive drum 1 with a predetermined contact width according to the image forming operation. Then, the developing roller 41 is moved at a peripheral speed greater than that of the photosensitive drum 1 so that the direction of surface movement of the developing roller 41 in the portion facing the photosensitive drum 1 is in the same direction as the direction of surface movement of the photosensitive drum 1 . rotationally driven. The developing unit 4 and the main body of the image forming apparatus 100 are provided with a mechanism for controlling the development contact/separation state between the developing roller 41 (not shown) and the photosensitive drum 1 . It is The developing roller 41 and the photosensitive drum 1 are brought into contact with each other according to the image forming operation or the like, and separated when the operation is stopped. 2, a predetermined DC voltage is applied to the core metal of the developing roller 41 from the developing voltage power supply 40 according to the image forming operation. ) is applied. The toner is a non-magnetic toner produced by a suspension polymerization method and having a negative charging property as a regular charging polarity, and has a volume average particle diameter of about 6.0 (μm). In order to improve the surface property, silicon oxide particles having a volume average particle diameter of about 20 (nm) are uniformly adhered to the toner surface in an amount of about 1.5% of the toner weight. Here, the volume average particle diameter of the toner is the volume average particle diameter measured by a laser diffraction particle size distribution analyzer LS-230 manufactured by Beckman Coulter, Inc. In the present embodiment, the toner produced by the suspension polymerization method is used, but the present invention is not limited to this. may be Further, in this embodiment, the image forming apparatus 100 that reversely develops the electrostatic latent image with the toner charged to the same negative polarity as the charging polarity of the photosensitive drum 1 is used. However, it can also be applied to the image forming apparatus 100 in which the charging polarity of the photosensitive drum 1 and the charging polarity of the toner are positive. The present invention can also be applied to an electrophotographic apparatus in which an electrostatic latent image is positively developed with toner charged to a polarity opposite to that of the photosensitive drum 1. In this case, the charging of the photosensitive drum is The toner charged to the opposite polarity is the normal polarity toner. Also, although non-magnetic toner is used, magnetic toner may be used.

The toner image formed on the photosensitive drum 1 is transferred onto the intermediate transfer belt 10 by a primary transfer roller 14 as a primary transfer member. The intermediate transfer belt 10 is arranged so as to be in contact with the photosensitive drums a, b, c, and d, and has a surface resistivity of 1×10 ¹¹ (Ω/□ ) was used. The intermediate transfer belt 10 has a thickness of 100 to 200 (μm), and is an endless resin film made of PVdf (polyvinylidene fluoride), nylon, PET (polyethylene terephthalate), PC (polycarbonate), or the like. The intermediate transfer belt 10 is stretched by a plurality of stretching members 11 , 12 , 13 , and rotates at a circumferential speed substantially equal to that of the photosensitive drum 1 in a direction of movement in the circumferential direction at the facing portion in contact with the photosensitive drum 1 . driven. The toner image formed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 10 while passing through the transfer nip portion, which is the contact portion between the photosensitive drum 1 and the intermediate transfer belt 10 . At the transfer nip portion, the primary transfer roller 14 is arranged so as to directly press the photosensitive drum 1 via the intermediate transfer belt 10 . In this embodiment, as shown in FIGS. 1 and 2, primary transfer is performed by applying a primary transfer voltage of 500 (V) from the primary transfer voltage power supply 140 to the primary transfer roller 14 .

However, part of the toner on the photosensitive drum 1 remains on the photosensitive drum 1 as untransferred toner without being transferred. The transfer residual toner remaining on the photosensitive drum 1 is a normal polarity toner having a weak charge amount or a reversed polarity toner having an opposite polarity charge. Further, the toner transferred to the intermediate transfer belt 10 by the primary transfer roller 14 was also discharged and charged with opposite polarity when passing through the transfer nip portion of the downstream station in the rotation direction of the intermediate transfer belt 10 . Inverted polarity toner may result. The reverse polarity toner electrically adheres to the photosensitive drum 1 of the downstream station as a retransfer toner. The transfer residual toner and the retransfer toner will be described later in detail.

Assuming that the exposure device 3 is the first exposure means, the pre-charging exposure device 5 serving as the second exposure means exposes the surface of the photosensitive drum 1 after the primary transfer to light and removes the static electricity over the entire length of the photosensitive drum 1 . It plays the role of reducing the absolute value of the surface potential of the surface and leveling the potential. As a result, the potential difference (hereinafter referred to as charging contrast) between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 immediately before the photosensitive drum 1 passes through the contact portion can be increased. Details of the charging contrast will be described later. As shown in FIG. 3, the surface of the photosensitive drum 1 is exposed by the pre-charging exposure device 5 . At this time, the area of the photosensitive drum 1 exposed by the pre-charging exposure device 5 is the area between the contact portion between the transfer portion and the charging roller 2 in the rotational direction of the photosensitive drum 1 . In this embodiment, as shown in FIG. 1, the pre-charging exposure device 5 is arranged downstream of the transfer section and upstream of the contact section in the rotational direction of the photosensitive drum 1 . If the charging contrast is increased by exposing the surface of the photosensitive drum 1 by the pre-charging exposure device 5, the amount of discharge in the vicinity of the upstream of the charging section between the charging roller 2 and the photosensitive drum 1 increases. When the amount of discharge increases, the transfer residual toner and the retransferred toner on the photosensitive drum 1 can be charged to the regular polarity. Since the transfer residual toner and the retransferred toner are charged to the normal polarity, an electrostatic repulsive force works with the charging roller 2 to which the voltage of the normal polarity side is applied, and the toner becomes difficult to adhere to the charging roller 2. , the toner contamination of the charging roller 2 is suppressed. In addition, the pre-charging exposure device 5 exposes the surface of the photosensitive drum 1 after the primary transfer to light over the entire length of the photosensitive drum 1 after the primary transfer, and removes static electricity from the surface of the photosensitive drum 1 after the primary transfer. This provides an effect of suppressing image defects such as image density unevenness due to potential unevenness. The surface of the photosensitive drum 1, which has been neutralized by the pre-charging exposure device 5, is re-charged to Vd, which is the dark potential, by the discharge when it enters the contact portion, and thereafter the steps of development, primary transfer, and neutralization are repeated. .

After the above operations are performed in the first, second, third, and fourth image forming stations a, b, c, and d, respectively, a yellow toner image of the first color, a magenta toner image of the second color, and a third color toner image are formed. A cyan toner image of the fourth color and a black toner image of the fourth color are formed on the intermediate transfer belt 10 . The toner images formed at each image forming station are sequentially transferred onto the intermediate transfer belt 10 to obtain a composite color image of a plurality of colors corresponding to the desired color image. The four-color toner image on the intermediate transfer belt 10 is fed by the paper feeding means 50 in the process of passing through the secondary transfer portion formed by the intermediate transfer belt 10 and the secondary transfer roller 15 as a secondary transfer member. It is transferred to the surface of the recording material P all at once. The secondary transfer roller 15 contacts the intermediate transfer belt 10 with a pressure of 50 (N) to form a secondary transfer portion. The secondary transfer roller 15 is driven to rotate with respect to the intermediate transfer belt 10 . In the present embodiment, when the toner on the intermediate transfer belt 10 is being secondarily transferred onto the recording material P, a voltage of 1000 (V) is applied to the secondary transfer roller 15 from the secondary transfer voltage power supply 150 as shown in FIG. voltage is applied.

After that, the recording material P bearing the four-color toner image is introduced into a fixing device 30 where the four-color toners are melted and mixed and fixed to the recording material P by being heated and pressurized. Toner remaining on the intermediate transfer belt 10 after the secondary transfer is cleaned and removed by the cleaning device 17 . The cleaning device 17 has a cleaning blade or the like that contacts the outer peripheral surface of the intermediate transfer belt 10 to scrape off toner remaining on the intermediate transfer belt 10 and collects the toner in the intermediate transfer belt cleaning device 17 . The intermediate transfer belt cleaning device 17 is disposed on the downstream side of the intermediate transfer belt 10 in the rotation direction of the intermediate transfer belt 10 from the secondary transfer portion so as to collect the toner adhering to the intermediate transfer belt 10 . there is

FIG. 2 is a block diagram showing a schematic control mode of the main part of the image forming apparatus 100 in this embodiment. The controller 200 exchanges various electrical information with the host apparatus, and controls the image forming operation of the image forming apparatus 100 by the control unit 202 via the interface 201 according to a predetermined control program and reference table. control effectively. The control unit 202 includes a CPU 151 which is a central element for performing various arithmetic processing, a memory 152 such as a ROM and a RAM which are storage elements, and the like. The RAM stores sensor detection results, counter count results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance through experiments, and the like. Control objects, sensors, counters, and the like in the image forming apparatus 100 are connected to the control unit 202 . A control unit 202 controls the transmission and reception of various electrical information signals, the timing of driving each unit, and the like, thereby controlling a predetermined image forming sequence. For example, the control unit 202 controls voltages and exposure amounts applied by the charging voltage power source 20, the developing voltage power source 40, the exposure device 3, the pre-charging exposure device 5, the primary transfer voltage power source 140, and the secondary transfer voltage power source 150. there is The image forming apparatus 100 forms an image on the recording material P based on an electrical image signal input from the host device to the controller 200 . Examples of host devices include image readers, personal computers, facsimiles, and smart phones.

2. Cleanerless System A phenomenon that occurs in the operation of individual process cartridges when the cleanerless system of this embodiment is implemented will be described with reference to FIG.

As shown in FIG. 4A, after the toner image developed on the photosensitive drum 1 is primarily transferred onto the intermediate transfer belt 10, part of the toner that has not been primarily transferred is primarily transferred onto the photosensitive drum 1. It remains as residual toner. If there is a cleaning member, this primary transfer residual toner is collected by the cleaning member, but in the case of a cleanerless system, there is no cleaning device for collecting the primary transfer residual toner. Therefore, the toner on the photosensitive drum 1 rushes into the charging roller 2 as it is without being cleaned. The primary transfer residual toner rushing into the charging roller 2 is normal polarity toner or reversed polarity toner with a small amount of electric charge. As shown in FIG. 4B, these primary transfer residual toners are discharged by an electric field due to the charging voltage in the air gap in front of the contact portion, and become negative, which is the same normal polarity as that of the photosensitive drum 1. charged. Since the primary transfer residual toner has a small amount of electric charge, it is easily affected by discharge, and tends to become negative polarity toner, which is normal polarity, due to discharge. Therefore, at the contact portion, the charging voltage is negative and larger than the surface potential of the photosensitive drum 1, so that the negatively charged primary transfer residual toner is attached to the charging roller 2 as shown in FIG. 4(c). It passes through the charging roller 2 without being attached. A part of the toner of the reversed polarity that rushes into the charging roller 2 without being discharged is electrically attracted to the charging roller 2, and the charging roller 2 may become dirty and cause image defects. The primary transfer residual toner that has passed through the contact portion reaches the laser irradiation position as the photosensitive drum 1 rotates. Since the primary transfer residual toner is not large enough to block the laser beam of the exposure device 3, it does not affect the process of forming the electrostatic latent image on the photosensitive drum 1, and the contact portion between the developing roller 41 and the photosensitive drum 1 is not affected. up to. As shown in FIG. 4D, the toner in the non-exposed area on the photosensitive drum 1 has a potential relationship between the surface potential of the photosensitive drum 1 and the development voltage (dark area potential (Vd) of the photosensitive drum 1=-500 ( V), developing voltage=−350 (V)), the toner is electrically recovered to the developing roller 41 side. In FIG. 4E, the toner in the exposed portion on the photosensitive drum 1 has a potential relationship between the surface potential of the photosensitive drum 1 and the development voltage (light portion potential (Vl) of the photosensitive drum 1=-150 (V), development voltage=−350 (V)), it remains on the photosensitive drum 1 without being collected by the developing roller 41 . However, the toner is also electrically supplied to the exposed portion on the photosensitive drum 1 from the developing roller 41 . Therefore, together with the toner supplied from the developing roller 41, the primary transfer residual toner is also transferred again. The development voltage in this embodiment is expressed as a potential difference from the ground potential. Therefore, the development voltage = -350 (V) is interpreted as having a potential difference of -350 (V) with respect to the ground potential (0 (V)) due to the development voltage applied to the core metal of the development roller 41. be done. The same applies to the charging voltage and the transfer voltage, which will be described later.

In this way, the primary transfer residual toner remaining on the photosensitive drum 1 without being transferred to the recording material P is collected by the developing roller 41 in the non-exposed area, and is removed from the photosensitive drum 1 together with the newly developed toner in the exposed area. be transcribed. The toner collected by the developing roller 41 is mixed with the toner in the developing section 4 and reused.

Next, a phenomenon that occurs when there are a plurality of image forming stations using a cleanerless system will be described with reference to FIG. In this embodiment, four image forming stations are arranged in parallel as shown in FIG. Consider the case where Here, the explanation of the phenomenon will use the image forming station a, which is arranged at the most upstream position, and the image forming station b, which is arranged at the downstream position. Image forming stations c and d arranged further downstream also cause the same phenomenon as in b, so the description is omitted.

The yellow toner on the intermediate transfer belt 10 that has been primarily transferred at the image forming station a located at the most upstream passes through the transfer nip portion of the image forming station b, which is a cartridge located downstream. Before passing through, as shown in FIG. 5A, the polarity of the yellow toner on the intermediate transfer belt 10 is partially reversed due to discharge at the transfer nip formed by the photosensitive drum 1b and the primary transfer roller 14b. do. This toner behaves as a retransfer toner. The re-transferred toner is a positive toner whose polarity is reversed, and is transferred onto the photosensitive drum 1b due to the potential difference between the photosensitive drum 1b and the primary transfer roller 14b. In a cleanerless system having no cleaning member, the retransferred toner transferred onto the photosensitive drum 1b directly rushes into the charging roller 2b.

During the image forming operation, the charging voltage applied to the charging roller 2b is of negative polarity and the retransferred toner is of positive polarity. An electric field acts in the direction in which the toner is electrically attracted to the charging roller 2b. However, before the toner rushes into the charging roller 2b, a discharge occurs in the gap between the photosensitive drum 1b and the charging roller 2b, and negative charges are supplied to the toner on the photosensitive drum 1b. As a result, the re-transferred toner changes to have the same polarity as that of the charging roller 2b, so that it passes through the charging roller 2b without electrically adhering to it. However, some of the retransferred toners of positive polarity may not be negatively charged due to discharge. The retransferred toner that is not negatively polarized electrically moves toward the charging roller 2b as shown in FIG. 5(c). As the image formation continues, the positive toner adhering to the charging roller 2b is gradually injected with charge due to the influence of the charging voltage applied to the charging roller 2b, and the positive polarity shifts to the negative polarity. . When the polarity shifts to the negative polarity, it repels the charging voltage applied to the charging roller 2b, so the retransferred toner is transferred onto the photosensitive drum 1b little by little. As a result, as shown in FIG. 5D, the retransferred toner is collected in the developing section 4b at the same time as the image is formed. However, if image formation is continued in a state in which the amount of toner adhering to the charging roller 2 is large, charging inhibition occurs due to the gradual accumulation of the retransferred toner. As a result, the surface of the photosensitive drum 1 cannot be uniformly charged to a desired potential, resulting in image defects due to charging failure. By executing the cleaning operation for cleaning the charging roller 2 before the image damage occurs, the image damage can be suppressed. However, when images are formed continuously, the cleaning operation of the charging roller 2 cannot be performed, which greatly affects the images. This is because if other operations are performed while image formation is being performed continuously, downtime will occur accordingly. Therefore, it is desirable to perform image formation in a state in which toner is not attached to the charging roller 2 and the cleaning operation is not performed as much as possible.

Therefore, in the present embodiment, as shown in FIG. 6, a current is supplied to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10 to suppress the generation of the retransferred toner. FIG. 6 is an enlarged view of the vicinity of the transfer nip of the image forming station b in FIG. As shown in FIG. 6, discharge is generated in the gap upstream of the transfer nip portion between the photosensitive drum 1 and the intermediate transfer belt 10, and most of the toner formed on the intermediate transfer belt 10 is made negative in advance, thereby removing the transfer nip portion. let it pass. Since the voltage applied from the primary transfer voltage power supply 140 is reflected on the intermediate transfer belt 10 by applying a current to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10, the upstream vicinity of the transfer nip portion is reflected. , discharge occurs between the photosensitive drum 1 and the intermediate transfer belt 10 . When the toner image formed in the upstream station is held on the intermediate transfer belt 10 when the discharge occurs, the toner image on the intermediate transfer belt 10 is removed by the discharge before the toner image enters the transfer nip portion. It is charged to the normal polarity side, that is, to the negative polarity. Therefore, it is possible to prevent the toner image on the intermediate transfer belt 10 from being re-transferred onto the photosensitive drum 1 when passing through the downstream transfer nip portion.

3. Adhesion of Toner to Charging Member As described above, in a cleanerless system without a cleaning member, residual transfer toner or retransferred toner on the photosensitive drum 1 comes into contact with the charging roller 2 and may adhere to the charging roller 2 depending on the conditions. do. Adhesion of toner to the charging roller 2 is mainly dominated by electrical adhesion. Therefore, the charge of the toner and the discharge of the charging roller 2 have a great influence. If the charge of the toner adhering to the surface of the photosensitive drum 1 is negative, which is the regular polarity, the amount of toner adhering to the charging roller 2 can be reduced. Even if the charge of the toner is unstable, the toner on the photosensitive drum 1 is negatively charged by the discharge from the charging roller 2 to which the negative voltage is applied to the surface of the photosensitive drum 1. You can pass through the junction. Therefore, the toner remaining on the photosensitive drum 1 can be prevented from adhering to the charging roller 2 by appropriately promoting the discharging of the toner with the charging roller 2 .

The discharge by the charging roller 2 is performed in the gap between the charging roller 2 and the photosensitive drum 1 immediately before the contact portion, as shown in FIG. Then, a negative potential is formed on the photosensitive drum 1 by discharging. Here, if toner adheres to the photosensitive drum 1 as shown in FIG. 3, how the discharge is received changes depending on the speed at which the toner passes through the gap. Specifically, when the peripheral speed of the toner passing through the gap on the photosensitive drum 1 is small, the toner is discharged for a long period of time, making it easy to optimize the polarity of the toner. As a result, the toner discharged by the charging roller 2 tends to be negatively charged. On the other hand, when the peripheral speed of the toner passing through the gap on the photosensitive drum 1 is high, the toner is discharged for a short period of time. Therefore, the toner that has not been negatively charged may adhere to the charging roller 2 as it is due to insufficient discharge. From the above phenomenon, it can be seen that the state of toner adhesion to the charging roller 2 changes depending on the peripheral speed of the photosensitive drum 1 .

Next, the charging contrast that affects toner adhesion to the charging roller 2 will be described. The amount of discharge between the charging roller 2 and the photosensitive drum 1 changes depending on the charging contrast expressed by the potential difference between the charging roller 2 and the photosensitive drum 1 . The greater the charging contrast, which is the difference between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1, the greater the amount of discharge. Considering that the toner is negatively charged by the discharge as described above, the polarity of the toner can be optimized when the charging contrast is large. On the other hand, when the charge contrast is small, the amount of discharge is insufficient, and there are cases where the polarity of the toner cannot be optimized. As a result, the toner cannot be electrically attracted onto the photosensitive drum 1 , which may cause the toner to adhere to the charging roller 2 .

However, if the discharge is repeated while the charging contrast is high, the photosensitive drum 1 may be deteriorated by discharge or scraped, resulting in deterioration of image quality. Therefore, in order to optimize the polarity of the toner, it is necessary to increase the charging contrast, but from the viewpoint of image quality, it is desirable to suppress the amount of discharge as much as possible.

Therefore, in order to suppress toner adhesion to the charging roller 2, it is necessary to control the peripheral speed of the photosensitive drum 1, the charging contrast, which is the potential difference between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1. .

Therefore, in this embodiment, the charging contrast is changed by the peripheral speed of the photosensitive drum 1 . As an example of a mode for changing the peripheral speed of the photosensitive drum 1 during image formation, changing according to the type of the recording material P can be mentioned. The photosensitive drum 1 is rotated by a main motor (not shown), and in this embodiment, the main motor also rotates the intermediate transfer belt 10 . When the toner image transferred on the intermediate transfer belt 10 is transferred onto the recording material P at the secondary transfer portion, the transfer performance differs depending on the recording material P. Therefore, it is necessary to change the rotation speed of the main motor according to the recording material P. There is For example, if the recording material P is thick and difficult to transfer, it is necessary to reduce the peripheral speed of the intermediate transfer belt 10 by means of a rotation speed switching unit (not shown) to change the condition to facilitate transfer. be. At that time, the peripheral speed of the photosensitive drum 1 is also reduced at the same time. Therefore, under the above conditions, the charging contrast is reduced in order to optimize the discharge conditions.

4. Photosensitive Drum Peripheral Speed and Charging Contrast Control The relationship between the peripheral speed of the photosensitive drum 1 during image formation and the charging contrast, which is a feature of the present embodiment, for suppressing the above-described image defects will be specifically described.

The image forming apparatus 100 in the present embodiment controls the exposure amount of the pre-charging exposure device 5 so that the charging contrast decreases as the peripheral speed of the photosensitive drum 1 decreases. As shown in Table 1, the image forming apparatus 100 in the present embodiment has five stages of peripheral speeds of the photosensitive drum 1 of 50, 75, 100, 125, and 150 (mm/sec). When using a recording material P that is difficult to transfer or fix toner, such as thick paper or rough paper, a mode with a low peripheral speed is adopted, and a recording material P that is easy to transfer or fix, such as thin paper or smooth paper, is selected. When used, image formation is executed in a mode with a high peripheral velocity. Then, the surface potential of the photosensitive drum 1 after exposure by the pre-charging exposure device 5 is controlled with respect to the peripheral speed of each photosensitive drum 1, and the charging contrast of the pre-charging exposure device 5 is controlled so that the relationship shown in Table 1 is obtained. Control the amount of light exposure. As described above, the smaller the peripheral speed, the longer the time during which the photosensitive drum 1 is subjected to discharge. On the other hand, the higher the peripheral speed, the shorter the time during which the photosensitive drum 1 receives the discharge, so the charging contrast is increased accordingly. Specifically, in Example 1, the charging contrast was 750, 800, 850, 900, and 950, respectively, with respect to five stages of peripheral speeds of the photosensitive drum 1c of 50, 75, 100, 125, and 150 (mm/sec). (V). FIG. 7 is a schematic diagram showing the charging contrast at 50 (mm/sec), which is the mode with the lowest peripheral velocity, and 150 (mm/sec), which is the mode with the highest peripheral velocity. Regardless of the peripheral speed of the photosensitive drum 1, the charging voltage is set to -1000 (V), and the pre-charging exposure amount is changed to change the charging contrast. At 50 (mm/sec), the surface potential of the photosensitive drum 1 after exposure before charging was set to -250 (V). On the other hand, at 150 (mm/sec), the surface potential of the photosensitive drum 1 after exposure before charging was set to -50 (V).

Next, the details of the operation of this embodiment will be described with reference to the flowchart of FIG.

First, the recording material P specified by the user is fed into the paper feed cassette 7 before image formation (S1). For the recording material P specified by the user in S1, conditions necessary for image formation are transmitted to the control unit 202 based on the information of the recording material P stored in advance in the memory 152 (S2). Specifically, the peripheral speed of the photosensitive drum 1, the charging voltage required for charging contrast formation, and the development voltage required for image formation are set. Thereafter, image formation is performed under conditions suitable for the recording material P sent in S2 (S3). Thereafter, image formation on the recording material P is completed, and the recording material P is discharged outside the image forming apparatus 100 by paper transport (S4).

Next, the effects of this embodiment will be described. In order to confirm the effects of this embodiment, the following tests were conducted to compare the effects of toner contamination and discharge.

First, in order to confirm toner contamination, a test was conducted by the following method. At the image forming station b, a solid black image with vertical stripes is printed at the center of the recording material P in the direction perpendicular to the conveying direction. print. As a result, after the vertical band image printed at the image station b is transferred to the intermediate transfer belt 10, it is retransferred to the photosensitive drum 1c of the image forming station c arranged downstream in the rotation direction of the intermediate transfer belt 10, and charged. It adheres to the roller 2c. Due to this toner adhesion, a toner-adhered portion and a non-toner-adhered portion are formed on the charging roller 2, and a difference in surface potential on the photosensitive drum 1 is generated between the two due to discharge. This difference in surface potential affects image formation, and a difference in density occurs on a full-surface halftone image, resulting in density unevenness. Utilizing the fact that the level of density unevenness worsens as the amount of toner adhering increases, the effect of suppressing toner contamination of the charging roller 2 is confirmed by comparing the level of density unevenness of full-surface halftone images.

Next, in order to confirm the influence of discharge, a test was conducted by the following method. At the image forming station b, an operation of printing an image with a coverage rate of 4 (%) on 100 sheets of continuous printing is repeated 200 times. When the influence of discharge is strong, image defects such as image smearing due to discharge products generated on the surface of the photosensitive drum 1 and streaks on halftone images due to scraping of the surface of the photosensitive drum 1 occur. do. Using this phenomenon, the effects of discharge can be compared.

As the conditions of Example 1, the test was conducted for each of the cases where the peripheral speed of the photosensitive drum 1 during image formation was set to 50, 75, 100, 125 and 150 (mm/sec). Also, the charging contrast was set to 750, 800, 850, 900 and 950 (V) by changing the exposure amount of the pre-charging exposure device 5 for each peripheral speed of the photosensitive drum 1 . Table 2 shows the test results of Example 1. As a comparison with Example 1, another test was conducted under the same conditions without changing the charging contrast according to the peripheral speed of the photosensitive drum 1 as shown in Tables 3 and 4. Table 3 shows Comparative Example 1 in which the charging contrast is relatively small (50 (mm/sec) in Example 1), and Table 4 shows Comparative Example 2 in which the charging contrast is relatively large (150 (mm/sec) in Example 1). (mm/sec) condition).

In the setting of the peripheral speed of the photosensitive drum 1 and the charging contrast in Example 1 shown in Table 2, the occurrence of density unevenness of the entire surface halftone due to the toner contamination of the charging roller 2 and the influence of discharge are suppressed at any peripheral speed of the photosensitive drum 1. can be suppressed. In Example 1, the discharge amount of the toner on the photosensitive drum 1 is appropriately controlled according to the peripheral speed of the photosensitive drum 1. This is because discharge deterioration of the photosensitive drum 1 can be suppressed.

On the other hand, under the conditions of Comparative Example 1 shown in Table 3, when the charging contrast was set to 750 (V) regardless of the peripheral speed of the photosensitive drum 1, the effect of discharge did not appear, but the Density unevenness occurs in the medium to high speed range. Under the conditions of Comparative Example 1, when the peripheral speed of the photosensitive drum 1 is high, the toner on the photosensitive drum 1 cannot be sufficiently charged, and some of the toner adheres to the charging roller 2 . This is because when the peripheral speed of the photosensitive drum 1 increases, the time during which the toner can be discharged by passing the toner through the charging roller 2 once becomes short, so that the discharging to the toner becomes insufficient and the toner adheres to the charging roller 2 . This suggests that it cannot be suppressed.

Therefore, as in Comparative Example 2 shown in Table 4, uniformly increasing the charging contrast regardless of the peripheral speed of the photosensitive drum 1 is considered. When the charging contrast is set to 950 (V) regardless of the peripheral speed of the photosensitive drum 1, density unevenness does not occur regardless of the peripheral speed of the photosensitive drum 1. FIG. This is because the charge contrast is large and the toner is sufficiently discharged. However, as compared with Example 1, the charging contrast in the middle to low peripheral speed range of the photosensitive drum 1 is increased, so that image defects due to the influence of discharge occur especially in the low speed range of the photosensitive drum 1. there is In the low-speed region of the photosensitive drum 1 of Comparative Example 2, the amount of discharge is greater than the amount of discharge required for image formation. It is considered that this excessive discharge has an adverse effect on the photosensitive drum 1 . From the above results, it can be seen that the greater the charge contrast, the greater the effect of discharge, and that the effect appears particularly when the peripheral speed of the photosensitive drum 1 is low. This is because, as described above, when the peripheral speed of the photosensitive drum 1 is low, the photosensitive drum 1 and the charging roller 2 are subjected to discharge for a long length, which makes deterioration more likely.

Therefore, setting the charging contrast in accordance with the peripheral speed of the photosensitive drum 1 as in the present embodiment suppresses image defects caused by toner contamination on the charging roller 2 and suppresses discharge deterioration of the photosensitive drum 1. It is found to be effective for The reason why the contamination of the charging roller 2 can be suppressed even if the charging contrast is reduced as the circumferential speed of the photosensitive drum 1 decreases is that the upstream discharge time per unit area of the photosensitive drum 1 increases as the circumferential speed decreases. It's for. That is, the amount of upstream discharge received per unit area of the photosensitive drum 1 is increased, and toner contamination of the charging roller 2 is suppressed.

Therefore, in the present embodiment, in which a first image forming mode in which an image is formed by rotating the photosensitive drum 1 and a second image forming mode in which the rotation speed is higher than that in the first image forming mode can be executed, the following will be described. to control as The difference between the absolute value of the voltage applied to the charging voltage power supply 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1 exposed by the pre-charging exposure device 5 is the difference between the first image forming mode and the second image forming mode. The charging voltage power source 20 and the pre-charging exposure device 5 are controlled so that the difference between the two is greater. The difference between the absolute value of the voltage applied to the charging voltage power supply 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1 exposed by the pre-charging exposure device 5 indicates the charging contrast. When controlling the pre-charging exposure device 5, the amount of exposure of the surface of the photosensitive drum 1 by the pre-charging exposure device 5 is controlled to be greater in the second image forming mode than in the first image forming mode. do. At this time, if there is a difference in charging contrast, a difference in the amount of discharge will occur. Therefore, the pre-charging exposure device 5 is controlled so that exposure is not performed in the first image forming mode and is performed only in the second image forming mode. may This is because when the pre-charging exposure is not performed, the absolute value of the surface potential of the photosensitive drum 1 becomes larger than when the pre-charging exposure is performed, thereby decreasing the charging contrast and reducing the amount of discharge. Therefore, the amount of discharge can be set larger in the second image forming mode in which pre-charging exposure is performed than in the first image forming mode in which pre-charging exposure is not performed.

In addition, the image forming station a, which is arranged most upstream in the rotation direction of the intermediate transfer belt 10, may be controlled as follows. The difference between the absolute value of the voltage applied to the charging voltage power source 20 and the absolute value of the surface potential of the surface of the photosensitive drum 1a exposed by the pre-charging exposure device 5a of the image forming station a arranged in the most upstream is defined as the second It is assumed that the first image forming mode and the second image forming mode are the same. Since no image forming station is arranged upstream of the image forming station a, which is arranged at the most upstream position, toner of other colors does not pass through the photosensitive drum 1a. That is, no retransfer toner is generated. Therefore, the charging roller 2a is less soiled than the other image forming stations b, c, and d. Therefore, it is not necessary to carry out control for suppressing contamination of the charging roller 2, which is carried out in the other image forming stations b, c, and d. Therefore, the image forming station a may be controlled according to the circumferential speed of the photosensitive drum 1 without controlling the charging contrast, or the change in the charging contrast due to the change in the circumferential speed may be controlled by another image forming station b. , c, and d.

As described above, by changing the charging contrast according to the peripheral speed of the photosensitive drum 1 and increasing the charging contrast as the peripheral speed increases, the toner contamination of the charging roller 2 is suppressed and the deterioration of the photosensitive drum due to discharge is suppressed. It is possible to suppress the deterioration of image quality associated with this.

Further, in the present embodiment, the charging contrast is controlled to a predetermined set value by changing the exposure amount of the photosensitive drum 1 after the primary transfer by the pre-charging exposure device 5, but the voltage applied to the charging roller 2 is By changing, the charging contrast may be controlled to a predetermined design value. Specifically, the voltage applied to the charging roller 2 by the charging voltage power source 20 is controlled so that the absolute value thereof is larger when the second image forming mode is performed than when the first image forming mode is performed. However, when the voltage applied to the charging roller 2 is changed, the surface potential of the photosensitive drum 1 after charging also changes, so the potential relationship between the photosensitive drum 1 and the developing roller 41 also changes, and the toner development characteristics change. put away. Therefore, when the charging contrast is controlled by changing the voltage applied to the charging roller 2, the surface potential of the photosensitive drum 1 when it reaches the developing roller 41 is constant regardless of the voltage applied to the charging roller 2. It is preferable to control For example, there is background control in which the surface potential of the photosensitive drum 1 is kept constant by performing non-image portion exposure of the non-image forming area by the exposure device 3 . The non-image portion exposure amount and the image portion exposure amount, which are the exposure amounts of the image forming area and the non-image forming area by the exposure device 3, are respectively controlled according to the magnitude of the surface potential of the photosensitive drum 1 after charging. Thereby, the surface potential of the photosensitive drum 1 when it reaches the developing roller 41 can be made constant regardless of the magnitude of the voltage applied to the charging roller 2 .

Also, the development voltage may be adjusted together with the adjustment of the background exposure. In that case, the exposure amount of the image portion to the image forming area of the exposure device 3 may be adjusted in accordance with the change in the developing voltage.

Alternatively, the transfer voltage may be changed as means for changing the charging contrast. By changing the transfer voltage, the potential of the surface of the photosensitive drum 1 discharged by the transfer voltage can be changed.

Moreover, although the image forming apparatus 100 having a plurality of image forming stations is employed in the present embodiment, it may be applied to an image forming apparatus having a single image forming station.

Further, although the present embodiment employs the configuration of the intermediate transfer member using a belt, the recording material P is conveyed between the photosensitive drum 1 and the primary transfer roller 14, and the toner image is transferred onto the recording material P at the transfer nip portion. A direct transfer method may be adopted.

In the first embodiment, when the peripheral speed of the photosensitive drum 1 is V,
50 (mm/sec) ≤ V ≤ 150 (mm/sec) Expression (1)
In this case, the charging contrast is controlled to suppress image deterioration due to toner adhesion to the charging roller 2 and the influence of discharge. In a second embodiment, a method of appropriately charging the toner and discharging the photosensitive drum 1 when the peripheral speed of the photosensitive drum 1 is set to be smaller than the range of formula (1) will be described. Specifically, the case of a peripheral velocity smaller than 50 (mm/sec) will be described.

First, in the peripheral speed range of the photosensitive drum 1 of Example 1, if the charging contrast is Δ, the charging contrast range is
750 (V) ≤ Δ ≤ 900 (V) Expression (2)
and Here, conditions under which discharge occurs between the charging roller 2 and the photosensitive drum 1 will be described. FIG. 9 is a graph showing the results of measuring the relationship between the charging voltage applied to the charging roller 2 and the surface potential of the photosensitive drum 1 under an environment of 23° C. temperature and 50% humidity. When the applied charging voltage is small, the surface potential on the photosensitive drum 1 does not increase at all, but the surface potential starts increasing from a certain voltage value. This is the discharge start voltage Vth. In this embodiment, −550 (V) is the discharge starting voltage Vth. The discharge start voltage Vth is determined from the gap between the charging roller 2 and the photosensitive drum 1 , the layer thickness of the photosensitive drum 1 , and the layer relative permittivity of the photosensitive drum 1 . When a charging voltage equal to or higher than the discharge start voltage Vth is applied to the charging roller 2, the photosensitive drum 1 is charged due to a discharge phenomenon based on Paschen's law. Therefore, when the charging voltage is Vpri, the range in which the discharge occurs between the charging roller 2 and the photosensitive drum 1 is expressed by the following equation (3).
|Vpri|>|Vth| Expression (3)

As shown in FIG. 9, when the absolute value of the charging voltage Vpri applied to the charging roller 2 is gradually increased from −400 (V), the amount of increase in the charging voltage Vpri is shown by a straight line with a slope of 1 starting from the discharge start voltage Vth. Accordingly, the dark potential Vd increases. Therefore, in the range of the charging voltage Vpri of formula (3), the dark area potential Vd formed on the photosensitive drum 1 is
|Vd|=|Vpri|-|Vth| Expression (4)
is represented by Since the dark potential Vd is 0 (V) when the discharge start voltage Vth is applied, discharge does not occur in the range of less than the charging contrast Δ=550 (V). Therefore, the range of charging contrast Δ necessary for image formation required under the conditions of Example 2 is expressed by the following formula.
550 (V) ≤ Δ ≤ 750 (V) Expression (5)

When the peripheral speed of the photosensitive drum 1 is set to 50 (mm/sec) or less, the range of formula (5) may be appropriately selected. Preferably, if the range of expression (5) is set such that the peripheral speed of the photosensitive drum 1 changes linearly from 50 (mm/sec) to 0 (mm/sec), the appropriate toner charging and the photosensitive drum can be suppressed, and image deterioration can be suppressed.

In the second embodiment, the range in which the peripheral speed of the photosensitive drum 1 is low has been described.

As in Example 3, when the peripheral speed is greater than the range of the peripheral speed of the photosensitive drum 1 of the formula (1), which is the condition of the first example, the charging contrast becomes even greater than the range of the formula (2). Otherwise, it becomes difficult to charge the toner. However, increasing the charging contrast causes deterioration due to discharge of the photosensitive drum 1 as described above. Accordingly, the third embodiment proposes a configuration in which the amount of toner re-transferred to the image forming station arranged downstream of the rotation of the intermediate transfer belt 10 is reduced without changing the charging contrast. This configuration is effective regardless of the range of the peripheral speed of the photosensitive drum 1, and by reducing the amount of retransferred toner, the amount of discharge necessary for charging the toner can be reduced. In particular, this is effective in a range where the peripheral speed of the photosensitive drum 1 is high, in which the charging contrast cannot be increased.

In Example 3, as shown in FIG. 10, the primary transfer roller 14 is arranged not in the vicinity of the transfer nip portion with the photosensitive drum 1 but at a position where it does not directly press the photosensitive drum 1 . At the time of primary transfer, a DC voltage of 1000 (V) is applied to the primary transfer roller 14 from the primary transfer voltage power supply 140, and a current required for primary transfer is supplied to the transfer nip portion via the circumferential direction of the intermediate transfer belt 10. supply. With this configuration, a voltage corresponding to the voltage applied from the primary transfer voltage power supply 140 is also applied to the intermediate transfer belt 10 in the vicinity of the gap formed between the primary transfer roller 14 and the photosensitive drum 1 . be. Therefore, as shown in FIG. 11, in the third embodiment, discharge is positively generated between the photosensitive drum 1 and the intermediate transfer belt 10 near the upstream of the transfer nip portion (hereinafter referred to as upstream discharge of the transfer nip portion). do. When a toner image exists at a position where an upstream discharge occurs in the transfer nip portion, negative charges are supplied from the photosensitive drum 1 to the toner image on the intermediate transfer belt 10 by this discharge. The toner image on 10 is charged to the normal polarity side. Then, the reverse polarity toner amount of the toner image on the intermediate transfer belt 10 is reduced by being charged to the normal polarity side. As a result, when the toner image on the intermediate transfer belt 10 passes through the transfer nip portion of the image forming station downstream of the rotation, retransfer of the toner from the toner image on the intermediate transfer belt 10 to the photosensitive drum 1 is suppressed. You will be able to

Since other configurations are the same as those of the first embodiment, description thereof is omitted.

In the configuration of Example 3, the same effects as in Example 1 were confirmed. The difference between the third embodiment and the first embodiment is only the difference in the peripheral configuration of the intermediate transfer belt 10 in the above-described image forming apparatus 100 and the set charging contrast value. As shown in Table 5, in Example 3, the charging contrast is set to be 100 (V) smaller than that in Example 1 with respect to each peripheral speed of the photosensitive drum 1 .

From the results of confirming the effects in Table 5, it is clear that in Example 3, as in Example 1, image defects due to toner contamination on the charging roller 2 are suppressed, and image quality due to deterioration of the photosensitive drum due to the pre-charging exposure device 5 is suppressed. It can be seen that deterioration can be suppressed. Further, it can be seen that in Example 3, even if the charging contrast is made even smaller than in Example 1, density unevenness can be suppressed. In particular, the charging contrast conditions when the peripheral speed of the photosensitive drum 1 was 100 (mm/sec) were the same as those in Comparative Example 1, and in Comparative Example 1, problems occurred under those conditions, whereas in Example 1 In 3, it can be seen that the problem can be solved even under the conditions. Further, in Example 1, when the charging contrast was 900 (V), image defects did not occur until the peripheral speed of the photosensitive drum 1 reached 125 (mm/sec). ) did not occur. This is because the retransfer of the toner from the intermediate transfer belt 10 to the photosensitive drum 1 can be further suppressed in the third embodiment. Specifically, the primary transfer roller 14 is not arranged in the vicinity of the transfer nip portion with the photosensitive drum 1, but is arranged at a position where it does not directly press the photosensitive drum 1, so that upstream discharge at the transfer nip portion is likely to occur. , and the amount of retransferred toner has decreased. This is because the upstream discharge time per unit area of the intermediate transfer belt 10 is extended, and the upstream discharge amount per unit area of the intermediate transfer belt 10 is increased. As a result, in the third embodiment, the toner on the intermediate transfer belt 10 is charged more to the normal polarity side than in the first embodiment, and retransfer of the toner is suppressed. From the above, it can be said that Example 3 is a particularly suitable configuration when the peripheral speed of the photosensitive drum 1 is greater than 150 (mm/sec).

As described above, by supplying current to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10, the toner on the charging roller 2 can be transferred even if the charging contrast is made smaller than in the first embodiment. It is possible to suppress the occurrence of image defects due to contamination.

Further, in the present embodiment, the primary transfer roller 14 is arranged at a position that does not directly press the photosensitive drum 1 , but the primary transfer roller 14 is moved from another contact member that contacts the intermediate transfer belt 10 to the transfer nip portion via the intermediate transfer belt 10 . and current may be supplied.

Further, in the third embodiment, the electric current is supplied from the primary transfer roller 14 to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10. It would be nice if the time could be extended. This is because the discharge time generated between the intermediate transfer belt 10 and the photosensitive drum 1 contributes to the retransfer toner amount. .

In Example 4, an intermediate transfer belt 10 having a lower resistance than that in Example 1 was used, in which the surface resistivity of the outer peripheral surface when measured with an applied voltage of 100 (V) was 2×10 ¹⁰ (Ω/□). . Here, by using the intermediate transfer belt 10 having a surface resistance lower than that of the first embodiment, the electric current is not supplied to the transfer nip portion through the circumferential direction of the intermediate transfer belt 10 as in the third embodiment. This is also because upstream discharge occurs in the transfer nip portion. In addition, since the resistance is low, the amount of current that flows, that is, the amount of charge to the toner can be increased. The surface resistivity is not limited to the surface resistivity of the outer peripheral surface, and the same effect can be obtained by using the intermediate transfer belt 10 having a surface resistivity of 2×10 ¹⁰ (Ω/□) on the inner peripheral surface. be done.

Specifically, as shown in FIG. 12, by using the intermediate transfer belt 10 through which current easily flows in the circumferential direction, the positive voltage applied from the secondary transfer voltage power supply 150 to the secondary transfer roller 15 causes the intermediate transfer belt 10 to move. It is transmitted and reflected in the vicinity of each primary transcription upstream. At this time, the transfer current flows as indicated by the black line in FIG. The image forming station a closest to the secondary transfer roller 15 receives the most transfer current. On the other hand, the farthest image forming station d has a structure in which the transfer current is least likely to flow. For example, a voltage may be applied to the tension member 11 in a superimposed manner so that the transfer current does not vary from image station to image station. As described above, the discharge is generated upstream of the transfer nip portion similarly to the third embodiment, and the effect of suppressing the retransfer can be obtained.

Since other configurations are the same as those of the first embodiment, description thereof is omitted.

In the configuration of Example 4, the same effects as in Examples 1 and 3 were confirmed. The difference between the fourth embodiment, the first embodiment, and the third embodiment is only the difference in the peripheral configuration of the intermediate transfer belt 10 in the above-described image forming apparatus 100 and the set charging contrast value. As shown in Table 6, in the fourth embodiment, the charging contrast is set 150 (V) smaller than in the first embodiment for each peripheral speed of the photosensitive drum 1 .

As described in the third embodiment, in the configuration in which the upstream discharge is generated at the transfer nip portion, retransfer of toner from the intermediate transfer belt 10 to the photosensitive drum 1 is suppressed by the upstream discharge at the transfer nip portion. As a result, in the fourth embodiment, the toner on the intermediate transfer belt 10 is charged more to the normal polarity side than in the first embodiment, and retransfer of the toner is suppressed. Furthermore, as compared with Example 3, the amount of discharge upstream of the transfer nip portion was increased, so that normal charging of the reversed polarity toner could be promoted. As a result, the amount of re-transferred toner is reduced, and the charging contrast can be reduced as a whole. In particular, the charging contrast conditions when the peripheral speed of the photosensitive drum 1 was 125 (mm/sec) were the same as those in Comparative Example 1, and in Comparative Example 1, problems occurred under those conditions, whereas in Example 1, there was a problem. In 4, it can be seen that the problem can be solved even under the conditions. In Example 1, when the charging contrast was 950 (V), the conditions were suitable for the peripheral speed of the photosensitive drum 1 of 150 (mm/sec), but in Example 4, the condition was 225 (mm/sec). , the peripheral speed of the photosensitive drum 1 is high. From the above, it can be said that the configuration is more suitable when the peripheral speed is greater than 150 (mm/sec).

As described above, by using the intermediate transfer belt 10 in which the current flows in the circumferential direction to some extent, even if the charging contrast is made smaller than that in the first embodiment, image defects due to toner contamination of the charging roller 2 occur. can be suppressed.

Further, in this embodiment, the intermediate transfer belt 10 is used to transfer the toner image onto the intermediate transfer belt 10, and then the toner image is transferred onto the recording material P. However, the recording material P is transported on the transport belt, A method of directly transferring the toner image onto the recording material P may be employed.

REFERENCE SIGNS LIST 1 photosensitive drum 2 charging roller 3 exposure device 4 developing section 5 pre-charging exposure device 10 intermediate transfer belt 14 primary transfer roller

Claims (12)

An image forming apparatus for forming a toner image on a recording material,
a rotatable image carrier; a charging member that forms an abutment portion in contact with the image carrier and charges the image carrier at the abutment; and the image charged by the charging member. An image having a first exposure means for exposing a carrier to form an electrostatic latent image, and a developing member for supplying toner to the electrostatic latent image and developing the toner image on the image carrier. a forming unit;
a voltage applying means for applying a voltage to the charging member;
an intermediate transfer member that forms a transfer portion in contact with the image carrier and carries the toner image formed on the image carrier at the transfer portion;
a transfer member for transferring the toner image carried on the intermediate transfer member onto a recording material;
second exposing means for exposing the image carrier downstream of the transfer section and upstream of the contact section in the rotation direction of the image carrier;
a control unit that controls the voltage application unit and the second exposure unit;
In an image forming apparatus having
A first image forming mode in which an image is formed by rotating the image carrier, and a second image forming mode in which an image is formed by rotating the image carrier at a rotation speed higher than that in the first image forming mode. is executable and
With the surface of the image carrier exposed by the second exposure means , the control unit controls the absolute value of the voltage to be applied to the voltage application means and the image carrier exposed by the second exposure means. The voltage applying means and the second exposure are arranged such that the difference between the absolute value of the surface potential of the body and the second exposure is larger when the second image forming mode is performed than when the first image forming mode is performed. An image forming apparatus characterized by controlling: The control unit performs control such that the amount of exposure of the image bearing member by the second exposure means is greater during execution of the second image forming mode than during execution of the first image forming mode. 2. The image forming apparatus according to claim 1, wherein: 2. The control unit performs control so that the exposure by the second exposure means is not performed when the first image forming mode is performed, but is performed when the second image forming mode is performed. 3. The image forming apparatus according to 2 above. The first exposure means exposes the image carrier charged by the charging member to form the surface potential that does not form the toner image, and exposes the image carrier to form the toner image. an image area exposure that exposes with an exposure amount larger than the exposure amount of the non-image area exposure in order to form a surface potential;
The control unit controls the exposure amount of the non-image portion exposure so as to form the surface potential of the image carrier so as not to form the toner image during image formation. 4. The image forming apparatus according to claim 1, wherein the exposure means is controlled. The controller controls the absolute value of the voltage to be applied to the voltage applying means of the image forming section arranged most upstream in the rotation direction of the intermediate transfer member, and the voltage of the image forming section arranged most upstream. executing control to make the difference between the absolute value of the surface potential of the image carrier exposed by the second exposure unit and the same between the first image forming mode and the second image forming mode; 5. The image forming apparatus according to claim 1, wherein: An image forming apparatus for forming a toner image on a recording material,
a rotatable image carrier;
a charging member that forms an abutment portion in contact with the image carrier and charges the image carrier at the abutment portion;
a first exposure unit that exposes the image carrier charged by the charging member to form an electrostatic latent image;
a developing member that supplies toner to the electrostatic latent image to develop the toner image on the image carrier;
a voltage applying means for applying a voltage to the charging member;
a transfer member that transfers the toner image developed on the image carrier onto a recording material at a transfer portion;
second exposing means for exposing the image carrier downstream of the transfer section and upstream of the contact section in the rotation direction of the image carrier;
a control unit that controls the voltage application unit and the second exposure unit;
In an image forming apparatus having
A first image forming mode in which an image is formed by rotating the image carrier, and a second image forming mode in which an image is formed by rotating the image carrier at a rotation speed higher than that in the first image forming mode. is executable and
With the surface of the image carrier exposed by the second exposure means , the control unit controls the absolute value of the voltage to be applied to the voltage application means and the image carrier exposed by the second exposure means. The voltage applying means and the second exposure are arranged such that the difference between the absolute value of the surface potential of the body and the second exposure is larger when the second image forming mode is performed than when the first image forming mode is performed. An image forming apparatus characterized by controlling: The control unit performs control such that the amount of exposure of the image bearing member by the second exposure means is greater during execution of the second image forming mode than during execution of the first image forming mode. 7. The image forming apparatus according to claim 6 , wherein: 6. The control unit performs control so that the exposure by the second exposure means is not performed when the first image forming mode is performed, but is performed when the second image forming mode is performed. 8. The image forming apparatus according to 7 above. The first exposure means exposes the image carrier charged by the charging member to form the surface potential that does not form the toner image, and exposes the image carrier to form the toner image. an image area exposure that exposes with an exposure amount larger than the exposure amount of the non-image area exposure in order to form a surface potential;
The control unit controls the exposure amount of the non-image portion exposure so as to form the surface potential of the image carrier so as not to form the toner image during image formation. 9. The image forming apparatus according to claim 6 , wherein the exposure means is controlled. The control section controls the voltage applied to the charging member by the voltage applying means so that the absolute value of the voltage is larger when the second image forming mode is performed than when the first image forming mode is performed. The image forming apparatus according to any one of claims 1 to 9 , characterized by: When either the first image forming mode or the second image forming mode is executed, residual toner remaining on the image carrier without being used for image formation is recovered by the developing member. The image forming apparatus according to any one of claims 1 to 10 . The image forming apparatus according to any one of claims 1 to 11 , wherein the toner is a one-component developer.

Images