JP2023096348A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that irradiates a photoconductor drum with light to eliminate static electricity, and can eliminate static electricity on the photoconductor drum without exposing the photoconductor drum more than necessary.SOLUTION: A control unit forms a test toner image to detect an electrification current of a toner part (I1) and an electrification current of a non-toner part (I2) (S102). The electrification current of the toner part is an electrification current when a contact area on a photoconductor drum brought into contact with the test toner image carried on an intermediate transfer belt passes through an electrifying unit, and the electrification current of the non-toner part is an electrification current when an area in non-contact with the test toner image passes through the electrifying unit. The control unit calculates a current ratio "(I1-I2)/I2" (S103), and when the absolute value of the current ratio is larger than a threshold (Yes in S104), performs settings to change the amount of pre-exposure of a pre-exposure device to increase the amount of pre-exposure of the pre-exposure device compared to that before the change to be "|(I1-I2)/I2|≤threshold" (S105). This can prevent the occurrence of a ghost image caused by transfer memory.SELECTED DRAWING: Figure 9

Description

The present invention relates to an image forming apparatus using electrophotographic technology, such as a printer, copier, facsimile machine, or multi-function machine.

As an electrophotographic image forming apparatus, there is an apparatus that sequentially superimposes and transfers toner images of respective colors from one or a plurality of photosensitive drums onto an intermediate transfer member to form an image. As an example, there is a tandem image forming apparatus in which a plurality of photosensitive drums are arranged along the rotation direction of an intermediate transfer belt. In the case of this image forming apparatus, the toner image transferred from the photosensitive drum on the upstream side in the rotational direction of the intermediate transfer belt, carried on the intermediate transfer belt, and conveyed downstream is brought into contact with the photosensitive drum on the downstream side and the intermediate transfer belt. When passing through the transfer section formed in contact with the photosensitive drum, it contacts the photosensitive drum.

On the downstream side of the photosensitive drum, the contact area that is in contact with the toner image on the intermediate transfer belt tends to retain charge (so-called transfer memory). can have a potential difference. If the potential difference on the photosensitive drum is large, an unexpected density difference may occur in, for example, a halftone image formed on the recording material (so-called ghost image).

In order to reduce the potential difference on the photosensitive drum before charging, a pre-exposure device is used that removes the charge from the photosensitive drum by irradiating the photosensitive drum with light. Conventionally, the charging current flowing in the above-mentioned contact area during charging of the photosensitive drum on the downstream side is detected, and based on the detected charging current, that is, based on the surface potential of the contact area, a pre-exposure device irradiates the photosensitive drum. It has been proposed to set the amount of light (pre-exposure amount) to be applied (Patent Document 1).

JP 2016-218155 A

However, with the device described in Japanese Patent Application Laid-Open No. 2002-200010, it is sometimes difficult to appropriately neutralize the photosensitive drum. That is, conventionally, the pre-exposure amount is set based on the surface potential of the contact area that contacts the toner image on the intermediate transfer belt. Therefore, when there is a large potential difference between the surface potential of the contact area on the photosensitive drum and the surface potential of the non-contact area that is not in contact with the toner image, the potential difference may not be eliminated even if the pre-exposure amount is adjusted. rice field. In this respect, it is conceivable to simply increase the pre-exposure amount regardless of the surface potential of the contact area. There is concern that the life of the photosensitive drum will be shortened.

According to the present invention, even when the potential difference between the surface potential of the contact area and the surface potential of the non-contact area on the photosensitive drum increases due to transfer memory, the photosensitive drum can be operated without increasing the pre-exposure amount more than necessary. An object of the present invention is to provide an image forming apparatus capable of removing static electricity.

An image forming apparatus according to an embodiment of the present invention is an image forming apparatus that forms a toner image on a recording material, and includes an intermediate transfer member that carries a test toner image and rotates, and an intermediate transfer member that corresponds to the intermediate transfer member. a photoreceptor that rotates in contact with the photoreceptor; a transfer rotator that forms a transfer portion of a toner image from the photoreceptor to the intermediate transfer member; a charging member that charges the photoreceptor at a charging position; an exposure charge removing means for removing charges by exposing the photosensitive member downstream from the transfer portion and upstream from the charging position; a power supply for applying voltage to the charging member; and a current detecting means for detecting current flowing through the charging member. Then, the test toner image carried on the intermediate transfer member and conveyed to the transfer portion and the contact area on the photoreceptor, which is in contact with the charging member while a voltage is applied, shifts the charging position. Assuming that the current detected when passing is I1, and the current detected when the non-contact area on the photoreceptor that is not in contact with the test toner image passes the charging position is I2, ( and setting means capable of executing a setting mode for setting the exposure amount of the exposure static elimination means so that the absolute value of I1-I2)/I2 is equal to or less than a threshold.

According to the present invention, even if the potential difference between the surface potential of the contact area on the photosensitive drum and the surface potential of the non-contact area on the photosensitive drum is large in the configuration in which the photoreceptor is irradiated with light to remove static electricity, pre-exposure is performed more than necessary. The photosensitive drum can be neutralized without increasing the amount.

1 is a schematic diagram showing an image forming apparatus according to an embodiment; FIG. FIG. 2 is a schematic diagram showing an image forming section; (a) A diagram for explaining a case where a ghost image does not occur, (b) a diagram for explaining a case where a ghost image occurs. (a) A diagram showing an example of a test toner image, (b) a diagram showing a ghost image generated in a halftone image. 5 is a graph showing the relationship between the amount of toner applied on a photosensitive drum and the charging current; 4 is a graph showing the relationship between pre-exposure amount and charging current; Graph for explaining the conditions under which ghost images occur. FIG. 3 is a control block diagram for explaining a control unit; 4 is a flowchart showing pre-exposure amount setting processing.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

<Image forming apparatus>
FIG. 1 is a schematic diagram showing an image forming apparatus 100 of this embodiment. The image forming apparatus 100 of this embodiment is a tandem-type laser beam printer that employs an intermediate transfer method and is capable of forming a full-color image using an electrophotographic method.

Image forming apparatus 100 has first, second, third, and fourth image forming units PY, PM, PC, and PK as a plurality of image forming units (stations). These image forming units PY, PM, PC, and PK are linearly arranged in this order along the rotating direction (moving direction) of an intermediate transfer belt 10 as an intermediate transfer member. Images of magenta (M), cyan (C), and black (K) are formed.

Note that elements having similar functions and configurations provided corresponding to the respective image forming units PY, PM, PC, and PK are shown to be elements for one of the colors unless particularly distinguished. The elements will be described comprehensively, omitting Y, M, C, and K at the end of the symbols. In addition, the elements for each color may be distinguished by adding Y, M, C, and K to the beginning of the word. Further, in this specification, each of the image forming units PY, PM, PC, and PK, which are arranged relatively upstream or downstream in the rotation direction of the intermediate transfer belt 10, is simply referred to as "upstream." They are sometimes called "downstream side" and "downstream side" to distinguish between them.

FIG. 2 is a schematic diagram showing one image forming section P in more detail as a representative. The image forming section P has a rotatable drum-shaped (cylindrical) photosensitive drum 1 . Around the photosensitive drum 1, a charging roller 2, an exposure device 3 (laser scanner), a developing device 4, a primary transfer roller 5 as a transfer rotary member, a pre-exposure device 6, and a cleaning device 7 are arranged.

A photosensitive drum 1 as a photosensitive member is an organic photosensitive drum in which an organic photosensitive layer and a surface protective layer are sequentially laminated on a conductive support. The surface protective layer contains fluororesin fine particles. The photosensitive drum 1 of the present embodiment uses aluminum with a thickness of 1 mm as a conductive support, and has an outer diameter of 30 mm by laminating a photosensitive layer and a surface protective layer thereon. Further, the photosensitive drum 1 is rotationally driven at a predetermined peripheral speed in the direction of arrow R1 by receiving the driving force of a motor (not shown).

A charging roller 2 as a charging member is arranged in contact with the photosensitive drum 1 . In the charging roller 2, a conductive cored bar 21 serving as a shaft portion is used as a base, and an elastic layer 22 is provided thereon. Metal materials such as iron, copper, stainless steel, and aluminum can be used for the conductive cored bar 21, and aluminum is used in this embodiment. The conductive core metal 21 may be plated for rust prevention and scratch resistance to the extent that the conductivity is not lost. The elastic layer 22 is thick in the longitudinal direction (rotational axis direction of the photosensitive drum 1) and thin at both ends in the longitudinal direction, in consideration of bending when the photosensitive drum 1 is pressed. Polished to shape. This is because both ends of the charging roller 2 in the longitudinal direction are configured to receive a predetermined pressing force toward the photosensitive drum 1 by the pressing mechanism. In other words, the contact pressure on the photosensitive drum 1 at the central portion in the longitudinal direction of the charging roller 2 tends to be smaller than at both ends, and this is to be suppressed. The elastic layer 22 is formed by dispersing carbon black, which is a conductive agent ^, in rubber (EPDM (ethylene-propylene-diene rubber)), which is an elastic material. Tuned and conductive.

As the conductive agent, an electronically conductive agent such as graphite or a conductive metal oxide, or an ionic conductive agent such as an alkali metal salt may be used. As the elastic material, synthetic rubber such as natural rubber, SBR, silicon rubber, urethane rubber, epichlorohydrin rubber, IR, BR, NBR and CR, polyamide resin, polyurethane resin and silicon resin may be used. The charging roller 2 uses, for example, a conductive core 21 having a diameter of 8 mm, and the elastic layer 22 is adjusted to have a volume resistivity of 1×10 ⁶ Ωcm by adding a conductive agent. is 14 mm. Further, the charging roller 2 is pressed toward the photosensitive drum 1 so as to have a predetermined contact pressure. The charging roller 2 rotates following the rotation of the photosensitive drum 1 . The charging roller 2 may be driven using a motor or gear (not shown).

A charging power source D3 is connected to the conductive core metal 21 of the charging roller 2 . In this embodiment, an ammeter 23 is connected between the charging power source D3 and the charging roller 2 to detect the current flowing through the charging roller 2 when the charging power source D3 applies a voltage to the charging roller 2 . The charging power source D3 applies an oscillating voltage obtained by superimposing a DC voltage and an AC voltage to the charging roller 2 as a charging voltage. In this embodiment, the charging voltage is an oscillating voltage in which a DC voltage of -600V and an AC voltage with a peak-to-peak voltage of 1700V are superimposed. The ammeter 23 as current detection means is capable of detecting a DC current component (hereinafter also simply referred to as "charging current") by time resolution (that is, capable of detecting a DC current component with respect to the time axis). . Here, it is desirable that the time resolution be at least 5 msec, particularly 1 msec or less.

The pre-exposure device 6 is an exposure charge removing unit (light charge remover) that irradiates the photosensitive drum 1 with light to remove at least part of the charge on the photosensitive drum 1 before charging. Although not shown, the pre-exposure device 6 includes a light guide, which is a light guide member arranged to face the photosensitive drum 1 along the longitudinal direction of the photosensitive drum 1, and a light guide member arranged at an end of the light guide in the longitudinal direction. and an LED lamp as a light source. The pre-exposure device 6 emits light from an LED lamp and irradiates the photosensitive drum 1 with the light reflected on the side surface of the light guide for exposure. As a result, at least part of the surface potential of the photosensitive drum 1 is removed. For the light guide, a resin (acrylic, polycarbonate, polystyrene, etc.) or glass having excellent light transmittance is used. One LED lamp may be provided at a position facing one end face of the light guide in the longitudinal direction. may be provided. The exposure amount of the photosensitive drum 1 by the pre-exposure device 6 can be changed. Here, the amount of exposure of the photosensitive drum 1 by the pre-exposure device 6 is the amount of light per unit time with which the surface of the photosensitive drum 1 (per unit area) is irradiated.

Returning to FIG. 1, the image forming apparatus 100 has an intermediate transfer belt 10 configured as an endless belt so as to face the photosensitive drums 1Y to 1K of all the image forming portions PY to PK. The intermediate transfer belt 10 is an example of a movable transfer member onto which toner images are transferred from the photosensitive drums 1Y-1K. The intermediate transfer belt 10 is stretched around a drive roller 11 , a tension roller 12 and a secondary transfer counter roller 13 . The intermediate transfer belt 10 is made of a polyimide resin and contains an appropriate amount of an antistatic agent such as carbon black to adjust the electric resistance. The intermediate transfer belt 10 is circulated (rotated) at a predetermined peripheral speed in the direction of the arrow R2 by the drive being transmitted to the drive roller 11 .

On the inner circumferential surface side of the intermediate transfer belt 10, the primary transfer rollers 5Y, 5M, 5C and 5K are arranged corresponding to the respective photosensitive drums 1Y, 1M, 1C and 1K. The primary transfer roller 5 is an example of supply means for supplying a transfer current for transfer between the photosensitive drum and the transfer body. The primary transfer rollers 5Y to 5K are urged (pressed) toward the photosensitive drums 1Y to 1K via the intermediate transfer belt 10. As shown in FIG. As a result, the primary transfer rollers 5Y to 5K sandwich the intermediate transfer belt 10 between the photosensitive drums 1Y to 1K, and as shown in FIG. A primary transfer portion T1 (primary transfer nip portion) is formed. A primary transfer power source D1 is connected to the primary transfer roller 5 . A primary transfer power source D1 applies a DC voltage having a polarity opposite to the charging polarity of the toner during development (regular charging polarity) as a primary transfer voltage to the primary transfer roller 5 . In the present embodiment, a constant voltage method is adopted in which the primary transfer voltage is controlled to a constant voltage during primary transfer.

Further, as shown in FIG. 1, a secondary transfer roller 14 as a secondary transfer member is arranged on the outer peripheral surface side of the intermediate transfer belt 10 at a position facing the secondary transfer facing roller 13 . The secondary transfer roller 14 is urged toward the secondary transfer opposing roller 13 via the intermediate transfer belt 10 . As a result, the secondary transfer roller 14 sandwiches the intermediate transfer belt 10 between itself and the secondary transfer opposing roller 13, and the secondary transfer portion, which is the contact portion between the intermediate transfer belt 10 and the secondary transfer roller 14, is formed. T2 (secondary transfer nip portion) is formed. A secondary transfer power source D2 is connected to the secondary transfer roller 14 . The secondary transfer power supply D2 applies a DC voltage having a polarity opposite to the normal charging polarity of the toner as a secondary transfer voltage to the secondary transfer roller 14 .

A belt cleaning device 19 is arranged at a position facing the tension roller 12 on the outer peripheral surface side of the intermediate transfer belt 10 .

During image formation, the surface of the rotating photosensitive drum 1 is substantially uniformly charged to a predetermined potential with a predetermined polarity (negative polarity in this embodiment) by the charging roller 2 to which a charging voltage is applied. As shown in FIG. 2, in the rotational direction of the photosensitive drum 1, the position where the charging roller 2 charges the photosensitive drum 1 is the charging position (charging portion a). In addition, in the rotation direction of the photosensitive drum 1 , minute gaps are formed between the photosensitive drum 1 and the charging roller 2 on the upstream side and the downstream side of the contact portion between the photosensitive drum 1 and the charging roller 2 . The charging roller 2 charges the surface of the photosensitive drum 1 by discharging generated in at least one of the gaps on the upstream side and the downstream side. However, here, for the sake of convenience, the contact portion between the photosensitive drum 1 and the charging roller 2 may be assumed to be the charging position (charging portion a).

The charged surface of the photosensitive drum 1 is scanned and exposed with laser light L based on image information by the exposure device 3 to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive drum 1 is developed into a toner image by the developing device 4 using toner as a developer. The developing device 4 has a developing roller 41 that carries toner and conveys it to a portion facing the photosensitive drum 1 . During development, an oscillating voltage in which a DC voltage and an AC voltage are superimposed is applied to the developing roller 41 as a developing voltage from the developing power source D4. In this embodiment, a toner image is formed by image portion exposure and reversal development. That is, the toner charged to the same polarity as the charge polarity of the photosensitive drum 1 adheres to the exposed portion of the photosensitive drum 1 where the absolute value of the potential is lowered by exposure after being substantially uniformly charged. In this embodiment, the exposure device 3 and the development device 4 constitute a toner image forming means for forming a toner image on the photosensitive drum 1 charged by the charging roller 2 .

The toner image formed on the photosensitive drum 1 is transferred (primary transfer) onto the intermediate transfer belt 10 by the primary transfer roller 5 to which the primary transfer voltage is applied at the primary transfer portion T1. In the rotation direction of the photosensitive drum 1, the position where the photosensitive drum 1 and the intermediate transfer belt 10 come into contact with each other and the toner image is transferred from the photosensitive drum 1 to the intermediate transfer belt 10 is the primary transfer position (primary transfer portion T1). be. For example, when forming a full-color image, yellow, magenta, cyan, and black toner images formed on the respective photosensitive drums 1 are superimposed and transferred onto the intermediate transfer belt 10 at the respective primary transfer portions T1.

As shown in FIG. 1, the toner image formed on the intermediate transfer belt 10 is transferred (2 next transcription). The recording material S is delivered from a cassette 18, which is a storage unit, transported by a transport roller pair 17, synchronized with the toner image on the intermediate transfer belt 10 by a registration roller pair 16, and transported to a secondary transfer portion T2. It's coming.

The recording material S onto which the toner image has been transferred is heated and pressurized by the fixing device 15 . As a result, the toner image is fixed on the recording material S. As shown in FIG. After that, the recording material S is discharged out of the apparatus, and a series of image forming processes are completed.

Toner remaining on the photosensitive drum 1 after the primary transfer process (primary transfer residual toner) is removed from the photosensitive drum 1 by a cleaning blade 71 provided in the cleaning device 7 and collected in a collection toner container 72 . (see Figure 2). A cleaning position b (cleaning portion) is a position in the rotation direction of the photosensitive drum 1 with which the cleaning blade 71 abuts on the photosensitive drum 1 . Toner remaining on the intermediate transfer belt 10 after the secondary transfer process (secondary transfer residual toner) is removed and collected by the belt cleaning device 19 .

At least part of the charge remaining on the photosensitive drum 1 after the primary transfer process (residual charge) is removed by the pre-exposure device 6 . A position in the rotation direction of the photosensitive drum 1 where the pre-exposure device 6 irradiates light on the photosensitive drum 1 is a pre-exposure position c (pre-exposure portion). In this embodiment, the pre-exposure device 6 exposes the photosensitive drum 1 downstream from the primary transfer portion T1 and upstream from the charging portion a (more specifically, the cleaning position b) in the rotational direction of the photosensitive drum 1 .

As described above, a toner image is formed on the photosensitive drum 1 by electrically attaching a predetermined amount of toner based on the potential of the electrostatic latent image formed by exposure. Then, the toner image is electrically transferred to the intermediate transfer belt 10 by the action of the primary transfer current supplied by the primary transfer voltage applied to the primary transfer portion T1.

<Transfer memory>
By the way, as the toner image transferred from the photosensitive drum 1 to the intermediate transfer belt 10 at the image forming portion P on the upstream side passes through the primary transfer portion T1 of the image forming portion P on the downstream side, A potential difference may occur on the photosensitive drum 1 . This is a contact area (called a toner portion) where the toner image on the intermediate transfer belt 10 comes into contact with the toner image on the photosensitive drum 1 (photoreceptor) on the downstream side when the toner image passes through the primary transfer portion T1. , and a non-contact area (referred to as a non-toner area), which is not in contact with the toner image. This phenomenon is called "transfer memory", and charge tends to remain in the contact area where the toner image on the intermediate transfer belt 10 contacts. Therefore, when the transfer memory occurs on the photosensitive drum 1, the image formed on the recording material S causes a toner image with an unexpected density difference (a so-called multi-color ghost image).

FIG. 3(a) is a diagram for explaining a case where no ghost image occurs, and FIG. 3(b) is a diagram for explaining a case where a ghost image occurs. In FIGS. 3A and 3B, the toner image from the image forming section P on the upstream side is charged again by the charging roller 2 after passing through the primary transfer section T1 of the image forming section P on the downstream side. The charging current at the time is schematically shown. In FIGS. 3(a) and 3(b), the charging current of the photosensitive drum 1 indicated by the diagram is "+ side" in the upper direction and "- side" in the lower direction. As shown in FIGS. 3A and 3B, when the difference in charging current between the toner portion and the non-toner portion is small, ghost images are less likely to occur.

Generally, in normal use of the image forming apparatus 100, the current flowing from the primary transfer roller 5 to the photosensitive drum 1 is set so as not to generate a ghost image caused by transfer memory. However, if, for example, the electrical resistance of the primary transfer roller 5 increases more than expected, or if an amount of toner more than expected passes through the primary transfer portion T1 on the downstream side, then the photosensitive drum 1 on the downstream side will transfer the primary The current flowing from the transfer roller 5 may be insufficient. In such a case, the potential difference (contrast ) becomes unsustainable. As a result, the potential difference on the photosensitive drum 1 generated between the toner portion and the non-toner portion cannot be eliminated, resulting in a transfer memory, which may cause a ghost image. Therefore, the pre-exposure device 6 removes the charge from the photosensitive drum 1 before it passes through the charging section, thereby making up for the decrease in contrast. However, if the amount of light (pre-exposure amount) with which the pre-exposure device 6 irradiates the photosensitive drum 1 is increased more than necessary, the current flowing into the photosensitive drum 1 at the charging portion a increases, resulting in abrasion of the photosensitive drum 1. In addition, continuous irradiation with a large amount of light can accelerate the deterioration of the sensitivity of the photosensitive drum 1 .

Alternatively, for example, a potential sensor may be provided to detect the surface potential of the photosensitive drum 1 before passing through the charging section, and the pre-exposure amount of the pre-exposure device 6 may be controlled according to the detection result. However, such a configuration requires space and cost for installing the potential sensor, and is difficult to adopt, contrary to recent demands for downsizing and cost reduction of the image forming apparatus 100 .

In this embodiment, in addition to detecting the charging current flowing through the toner portion, the charging current flowing through the non-toner portion is detected, and the pre-exposure amount of the pre-exposure device 6 can be set based on the detected charging currents. ing. This will be explained below.

First, the principle of pre-exposure amount setting control for suppressing the occurrence of ghost images caused by transfer memory will be described. FIG. 4(a) shows an example of a test toner image, and FIG. 4(b) shows an example of a ghost image generated in a halftone image.

First, the following experiments were conducted. The red solid image formed by the YM image forming units PY and PM is sent to the primary transfer unit T1 of the C image forming unit PC where only the photosensitive drum 1 is charged and the electrostatic latent image is not formed. Then, on the photosensitive drum 1 of the C image forming portion PC, the contact area (toner portion, red solid image portion) in contact with the solid red image at the primary transfer portion T1 and the non-contact region (non-toner portion) not in contact with the solid image at the primary transfer portion T1. A charging current in the charging portion a is measured by an ammeter 23 (see FIG. 2). As shown in FIG. 4A, the red solid image was formed over the entire image forming area (area where a toner image can be formed) in the longitudinal direction of the photosensitive drum 1 . In this embodiment, unless otherwise specified, a test toner image (referred to as a test toner image) formed for detecting the charging current is formed over the entire image forming area in the longitudinal direction of the photosensitive drum 1. It is a solid image.

For example, the photosensitive drum 1 of the C image forming portion PC is charged substantially uniformly to "-700 V" by the charging roller 2, and the photosensitive drum 1 passes through the primary transfer portion T1 and is again charged by the charging roller 2 at the charging portion a. The charging current was "-26 .mu.A" in the red solid image portion and "-30 .mu.A" in the non-toner portion. At this time, due to the images formed by the YM image forming stations PY and PM on the upstream side, a potential difference that causes a ghost image occurred in the C image forming station PC on the downstream side. Even if it occurred, the ghost image on the next image was within the allowable range. It is considered that this is because the potential difference between the toner portion and the non-toner portion after passing through the transfer portion is small, and the potential difference of the surface potential can be eliminated by charging (see FIG. 3A).

In a similar experiment, while maintaining the transfer current flowing from the primary transfer roller 5 to the photosensitive drum 1 in the C image forming section PC, the amount of toner borne on the red solid image formed in the YM image forming sections PY and PM on the upstream side was changed. I increased it by 1.4 times. In this case, when the photosensitive drum 1 of the C image forming portion PC passes through the primary transfer portion T1 and is again charged by the charging roller 2 at the charging portion a, the charging current is “−24 μA” at the solid red image portion, and “−24 μA” at the non-toner It was "-30 μA" at the part. At this time, the ghost image in the next image exceeded the allowable range (see FIG. 4(b)). This is probably because the potential difference between the toner portion and the non-toner portion after passing through the transfer portion was large, and the potential difference in the surface potential could not be eliminated by charging (see FIG. 3B).

Next, the relationship between the charging current (charging DC current) and the ghost image will be described. FIG. 5 shows the amount of toner applied (mg/cm ² ) of the toner image formed on the photosensitive drum 1, and the amount of toner image formed by the image forming section P on the upstream side is the primary toner image formed by the image forming section P on the downstream side. After passing the transfer portion T1, in the photosensitive drum 1 on the downstream side, the charging current (I1) that flows when the toner portion passes the charging portion a and the charging current that flows when the non-toner portion passes the charging portion a (I2) and the difference "I1-I2" (μA). As can be understood from FIG. 5, as the amount of applied toner increases, the difference "I1-I2" between the charging currents also increases. In other words, the larger the toner borne-on amount of the toner image formed in the image forming portion P on the upstream side, the greater the potential difference between the toner portion and the non-toner portion after passing through the charging portion, that is, after recharging, on the downstream side photosensitive drum 1 . As a result, the possibility of generating a ghost image increases accordingly. In other words, transfer memory is likely to occur when the amount of toner on the toner image carried on the intermediate transfer belt 10 is large.

FIG. 6 shows the relationship between the pre-exposure amount of the pre-exposure device 6 and the charging current (charging DC current) when the charging voltage applied by the charging power source D3 is constant. As can be understood from FIG. 6, even if the charging voltage is constant, the charging current increases as the pre-exposure amount increases. This is because the photosensitive drum 1 is sufficiently neutralized when the pre-exposure amount increases. The increase in the charging current becomes small from around 50% of the pre-exposure amount, because the surface potential of the photosensitive drum 1 after static elimination is saturated. From this, it can be seen that increasing the pre-exposure amount of the pre-exposure device 6 is effective in suppressing the occurrence of ghost images. However, as already described, if the pre-exposure amount of the pre-exposure device 6 is increased, the current flowing through the photosensitive drum 1 at the charging portion a increases and the life of the photosensitive drum 1 can be shortened. It is preferable not to increase the amount.

Next, the conditions under which a ghost image occurs, more specifically, the charging current conditions under which transfer memory occurs will be described. FIG. 7 is a graph showing the relationship between the charging current and the occurrence of ghost images, taking the C image forming station PC as an example. "I1-I2" represents the difference between the charging current (I1) of the toner portion (red solid image portion) of the photosensitive drum 1 and the charging current (I2) of the non-toner portion, and "(I1-I2)/I2" is the difference. and the charging current of the non-toner portion. In FIG. 7, the horizontal axis indicates the ratio "(I1-I2)/I2", and the vertical axis indicates "potential difference between the surface potential of the charged toner portion and the surface potential of the non-toner portion after charging.” .

As shown in FIG. 7, here, when the "potential difference between the surface potential of the toner portion after charging and the surface potential of the non-toner portion after charging" is greater than "4.0 V", a ghost image occurs, and "4. 0 V" or less, no ghost image occurs. When the ratio "(I1-I2)/I2" is greater than "0.18", the "potential difference between the toner portion and the non-toner portion" is greater than "4.0V". Then, as the ratio "(I1-I2)/I2" increases, the potential difference between the toner portion and the non-toner portion increases exponentially, making the ghost image more likely to occur. In the case of this embodiment, a ghost image may occur when "(I1-I2)/I2>threshold (Icg=0.18)".

Therefore, in the present embodiment, when a charging voltage is applied (at the time of charging) by the charging power source D3, the charging current of the toner portion and the charging current of the non-toner portion are detected by the ammeter 23 (see FIG. 2). The detection result is fed back to the pre-exposure amount of the pre-exposure device 6 . That is, when the pre-exposure amount of the pre-exposure device 6 is adjusted, the charging current of the toner portion during charging and the charging current of the non-toner portion change. ≤threshold value (Icg)" (in this embodiment, the current ratio is 0.18 or less), it is possible to suppress the occurrence of a ghost image.

<Control mode>
FIG. 8 is a block diagram showing a control mode of main parts of the image forming apparatus 100 of this embodiment. The image forming apparatus 100 is provided with a control section 110 that controls each section of the image forming apparatus 100 in an integrated manner. The control unit 110 includes a CPU (Central Processing Unit) 111, a ROM (Read Only Memory) 112, a RAM (Random Access Memory) 113, and the like. The RAM 113 stores the detection result of the ammeter 23, the calculation result, and the like, and the ROM 112 stores the control program, the data table obtained in advance, and the like. In relation to this embodiment, the control unit 110 is connected to the ammeter 23, the pre-exposure amount control circuit 120, and the like.

In this embodiment, the control unit 110 sets the pre-exposure amount in the pre-exposure amount control circuit 120, so that the pre-exposure device 6 emits light with a predetermined amount of light. The pre-exposure amount control circuit 120 can control the current (driving current) supplied to the pre-exposure device 6, for example, within a range of "0 mA (minimum) to 20 mA (maximum)" by PWM control. There is a substantially linear relationship between the drive current and the pre-exposure amount, and there is a substantially linear relationship between the drive current and the pre-exposure duty (PWM duty). That is, the drive current is "0 mA" at a pre-exposure duty of "0%", and the drive current is "20 mA" at a pre-exposure duty of "100%".

Here, the image forming apparatus 100 performs a "job", which is a series of image output operations for forming and outputting an image on a single or a plurality of recording materials S, which is started by one start instruction. A job generally includes an image forming process (printing process), a pre-rotation process, an inter-paper process when forming images on a plurality of recording materials S, and a post-rotation process. The image forming process is a period during which an electrostatic latent image of an image to be actually formed and output on the recording material S is formed, a toner image is formed, and the toner image is transferred. say. The pre-rotation process is a period from when the start instruction is input to when the image formation is actually started, during which preparatory operations are performed before the image forming process. The paper interval process is a period corresponding to a period between recording materials S when performing continuous image formation in which images are continuously formed on a plurality of recording materials S. FIG. The post-rotation process is a period during which an arrangement operation (preparation operation) is performed after the image forming process. The non-image forming time is a period other than the image forming time, and includes the pre-rotation process, the paper interval process, the post-rotation process, and the preparatory operation when the image forming apparatus 100 is turned on or returned from the sleep state. is included during the pre-multi-rotation process.

In the case of the present embodiment, the control unit 110 as a setting unit performs a pre-exposure amount setting process (setting mode, which will be described later) in the post-rotation of the job every time the number of sheets of recording material S on which images have been formed becomes equal to or greater than a predetermined threshold value. 9) can be executed. The control unit 110 sets the pre-exposure amount of the pre-exposure device 6 based on the detection result of the charging current by the ammeter 23, as described later in detail. However, the present invention is not limited to this, and the pre-exposure amount setting process can also be performed at any timing during non-image formation.

<Pre-exposure amount setting processing>
Next, the pre-exposure setting process (setting mode) of this embodiment will be described. FIG. 9 is a flow chart showing the pre-exposure amount setting process of this embodiment. Here, the pre-exposure amount setting process will be described using the C image forming unit PC as an example.

For example, when the post-rotation process at the end of the job is started, the control unit 110 determines whether image formation has been performed on recording materials S of 100 sheets (e.g., A4 size conversion value) or more since the previous pre-exposure adjustment process. (S101). If image formation has not been performed on 100 or more sheets of recording material S (No in S101), the control unit 110 does not perform the operation of forming the test toner image or the operation of detecting the charging current, and performs the predetermined post-rotation process. When the operation of is completed, the process is terminated.

On the other hand, when image formation is performed on 100 or more recording materials S (Yes in S101), the control unit 110 forms a test toner image and detects the charging current of the toner portion and the charging current of the non-toner portion. (S102). For example, as a test toner image, a solid red image is formed on the intermediate transfer belt 10 by the YM image forming units PY and PM, and this solid red image is sent to the primary transfer unit T1 of the C image forming unit PC. In the C image forming portion PC, an ammeter 23 measures the charging current I1 when the contact area (toner portion) on the photosensitive drum 1c that contacts the red solid image at the primary transfer portion T1 passes through the charging portion a. detected by In addition, the ammeter 23 detects the charging current I2 during charging when the area (non-toner portion) that is not in contact with the solid red image at the primary transfer portion T1 passes through the charging portion a.

Next, based on the charging current detected by the ammeter 23, the control unit 110 obtains a current difference (I1-I2) between the charging current of the toner portion and the charging current of the non-toner portion. A ratio "(I1-I2)/I2" to the charging current of the part is calculated (S103). The calculated ratio “(I1−I2)/I2” (referred to as current ratio for convenience) is stored in RAM 113 . The control unit 110 compares the absolute value of the calculated current ratio “(I1−I2)/I2” with a threshold (Icg) stored in advance in the ROM 112 at which a ghost image may occur (S104). As described above, the threshold value (Icg) is a boundary indicating whether or not the "potential difference between the surface potential of the toner portion after charging and the surface potential of the non-toner portion after charging" is large enough to cause a ghost image. value, for example "0.18" (see FIG. 7).

If the absolute value of the current ratio is equal to or less than the threshold value (No in S104), the "potential difference between the surface potential of the toner portion after charging and the surface potential of the non-toner portion after charging" is not large enough to cause a ghost image. Therefore, the control unit 110 ends the processing. In this case, the control unit 110 does not change the pre-exposure amount of the pre-exposure device 6, and terminates the process when the operation of the predetermined post-rotation process is completed.

On the other hand, if the absolute value of the current ratio is greater than the threshold (Yes in S104), the control unit 110 increases the pre-exposure amount of the pre-exposure device 6 from before the change, and "|(I1−I2)/I2|≦threshold (Icg)", the pre-exposure amount of the pre-exposure device 6 is changed and set (S105). Specifically, according to the relationship shown in FIG. Change the drive current setting. Based on the charging current I1 flowing in the toner portion and the charging current I2 flowing in the non-toner portion, that is, based on the surface potential of the contact area, the pre-exposure amount is set such that the current ratio becomes constant (0.18). is preferred. As a result, it is possible to apply a charge removing current that equalizes the difference in surface potential between the toner portion and the non-toner portion, which causes ghost images, thereby suppressing the generation of ghost images caused by transfer memory.

In the above-described embodiment, the case where a solid red image is formed as a test toner image and the pre-exposure amount of the pre-exposure device 6C in the C image forming section PC is set has been described as an example. In the image forming apparatus 100 of the present embodiment, a ghost image due to transfer memory is likely to occur in the K image forming station PK as well as in the C image forming station PC. Therefore, the setting of the pre-exposure amount of the pre-exposure device 6K in the K image forming section PK is performed in the same manner as the setting of the pre-exposure amount of the C image forming section PC. In that case, as in the case of the C image forming part PC, a red solid image is used as a test toner image (whether it is formed for the pre-exposure amount setting control of the C image forming part PC or is newly formed separately). good). However, it is not limited to this, and a solid green image or a solid blue image may be used as the test toner image.

In addition, since the increase in the absolute value of the surface potential of the photosensitive drum 1 after passing through the transfer portion is small compared to the secondary color image, ghost images are less likely to occur. A charging current may be detected in the same manner as described above at the image forming station P on the downstream side using a monochrome image. In other words, the pre-exposure setting process of the present embodiment is performed in the pre-exposure setting process provided corresponding to the photosensitive drum 1 downstream of the uppermost photosensitive drum 1 in the rotation direction of the intermediate transfer belt 10 among the plurality of photosensitive drums 1. It can be applied to the device 6.

Furthermore, when a solid (particularly multi-color solid) image is formed in the image forming section P on the upstream side, a ghost image due to transfer memory is likely to occur in the image forming section P on the downstream side. Therefore, a solid image is preferable as the test toner image. However, since the toner borne-on amount of the test toner image should be at least a predetermined amount that may cause a ghost image due to transfer memory, if the toner borne-on amount satisfies this, the test toner image is a halftone image. may be

As can be understood from FIG. 7, when the current ratio "(I1-I2)/I2" exceeds a certain threshold value, the difference between the toner portion and the non-toner portion when passing through the transfer portion and being recharged. The potential difference exceeds the ghost generation threshold, and a ghost image due to transfer memory is likely to occur. On the other hand, in the present embodiment, by detecting the charging current (I1, I2) in the image forming section P on the downstream side, the generation level of the ghost image caused by the transfer memory is predicted and fed back to the pre-exposure device 6. . Specifically, the pre-exposure amount of the pre-exposure device 6 is set to be increased when the current ratio "(I1-I2)/I2" exceeds a predetermined threshold value.

As described above, in this embodiment, a test toner image is carried on the intermediate transfer belt 10 from the photosensitive drum 1 on the upstream side, and the toner portion in contact with the test toner image on the photosensitive drum 1 on the downstream side passes the charging portion a. The charging current (I1) at the time of charging is detected. In addition, the charging current (I2) is detected when the non-toner portion, which is not in contact with the test toner image, passes through the charging portion a on the photosensitive drum 1 on the downstream side. Then, the pre-exposure amount of the pre-exposure device 6 is set so that the absolute value of the current ratio "(I1-I2)/I2" based on these charging currents is equal to or less than the threshold. As a result, the charge of the photosensitive drum 1 can be eliminated without increasing the pre-exposure amount of the pre-exposure device 6 more than necessary, thereby suppressing the generation of a ghost image due to transfer memory in the photosensitive drum 1 on the downstream side. becomes possible. Further, in the present embodiment, since it is not necessary to irradiate pre-exposure more than necessary, the life of the photosensitive drum 1 is not shortened.

<Other embodiments>
Although the present invention has been described with reference to specific embodiments, the present invention is not limited to the above-described embodiments.

For example, in the above embodiment, the case where the charging roller 2 is in contact with the photosensitive drum 1 has been described, but the present invention is not limited to this. The problem of ghost images such as multi-color ghost images due to transfer memory is a common problem regardless of charging methods such as non-contact and proximity. Therefore, the charging member such as the charging roller 2 does not necessarily have to be in contact with the surface of the photosensitive drum 1, which is the member to be charged. may be arranged in non-contacting proximity with each other. In this way, the charging member is arranged in the vicinity of the photosensitive drum, and the discharge in the vicinity thereof (in the above-described embodiment, corresponding to the gaps upstream and downstream of the contact portion between the charging roller 2 and the photosensitive drum 1) The present invention can also be applied to a configuration in which the photosensitive drum 1 is charged.

Further, in the above-described embodiment, the AC charging method using an oscillating voltage obtained by superimposing a DC voltage and an AC voltage as the charging voltage is adopted, but the present invention is not limited to this. The problem of ghost images such as multi-color ghost images due to transfer memory tends to occur more remarkably in the DC charging method using a charging voltage consisting of only a DC voltage, which has lower potential convergence of the photosensitive drum than in the AC charging method. There is Therefore, by applying the present invention to the DC charging method, a higher effect can be obtained.

In the above-described embodiment, a tandem image forming apparatus has been described as an example, but the present invention can also be applied to a so-called one-drum image forming apparatus. A single-drum type image forming apparatus has a plurality of developing devices for one photosensitive drum. Develop sequentially. Then, each time a toner image of each color is formed on the photosensitive drum, the toner image is primarily transferred from the photosensitive drum to the intermediate transfer member in the primary transfer portion so as to be sequentially superimposed. The intermediate transfer member bears the transferred toner image and repeatedly passes through the primary transfer portion. Then, the finally formed multiple toner image is secondarily transferred onto the recording material at the secondary transfer portion. In such an image forming apparatus as well, the problem of ghost images such as multi-color ghost images due to transfer memory may occur in the primary transfer process for the second and subsequent colors, as in the image forming apparatus of the above-described embodiments.

Therefore, the present invention can also be applied to such an image forming apparatus, and effects similar to those of the above-described embodiments can be obtained. In other words, the test toner image transferred from the photosensitive drum to the intermediate transfer member in the primary transfer portion is conveyed to the transfer portion again, and the area of the photosensitive drum that is in contact with the test toner image and the area of the photosensitive drum that is not in contact with the test toner image are respectively transferred. Detects charging current during charging. This makes it possible to adjust the amount of pre-exposure after at least a toner image of a color other than the color to be formed first is transferred to the intermediate transfer member when forming an image with toners of a plurality of colors.

1Y (1M, 1C, 1K)...photoreceptor (photosensitive drum), 2Y (2M, 2C, 2K)...charging member (charging roller), 5Y (5M, 5C, 5K)...transfer rotator (primary transfer roller) , 6Y (6M, 6C, 6K)...exposure charge removing means (pre-exposure device), 10...intermediate transfer member (intermediate transfer belt), 23...current detecting means (ammeter), 100...image forming apparatus, 110...setting means (control section), D3... power supply (charging power supply), S... recording material, T1... transfer section (primary transfer section)

Claims (6)

An image forming apparatus for forming a toner image on a recording material,
an intermediate transfer member that carries a test toner image and rotates;
a photosensitive member that rotates in contact with the intermediate transfer member;
a transfer rotator forming a transfer portion of the toner image from the photoreceptor to the intermediate transfer member;
a charging member that charges the photoreceptor at a charging position;
exposure charge removing means for removing charges by exposing the photoreceptor downstream from the transfer unit and upstream from the charging position with respect to the rotation direction of the photoreceptor;
a power supply that applies a voltage to the charging member;
current detection means for detecting the current flowing through the charging member;
A test toner image carried on the intermediate transfer member and conveyed to the transfer unit and a contact area on the photosensitive member in contact with the charging member while a voltage is applied pass through the charging position. Let I1 be the current detected when the test toner image passes through, and I2 be the current detected when the non-contact area on the photoreceptor that is not in contact with the test toner image passes the charging position. I2) setting means capable of executing a setting mode for setting the exposure amount of the exposure static elimination means so that the absolute value of /I2 is equal to or less than a threshold;
An image forming apparatus characterized by: The setting means sets the exposure amount of the exposure static elimination means such that the absolute value of (I1-I2)/I2 is 0.18 or less.
2. The image forming apparatus according to claim 1, wherein: When the current detected by the current detection means is greater than the threshold, the setting means increases the exposure amount of the photoreceptor by the exposure static elimination means to a level greater than that before the change.
3. The image forming apparatus according to claim 1, wherein: comprising another photoreceptor on the upstream side of the photoreceptor, which abuts on the intermediate transfer body and forms a toner image to be transferred to the intermediate transfer body;
The test toner image carried on the intermediate transfer member is formed on the another photoreceptor, transferred from the another photoreceptor, and conveyed to the transfer unit.
4. The image forming apparatus according to claim 1, wherein: further comprising another photoreceptor in contact with the intermediate transfer member on the upstream side of the another photoreceptor and forming a toner image to be transferred to the intermediate transfer member;
The test toner images are formed on a plurality of the different photoreceptors, transferred from the plurality of the different photoreceptors to the intermediate transfer member in a superimposed manner, and conveyed to the transfer unit.
5. The image forming apparatus according to claim 4, wherein: the power supply applies only a DC voltage to the charging member;
6. The image forming apparatus according to claim 1, wherein:

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