JP7207958B2 - image forming device - Google Patents

Description

The present invention relates to image forming apparatuses such as laser beam printers, copiers, facsimiles, etc. that use an electrophotographic recording method.

Conventionally, a toner image formed on a drum-shaped photosensitive member (hereinafter referred to as a photosensitive drum) as an image bearing member is primarily transferred to an intermediate transfer member and then secondarily transferred from the intermediate transfer member to a transfer material such as paper. An image forming apparatus using an intermediate transfer system is known. The intermediate transfer system has high media flexibility that allows image formation on a wide variety of transfer materials. In particular, in color image forming apparatuses, an in-line method is widely used in which toner images formed on respective photosensitive drums provided in a plurality of image forming units are sequentially primary-transferred so as to be superimposed on an intermediate transfer member.

In an intermediate transfer type image forming apparatus, it is necessary to clean the toner remaining on the intermediate transfer body without being transferred to the transfer material during secondary transfer (hereinafter referred to as transfer residual toner). As a cleaning method for the residual toner after the transfer, a charging member is provided to charge the residual toner after the secondary transfer. Methods for recovery are known. In such a method, the residual toner after transfer is charged by a charging member to a polarity opposite to the normal charging polarity of the toner, and is transferred from the intermediate transfer member to the photosensitive drum by a potential difference at the primary transfer portion where the intermediate transfer member and the photosensitive drum are in contact. Move electrostatically to the drum.

In the cleaning method described above, when the transfer residual toner is charged to a polarity opposite to the normal charge polarity of the toner, the transfer residual toner may adhere to the charging member. , it becomes difficult to sufficiently charge the transfer residual toner. Japanese Patent Application Laid-Open No. 2002-200002 discloses a process of switching the polarity of the voltage applied to the charging member before the amount of toner adhering to the charging member increases to move the toner from the charging member to an intermediate transfer member and collect the toner with a photosensitive drum ( Disclosed is a configuration in which a discharge step is provided. More specifically, by reversing the direction of the electric field formed in the primary transfer portion to that during image formation, the toner transferred from the charging member to the intermediate transfer member is electrostatically transferred from the intermediate transfer member to the photosensitive drum. A configuration is disclosed in which the particles are moved to and recovered by a photosensitive drum.

JP 2016-142763 A

In Japanese Patent Application Laid-Open No. 2002-100000, for example, when the discharging process is performed in a color mode in which a plurality of photosensitive drums are in contact with the intermediate transfer body, a plurality of toners (hereinafter referred to as discharged toner) that have moved from the charging member to the intermediate transfer body are discharged. are collected by a plurality of photosensitive drums when passing through the primary transfer portion. On the other hand, when the discharging process is executed in a monochrome mode in which only one photosensitive drum is in contact with the intermediate transfer member, the discharged toner is collected by one primary transfer portion. That is, there is a possibility that the discharge toner collection efficiency in the monochrome mode will be lower than the discharge toner collection efficiency in the color mode. This is particularly noticeable when the amount of ejected toner is large.

Toner that has not been collected on the photosensitive drum in the discharging process remains on the intermediate transfer belt 20, and as the intermediate transfer belt 20 rotates, it is collected and accumulated on the charging member again. Therefore, when a large amount of toner is not collected on the photosensitive drum in the discharging process (when collection efficiency is low), the amount of toner accumulated in the charging member decreases, and the degree of recovery of the charging performance of the charging member decreases. put away. In such a case, in order to suppress the deterioration of the charging performance of the charging member, it is necessary to increase the frequency of performing the discharging process during the image forming operation. However, when the frequency of the discharge process increases, there is a possibility that the standby time for image formation increases.

SUMMARY OF THE INVENTION Accordingly, it is an object of the present invention to suppress an increase in waiting time for image formation when performing a step of collecting toner on an image carrier after transferring toner from a charging member to an intermediate transfer member.

In the present invention, a first image carrier carrying a black toner image and a second image carrier carrying a toner image of a color different from that of the first image carrier are movable, an endless intermediate transfer member onto which a toner image carried on at least one of the first image carrier and the second image carrier is primarily transferred; and a toner image primarily transferred to the intermediate transfer member. to a transfer material, and downstream of the secondary transfer portion, which is a contact portion between the intermediate transfer member and the secondary transfer member, with respect to the moving direction of the intermediate transfer member. and the first image carrier and the second image carrier are provided upstream of a position where they contact the intermediate transfer member, and form a charging section in contact with the intermediate transfer member. a charging member for charging the toner remaining on the intermediate transfer member at the charging portion; a charging power source for applying a voltage to the charging member; A control capable of executing an image forming operation by selecting a first mode in which only the first image bearing member is brought into contact with the transfer member and a second mode in which only the first image bearing member is brought into contact with the intermediate transfer member. wherein the control means applies a voltage having the same polarity as the normal charging polarity of the toner from the charging power source to the charging member, thereby removing the toner adhering to the charging member from the charging member. to the intermediate transfer member, the toner transferred from the charging member to the intermediate transfer member is transferred to at least one of the first image carrier and the second image carrier in contact with the intermediate transfer member. wherein, in the first mode, when the amount of toner adhering to the charging member exceeds a first threshold value, the first image carrier and In the case where the discharging step is performed while the second image bearing member is in contact with the intermediate transfer member, and the mode is switched from the first mode to the second mode, the toner adheres to the charging member. when the amount of toner applied is equal to or greater than a second threshold value smaller than the first threshold value, switching to the second mode after executing the discharging step in the first mode, and switching to the second mode; wherein the amount of toner adhering to the charging member is smaller than the first threshold value and exceeds a third threshold value equal to or greater than the second threshold value, the first image carrier; is in contact with the intermediate transfer member, and the discharging step is performed .

According to the present invention, it is possible to suppress an increase in waiting time for image formation when performing the step of collecting toner on the image carrier after transferring the toner from the charging member to the intermediate transfer member.

1 is a schematic cross-sectional view for explaining the configuration of an image forming apparatus; FIG. 3 is a block diagram of a control unit of the image forming apparatus; FIG. 4A and 4B are schematic diagrams for explaining contact states between an image carrier and an intermediate transfer member in a full-color mode and a monochrome mode in Example 1. FIG. 4 is a schematic diagram for explaining the configuration around the contact portion between the charging member and the intermediate transfer body in Example 1. FIG. FIG. 5 is a schematic diagram illustrating a discharge process in Comparative Example 1 and Example 1; 10 is a graph for explaining changes in count values before and after switching to full mono in Comparative Example 1 and Example 1; FIG. 8 is a schematic diagram illustrating a peripheral configuration of a contact portion between a charging member and an intermediate transfer body in a modified example of Embodiment 1;

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 arrangement of the components described in the following examples should be appropriately changed according to the configuration of the device to which the invention is applied and various conditions. It is not intended to be limited to the following examples.

(Example 1)
[Configuration of Image Forming Apparatus]
FIG. 1 is a schematic cross-sectional view of an image forming apparatus 10 in this embodiment. FIG. 2 is a block diagram of the control system of the image forming apparatus 10 of this embodiment. As shown in FIG. 2, the image forming apparatus 10 is connected to a personal computer 200 as a host device. An operation start command and an image signal from the personal computer 200 are transmitted to the controller 110 as control means, and the image forming apparatus 10 performs image formation by the controller 110 controlling various means.

As shown in FIG. 1, the image forming apparatus 10 of this embodiment is an intermediate transfer type color image forming apparatus using an electrophotographic method. , and fourth image forming units 1a, 1b, 1c, and 1d. The first, second, third and fourth image forming sections 1a, 1b, 1c and 1d are for forming yellow, magenta, cyan and black images, respectively. These four image forming units 1a, 1b, 1c, and 1d are arranged in a row at regular intervals. In this embodiment, the configurations of the first to fourth image forming units 1a to 1d are substantially the same except that the colors of toners used are different. Therefore, hereinafter, the suffixes a, b, c, and d given to the reference numerals in the drawings are omitted to indicate that the elements are provided for one of the colors, unless a particular distinction is required. explained in detail.

As shown in FIG. 1, an image forming unit 1 includes a drum-type electrophotographic photosensitive member (hereinafter referred to as a photosensitive member) which is rotatable in the direction of arrow R1 and serves as a first image carrier on which a toner image is formed. (referred to as a drum) 2 is installed. Around the photosensitive drum 2, a drum charging roller 3 as means for charging the photosensitive drum 2, developing means 4, and cleaning means 6 are installed. In addition, exposure means 7 (laser scanner) is arranged downstream of the drum charging roller 3 and upstream of the developing means 4 with respect to the rotational direction of the photosensitive drum 2 .

The photosensitive drum 2 in this embodiment is a negatively charged OPC (organic photoconductor) photosensitive member, and has a photosensitive layer on an aluminum drum substrate. The photosensitive drum 2 is rotationally driven in the direction indicated by an arrow R1 (clockwise) at a predetermined peripheral speed (surface movement speed) by a driving device (not shown). In this embodiment, the peripheral speed of the photosensitive drum 2 corresponds to the process speed of the image forming apparatus 10 .

The drum charging roller 3 is in contact with the photosensitive drum 2 with a predetermined pressing force, and is applied with a predetermined voltage from a power source (not shown) to uniformly charge the surface of the photosensitive drum 2 to a predetermined potential. In this embodiment, the photosensitive drum 2 is negatively charged by the drum charging roller 3 .

The exposing means 7 forms an electrostatic latent image corresponding to image information on the surface of the photosensitive drum 2 charged by the drum charging roller 3 by exposing the surface of the photosensitive drum 2 to light. That is, in the exposure means 7, a laser beam modulated corresponding to time-series electric digital pixel signals of image information inputted from a personal computer 200 is outputted from a laser output section, and this laser beam is exposed through a reflecting mirror. The surface of the drum 2 is irradiated.

The developing means 4 in this embodiment employs a one-component contact developing system as a developing system, and has a developing roller 8 as a toner carrier. The toner carried on the developing roller 8 in a thin layer is transferred to a facing portion (developing portion ). Then, the electrostatic latent image formed on the photosensitive drum 2 by the exposing means 7 is developed as a toner image by applying a voltage from a developing power source (not shown) to the developing roller 8 . In this embodiment, the normal charge polarity of the toner is negative, and the toner charged to the same polarity as the charge polarity of the photosensitive drum 2 is placed at a position corresponding to the electrostatic latent image formed by the exposure means 7. A toner image is developed on the photosensitive drum 2 by a reversal development method in which the toner image is adhered to the surface of the photosensitive drum 2 .

Further, each developing means 4a, 4b, 4c and 4d contains yellow, magenta, cyan and black toners, respectively. In the configuration of the image forming apparatus 10 of this embodiment, in the full-color image forming mode in which image formation is performed using all of the image forming units 1a to 1d, all the developing rollers 8a to 8d of the developing means 4a to 4d are used as photosensitive drums. 2a to 2d. On the other hand, in a monocolor (single-color) image forming mode in which image formation is performed using only the image forming unit 1d, the developing roller 8d is brought into contact with the photosensitive drum 2d, and the developing rollers 8a to 8c are in contact with the photosensitive drums 2a to 2c. Separate. This is to prevent deterioration and consumption of the developing rollers 8a to 8c and toner in the image forming sections 1a to 1c which do not perform image formation.

An intermediate transfer belt 20 that is a movable endless intermediate transfer member is provided at a position facing the photosensitive drum 2 of each image forming unit 1 . The intermediate transfer belt 20 is stretched around a drive roller 21, a tension roller 22, and a counter roller 23 as a plurality of supporting members. As the driving roller 21 is rotationally driven in the illustrated arrow R2 direction, the intermediate transfer belt 20 receives the driving force transmitted from the driving roller 21 and moves in the illustrated arrow R3 direction at approximately the same circumferential speed as the photosensitive drum 2. Circular movement (rotation) is performed at a high speed, that is, a predetermined process speed. Incidentally, the drive roller 21, the tension roller 22, and the opposing roller 23 are connected to the ground.

Primary transfer rollers 5 a to 5 d as primary transfer members (contact members) are arranged on the inner peripheral surface of the intermediate transfer belt 20 so as to correspond to the respective photosensitive drums 2 of the image forming units 1 . The primary transfer roller 5 contacts the inner circumferential surface of the intermediate transfer belt 20 and rotates following the movement of the intermediate transfer belt 20 . Further, the primary transfer roller 5 urges the intermediate transfer belt 20 against the photosensitive drum 2 to form a primary transfer portion N1 (contact portion) at a position where the intermediate transfer belt 20 and the photosensitive drum 2 contact each other. do. Further, a primary transfer power supply 40 is connected to the primary transfer roller 5 , and the primary transfer power supply 40 can apply a positive or negative voltage to the primary transfer roller 5 . During image formation, the primary transfer power source 40 applies a voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in this embodiment) to the primary transfer roller 5, thereby forming an image on the photosensitive drum 2. The resulting toner image is primarily transferred onto the intermediate transfer belt 20 .

A secondary transfer roller 24 as a secondary transfer member is arranged on the outer peripheral surface side of the intermediate transfer belt 20 so as to face the opposing roller 23 , and the position where the secondary transfer roller 24 and the intermediate transfer belt 20 contact each other. , a secondary transfer portion N2 is formed. The secondary transfer roller 24 rotates following the movement of the intermediate transfer belt 20 or the transfer material P conveyed to the secondary transfer portion N2. A secondary transfer power supply 44 is connected to the secondary transfer roller 24 , and the secondary transfer power supply 44 can apply a positive or negative voltage to the secondary transfer roller 24 . During image formation, the secondary transfer power source 44 applies a voltage having a polarity opposite to the normal charging polarity of the toner (positive polarity in the present embodiment) to the secondary transfer roller 24, so that the toner is transferred from the intermediate transfer belt 20 to the secondary transfer roller 24. A toner image is secondarily transferred onto the transfer material P.

In this embodiment, PEN (polyethylene naphthalate) resin is used as the intermediate transfer belt 20 as the second image bearing member that carries the toner image. The intermediate transfer belt 20 has a surface resistivity of 5.0×10 ¹⁰ Ω/□ and a volume resistivity of 8.0×10 ¹⁰ Ωcm. The resistivity was measured at an applied voltage of 100 V using a resistivity meter Hiresta UP manufactured by Mitsubishi Chemical Analytech Co., Ltd. and a URS probe dedicated to Hiresta UP as a measuring electrode.

As the intermediate transfer belt 20, an endless belt made of resin such as vinylidene fluoride resin (PVDF), ethylene tetrafluoride-ethylene copolymer resin (ETFE), polyimide, polyethylene terephthalate (PET), and polycarbonate is used. be able to. Alternatively, as the intermediate transfer belt 20, a rubber base layer such as ethylene propylene diene rubber (EPDM) is coated with, for example, a fluorine resin such as PTFE dispersed in urethane rubber to form an endless belt. be able to.

The primary transfer roller 5 is made of an elastic member such as sponge rubber. In this embodiment, as the primary transfer roller 5, a nickel-plated steel bar with a diameter of 6 mm was coated with NBR hydrin rubber to a thickness of 4 mm. The electrical resistance value of the primary transfer roller 5 is 1.0×10 when the primary transfer roller is pressed against the aluminum cylinder with a force of 9.8 N and rotated at 50 mm/sec and 100 V is applied. ⁵ Ω.

The image forming apparatus 10 has means (not shown) for bringing the primary transfer rollers 5a, 5b, 5c, and 5d into contact with or away from the photosensitive drum 2 via the intermediate transfer belt 20. FIG. The controller 110 can select either a full-color image forming mode (hereinafter referred to as a full-color mode) or a monochromatic image forming mode (hereinafter referred to as a monochrome mode) to form an image. . FIG. 3A is a schematic diagram for explaining the full-color mode (first mode) of this embodiment, and FIG. 3B is a schematic diagram for explaining the monochrome mode (second mode) of this embodiment. It is a diagram.

As shown in FIG. 3A, in the full-color mode, the primary transfer rollers 5a to 5d are pressed against the photosensitive drums 2a to 2d by a contact/separation mechanism (not shown). Image formation is performed by forming the next transfer portions N1a to N1d. On the other hand, in the monochrome mode, as shown in FIG. 3B, only the primary transfer roller 5d is pressed against the photosensitive drum 2d by a contact/separation mechanism (not shown) to form an image forming portion containing black toner. Image formation is performed by forming the primary transfer portion N1d only in 1d.

In the monochrome mode, as shown in FIG. 3B, the contact/separation mechanism (not shown) releases the primary transfer rollers 5a-5d from being pressed against the photosensitive drums 2a-2c. As a result, the photosensitive drums 2a to 2c are separated from the intermediate transfer belt 20, so that the primary transfer portions N1a to N1c are not formed. That is, when performing image formation in the monochrome mode, driving of each member in each of the image forming sections 1a to 1c can be stopped. By adopting such a configuration, it is possible to suppress unnecessary operations and extend the life of each of the image forming units 1a to 1c.

As for the mechanism for contacting or separating the photosensitive drum 2 and the intermediate transfer belt 20, the primary transfer roller 5 is pressed toward the photosensitive drum 2 via the intermediate transfer belt 20 by an urging means such as a spring. can be considered. In such a configuration, it is possible to separate the primary transfer roller 5 and the intermediate transfer belt 20 from the photosensitive drum 2 by releasing the biased state of the spring.

The secondary transfer roller 24 is made of, for example, an elastic member such as sponge rubber. In this embodiment, as the secondary transfer roller 24, a nickel-plated steel bar with a diameter of 6 mm was coated with NBR hydrin rubber to a thickness of 6 mm. The electrical resistance value of the secondary transfer roller 24 is 3.0× when the secondary transfer roller 24 is pressed against the aluminum cylinder with a force of 9.8 N and rotated at 50 mm/sec and 1000 V is applied. 10 ⁷ Ω.

Toner remaining on the intermediate transfer belt 20 is charged downstream of the secondary transfer portion N2 and upstream of the image forming portion 1a provided most upstream with respect to the movement direction of the intermediate transfer belt 20. A conductive brush 31 is provided as charging means. Details of the configuration and operation of the conductive brush 31 will be described later.

With respect to the conveying direction of the transfer material P, on the upstream side of the secondary transfer portion N2 are a registration roller 13, a conveying roller 15, a feed roller 14, and a paper feed cassette 16 as a storage portion for storing the transfer material P. and is provided. The transfer material P accommodated in the paper feed cassette 16 is fed toward the transport roller 15 by the rotation of the feed roller 14, and then transported toward the secondary transfer portion N2 by the transport roller 15 and the registration roller 13. be.

A fixing roller 12A having a heat source and a pressure roller 12B that contacts the fixing roller 12A with a predetermined pressure are provided downstream of the secondary transfer portion N2 with respect to the conveying direction of the transfer material P. A fixing means 12 is provided having a fixing means.

[Image forming operation]
Next, the image forming operation of the image forming apparatus 10 of this embodiment will be described by taking the full-color image forming mode as an example.

First, when an image forming operation start signal is issued, the photosensitive drum 2 is rotationally driven at a predetermined peripheral speed in the direction indicated by the arrow R1, and is charged by the drum charging roller 3 in the course of rotation, thereby providing a uniform surface. An electric potential is formed. After an electrostatic latent image is formed on the surface of the photosensitive drum 2 by the exposing means 7 in the course of rotation, the electrostatic latent image is developed by the toner stored in the developing means 4, whereby the photosensitive drum 2 is exposed. A toner image corresponding to the image information is formed on the surface. In this embodiment, as the developing method, a contact developing method is used in which the developing roller 8 carrying toner is brought into contact with the photosensitive drum 2 to develop the electrostatic latent image. A development method may also be used. In this embodiment, the electrostatic latent image is developed using the reversal development method. The present invention can also be applied to an image forming apparatus for positive development.

The toner image developed on the photosensitive drum 2 is formed by applying a positive voltage opposite to the normal charging polarity of the toner from the primary transfer power source 40 to the primary transfer roller 5 at the primary transfer portion N1. , the image is primarily transferred from the photosensitive drum 2 to the intermediate transfer belt 20 . In this way, at each primary transfer portion N1, the toner images of the respective colors are sequentially superimposed and primarily transferred onto the intermediate transfer belt 20, and a multiple toner image composed of toner images of a plurality of colors is formed on the intermediate transfer belt 20. be done.

The registration roller 13 conveys the transfer material P to the secondary transfer portion N2 at the timing when the leading edge of the multi-color toner image primarily transferred to the intermediate transfer belt 20 reaches the secondary transfer portion N2. Then, by applying a positive voltage, which is opposite in polarity to the normal charge polarity of the toner, from the secondary transfer power source 44 to the secondary transfer roller 24, the transfer material is transferred from the intermediate transfer belt 20 to the secondary transfer portion N2. A plurality of color toner images are collectively secondarily transferred to P.

After that, the transfer material P on which the toner images of multiple colors are secondarily transferred is conveyed to the fixing means 12, and is heated and pressed by the fixing roller 12A and the pressure roller 12B to melt and mix the toner images of multiple colors. is fixed on the transfer material P. Then, the transfer material P on which the toner images of multiple colors are fixed is discharged outside the image forming apparatus 10, and a series of image forming operations is completed.

Toner remaining on the photosensitive drum 2 after the primary transfer is removed from the photosensitive drum 2 by a cleaning blade 61 as a contact member made of an elastic material such as urethane rubber. to be recovered.

Toner remaining on the intermediate transfer belt 20 without being secondarily transferred to the transfer material P (hereinafter referred to as transfer residual toner) moves together with the intermediate transfer belt 20 and is charged by the conductive brush 31 . After that, the transfer residual toner moves together with the intermediate transfer belt 20 and is electrostatically transferred from the intermediate transfer belt 20 to the photosensitive drum 2 due to the potential difference between the photosensitive drum 2 and the intermediate transfer belt 20 when passing through the primary transfer portion N1. , and is collected by the cleaning means 6 .

[Recovery operation of residual toner]
The recovery operation of the transfer residual toner in the full color mode in this embodiment will be described in detail with reference to FIG. FIG. 4 is a schematic diagram for explaining the peripheral configuration of the conductive brush 31 in this embodiment. As shown in FIG. 4, the conductive brush 31 is arranged downstream of the secondary transfer portion N2 and upstream of the primary transfer portion N1a with respect to the moving direction of the intermediate transfer belt 20. A charging portion C is formed in contact with the transfer belt 20 .

^The conductive brush 31 is a brush member using nylon whose material is conductive. width) is 5 mm. The electric resistance value of the conductive brush 31 is 1.0 when the conductive brush 31 is pressed against the aluminum cylinder with a force of 9.8 N and rotated at 50 mm/sec and a voltage of 500 [V] is applied. ×10 ⁶ Ω.

As shown in FIG. 4, the conductive brush 31 is electrically connected to the charging power source 51 via the current detection means 71, and the charging power source 51 applies a positive or negative voltage to the conductive brush 31. It is possible. When the operation of collecting residual toner is performed, a positive voltage is applied from the charging power supply 51 to the conductive brush 31 . The output value of the voltage output from the charging power supply 51 is controlled (constant current control) by the controller 110 based on the current value detected by the current detection means 71 so as to reach a preset target current value. The target current value is appropriately set according to the design and environment of the image forming apparatus 10 so as not to excessively charge the transfer residual toner and to prevent cleaning failure due to insufficient charging of the transfer residual toner. In this embodiment, the target current value of the conductive brush 31 is set to 20 μA.

The toner carried on the intermediate transfer belt 20 before passing through the secondary transfer portion N2 is charged with the same negative polarity as the charge on the surface of the photosensitive drum 2, and in a state in which variations in charge distribution are small. are doing. On the other hand, the residual toner after passing through the secondary transfer portion N2 has a broad charge distribution and a distribution in which the peak shifts to the positive polarity side, which is the opposite polarity to the normal charging polarity of the toner. tend to have That is, the transfer residual toner is in a state in which negatively charged toner, hardly charged toner, and positively charged toner are mixed.

In the recovery operation of the transfer residual toner in this embodiment, the transfer residual toner passing through the position where the conductive brush 31 and the intermediate transfer belt 20 are in contact is removed by applying a positive voltage from the charging power supply 51 to the conductive brush 31 . Charge positively. At this time, part of the negatively charged toner among the transfer residual toner is electrostatically collected by the conductive brush 31 to which a positive voltage is applied.

As the intermediate transfer belt 20 moves, the toner positively charged by the conductive brush 31 reaches the primary transfer portion N1a of the image forming portion 1a arranged most upstream. At this time, by applying a positive voltage to the primary transfer roller 5a, the positively charged residual toner is electrostatically transferred from the intermediate transfer belt 20 to the photosensitive drum 2a. After that, the transfer residual toner that has moved to the photosensitive drum 2a moves with the rotation of the photosensitive drum 2a, and is collected by the cleaning means 6a by the cleaning blade 61a. By the operation described above, the transfer residual toner is removed from the intermediate transfer belt 20 .

In the full-color mode, transfer residual toner is removed from the intermediate transfer belt 20 in this manner. Note that, in the full-color mode, transfer residual toner that has not been collected by the image forming section 1a is collected by the image forming sections 1b to 1d in the same manner.

On the other hand, in the monochrome mode, as shown in FIG. 20 and reaches the primary transfer portion N1d. At this time, by applying a positive voltage from the primary transfer power source 40d to the primary transfer roller 5d, the positively charged residual toner is electrostatically transferred from the intermediate transfer belt 20 to the photosensitive drum 2d. . After that, the transfer residual toner that has moved to the photosensitive drum 2d moves with the rotation of the photosensitive drum 2d, and is collected by the cleaning means 6d by the cleaning blade 61d. In the monochrome mode, transfer residual toner is cleaned from the intermediate transfer belt 20 in this way.

[Blowing process]
If the recovery operation of the transfer residual toner is repeated and the amount of negative toner adhering to the conductive brush 31 increases, the charging performance of the transfer residual toner by the conductive brush 31 may deteriorate. Therefore, in the configuration of this embodiment, in order to suppress the deterioration of the charging performance of the conductive brush 31 due to the accumulation of toner on the conductive brush 31, the toner adhering to the conductive brush 31 is removed at a predetermined timing other than during image formation. An operation of discharging onto the intermediate transfer belt 20 is executed.

Specifically, when an image is not formed after an image forming operation is completed, a voltage having the same polarity as the normal charging polarity of the toner (negative polarity in this embodiment) is applied to the conductive brush 31 so that the conductive brush 31 is The negative toner adhering to the intermediate transfer belt 20 is transferred to the intermediate transfer belt 20 . Negative toner (hereinafter referred to as discharged toner) electrostatically moved from the conductive brush 31 to the intermediate transfer belt 20 reaches the primary transfer portion N1 as the intermediate transfer belt 20 moves. At this time, by applying a negative voltage from the primary transfer power supply 40 to the primary transfer roller 5 , the discharged toner that has moved from the conductive brush 31 to the intermediate transfer belt 20 is electrostatically transferred from the intermediate transfer belt 20 to the photosensitive drum 2 . to move. The discharged toner that has moved to the photosensitive drum 2 moves with the rotation of the photosensitive drum 2 and is collected by the cleaning means 6 by the cleaning blade 61 . By the above operation, the step of ejecting the negative toner adhering to the conductive brush 31 is executed.

As described above, in this embodiment, by executing the discharge process at a predetermined timing, the negative toner accumulated in the conductive brush 31 is discharged, and good cleaning performance is maintained. The toner ejected onto the intermediate transfer belt 20 in the ejection step may be collected by the photosensitive drum of at least one of the image forming units 1a to 1d.

The discharge process in the full-color mode and the discharge process in the monochrome mode of this embodiment will be described in more detail below with reference to FIGS. FIG. 5A is a schematic diagram for explaining the ejection process in the full-color mode of this embodiment, and FIG. 5B is a schematic diagram for explaining the ejection process in the monochrome mode of this embodiment.

As shown in FIG. 5A, a negative voltage is applied to the conductive brush 31, the primary transfer rollers 5a and 5d, and the secondary transfer roller 24 in the discharge process in the full color mode. That is, in this embodiment, the negative toner discharged from the conductive brush 31 is collected by the image forming units 1a and 1d. Although most of the discharged toner is collected by the image forming section 1a arranged on the upstream side in the rotation direction of the intermediate transfer belt 20, if the amount of discharged toner is large and is not collected by the image forming section 1a, It is also collected in the downstream image forming section 1d. As described above, in the full-color mode, toner discharged from the two image forming units 1a and 1d is collected.

The reason why a positive voltage is applied to the primary transfer rollers 5b and 5c is to reversely transfer a small amount of positive toner existing on the intermediate transfer belt 20 onto the photosensitive drums 2b and 2c for cleaning. .

The reason why the negative voltage is applied to the secondary transfer roller 24 in the ejection step is that if all the negative toner cannot be collected in the image forming units 1a and 1d, two negative toners are collected. This is to prevent adhesion to the next transfer roller 24 . As a result, it is possible to prevent the transfer material P conveyed to the secondary transfer portion N2 after the discharge process from being soiled by the toner adhering to the secondary transfer roller .

Here, in this embodiment, regarding the moving direction of the intermediate transfer belt 20, the configuration for collecting discharged toner in the most upstream image forming section 1a and the most downstream image forming section 1d has been described. However, in the full-color mode, primary transfer portions N1a to N1d are formed in all four image forming portions 1a to 1d. Therefore, discharged toner can be collected in any of the image forming units 1a to 1d, and the combination thereof may be freely set.

On the other hand, as shown in FIG. 5B, negative voltage is applied to the conductive brush 31, the primary transfer roller 5d, and the secondary transfer roller 24 in the ejection process in the monochrome mode. In the monochrome mode, since the primary transfer roller 5d is the only one in contact with the photosensitive drum 2d, the discharged toner discharged from the conductive brush 31 is collected only by the image forming portion 1d. As described above, in the monochrome mode, there are fewer image forming units for collecting discharged toner than in the full-color mode, and collection efficiency of the discharged toner tends to be lower than in the full-color mode.

[Conditions for the discharge process]
Next, the conditions for carrying out the discharge process in this embodiment will be described. In this embodiment, the toner adhering to the conductive brush 31 when forming an image on the transfer material P by secondarily transferring the toner image from the intermediate transfer belt 20 to the transfer material P (hereinafter simply referred to as image formation) Predict quantity. Then, when the amount of toner adhering to the conductive brush 31 exceeds a predetermined threshold value, it is determined that the charging performance of the conductive brush 31 is degraded, and the discharging step is performed.

First, a method for predicting the amount of toner adhering to the conductive brush 31 will be described. In the present embodiment, the average print rate of the image formed on the transfer material P (hereinafter referred to as a print image) is integrated for each number of the transfer material P on which the image is formed, so that the image adheres to the conductive brush 31 . Predict the amount of toner. The higher the average coverage rate of the printed image, the greater the amount of residual toner after transfer, and the greater the amount of toner adhering to the conductive brush 31 . Therefore, in this embodiment, the total amount of transfer residual toner reaching the conductive brush 31 is estimated by accumulating the average coverage rate of the printed image for each number of transfer materials P on which the image is formed by the controller 110. The amount of toner adhering to the conductive brush 31 is predicted.

Here, the average print rate of print images is calculated as follows. First, image information input from the personal computer 200 to the controller 110 is color-separated into time-series color image signals for each of the image forming units 1a to 1d in the controller 110. FIG. Next, in each image forming section 1, the ratio of the number of pixels emitted by the exposure means 7 (that is, the number of pixels forming an image) to the total number of image pixels is calculated, and the printing rate in each image forming section 1 is calculated. calculate. Then, by averaging the printing rate of each image forming unit 1, the average printing rate of the printed image is calculated. In the monochrome mode, the value obtained by dividing the printing rate of the fourth image forming section by 4 is taken as the average printing rate.

Table 1 is a table for explaining the method of accumulating the average coverage rate of printed images in this embodiment. As shown in Table 1, in this embodiment, the count is added for each page based on the average printing rate of the printed image, and the integrated value of the count (hereinafter simply referred to as the count value) is used as the conductive brush. The amount of toner adhering to 31 is predicted. That is, the larger the count value, the larger the amount of toner adhering to the conductive brush 31 . For example, when 10 images with an average print rate of 7.5% are formed, the count value is 20 because +2 counts are added per page.

Note that when the discharging process is performed, the toner is discharged from the conductive brush 31 onto the intermediate transfer belt 20, and the amount of toner adhering to the conductive brush 31 is reduced. Therefore, the count value is subtracted when the discharge process is performed. In this embodiment, the count value is decremented by 100 when the discharge process is performed in the full-color mode, and the count value is decremented by 50 when the discharge process is performed in the monochrome mode.

The reason why the amount of subtraction in the monochrome mode is smaller than that in the full-color mode is that, as described above, in the monochrome mode, only the image forming unit 1d collects discharged toner, and compared to the full-color mode, the amount of discharged toner collected is smaller. This is because the efficiency is low. In the discharging process, when the amount of discharged toner is large, part of the toner remains on the intermediate transfer belt 20 without being collected in the image forming section 1d in the monochrome mode. The toner remaining on the intermediate transfer belt 20 is collected again by the conductive brush 31 as the intermediate transfer belt 20 rotates during image formation after the ejection process. In other words, part of the toner ejected from the conductive brush 31 is collected by the conductive brush 31 again, so the amount of decrease in the amount of toner accumulated on the conductive brush 31 during the ejection process is small. Therefore, in this embodiment, the count subtraction amount in the monochrome mode is set to a smaller value than the count subtraction amount in the full-color mode.

As described above, in this embodiment, the amount of toner adhering to the conductive brush 31 is predicted by using the count value. Then, the controller 110 sequentially refers to the count value during image formation and compares it with a preset threshold value. When the count value exceeds the threshold value, it is judged that the amount of toner adhering to the conductive brush 31 exceeds the allowable amount and the charging performance is deteriorated, the interval between sheets is extended during image formation, and a discharge process is carried out.

In this embodiment, the thresholds are different between the full-color mode and the monochrome mode. The threshold Xfc (first threshold) is 200 in the full-color mode, and the threshold Xmc (third threshold) is 150 in the monochrome mode. The reason why the threshold value Xmc for the monochrome mode is set to a smaller value than the threshold value Xfc for the full-color mode is as follows.

As shown in FIGS. 3A and 3B, in the full-color mode, there are four image forming units 1a to 1d that collect the transfer residual toner positively charged by the conductive brush 31. There is only one 1d in monochrome mode. That is, even when the transfer residual toner is collected, the recovery efficiency of the transfer residual toner tends to be lower in the monochrome mode than in the full color mode. If the transfer residual toner remains on the intermediate transfer belt 20 without being completely collected, an image defect occurs when a toner image corresponding to the printed image is primarily transferred to the position where the transfer residual toner remains. end up That is, even if the charging performance of the conductive brush 31 is the same, image defects due to residual toner tend to occur more easily in the monochrome mode than in the full-color mode.

For the above reasons, it is necessary to keep the charging performance of the conductive brush 31 higher in the monochrome mode than in the full-color mode. Therefore, in this embodiment, the threshold value Xmc in the monochrome mode is set to a smaller value than the threshold value Xfc in the full-color mode, and the charging performance of the conductive brush 31 is maintained at a higher level.

As a result, in this embodiment, when forming a print image with the same average printing rate, the number of pages of the transfer material P to be discharged differs between the full-color mode and the monochrome mode, and the monochrome mode requires a smaller number of pages. A spitting step is performed. For example, when continuously forming images with an average print rate of 7.5%, the discharge process is performed after 100 pages in the full-color mode, whereas the discharge process is performed after 75 pages in the monochrome mode.

[Exhaling process when switching from full color mode to monochrome mode]
Next, a description will be given of the discharge process when switching from the full-color mode to the monochrome mode (full-mono switching), which is a feature of this embodiment. When a monochrome mode image is formed after a full-color image is formed, the contact state of the primary transfer roller is switched from the state shown in FIG. 3A to the state shown in FIG. 3B.

In this embodiment, the count value is referenced by the controller 110 immediately before the full mono switching is performed. Then, if the count value is equal to or greater than the threshold value Xch (second threshold value), the ejection process is performed in the full-color mode before shifting to the monochrome mode. Note that the threshold Xch is set to a value lower than the threshold Xfc in the full-color mode, and is set to 100 in this embodiment.

That is, in this embodiment, when forming images with the same average print rate, the discharge process with fewer pages is performed in the case of performing full-mono switching midway, compared to the case of continuously forming images only in the full-color mode. to implement. For example, when continuously forming images with an average print rate of 7.5%, the number of pages for which the ejection process is performed is 100 pages when the image formation is continuously performed only in the full-color mode. On the other hand, when performing the full mono switching after page 50, the discharge process is performed before performing the full mono switching. For example, when full mono switching is performed on page 75, the discharge process is performed on page 75.

The reason why the discharge process is performed in the full-color mode and then transitions to the monochrome mode at the time of full-mono switching is that the number of times the discharge process is performed in the entire image forming job is reduced, and the waiting time for image formation is suppressed. That's what it is. A detailed description will be given with reference to FIG. FIG. 6A is a graph showing the transition of the count value during image formation in Comparative Example 1, and FIG. 6B is a graph showing the transition of the count value during image formation in this embodiment. be. Here, Comparative Example 1 has a configuration in which the discharge process is not performed before switching to full mono. Further, Comparative Example 1 has many parts in common with the present embodiment except that the discharging process is not performed before the full mono switching. Therefore, in the following description, the same reference numerals as in the present embodiment are assigned to the configurations common to Comparative Example 1 and the present embodiment, and the description thereof is omitted.

FIGS. 6A and 6B show the transition of the count when the image is first formed in the full-color mode, then the full-mono switching is performed, and then the image is formed in the monochrome mode. In the full-color mode, 33 pages of images with an average print rate of 15% are formed, and in the subsequent monochrome mode, 30 pages of images with an average print rate of 7.5% (black print rate of 30%) are formed.

First, transition of the count value in Comparative Example 1 will be described with reference to FIG. As shown in FIG. 6A, in Comparative Example 1, an image with an average print rate of 15% is formed in the full-color mode, and +5 counts are added per page. count is added. After that, when the full-mono switching is performed and the mode is shifted to the monochrome mode, the count value 165 exceeds the count threshold Xmc of 150 in the monochrome mode, so the discharge process is performed. When the discharge process is performed in monochrome mode, the count value is 115 because 50 is subtracted from the count value.

In Comparative Example 1, 30 pages of images with an average print rate of 7.5% (black print rate of 30%) are further formed in the monochrome mode after the discharge process is completed. For an image with an average print rate of 7.5%, +2 counts are added per page. Therefore, when 18 pages of images are formed, the count value exceeds the threshold Xmc of 150 again. Therefore, the discharge process is performed again, and in Comparative Example 1, the discharge process is performed a total of two times in the current image forming job.

Next, transition of the count value in this embodiment will be described with reference to FIG. 6(b). As shown in FIG. 6B, in the full-color mode, an image with an average print rate of 15% is formed, and +5 counts are added per page, so a total of 165 counts are added for 33 pages. . After that, full-mono switching is attempted, but since the count value is equal to or greater than the threshold value Xch, the discharge process is performed in full-color mode before performing full-mono switching. When the discharge process is performed in full color mode, the count value is 65 because 100 is subtracted from the count value.

In this embodiment, after that, full mono switching is performed, and 30 more pages of images with an average print rate of 7.5% (black print rate of 30%) are formed in monochrome mode. For an image with an average print rate of 7.5%, +2 counts are added per page, and a total of 60 counts are added for 30 pages, but the count value does not exceed the threshold Xmc of 150. Therefore, in this embodiment, the ejection process is performed once in the current image forming job.

Both the time Tm required for the ejection process in the monochrome mode and the time Tf required for the ejection process in the full-color mode are determined by the time required for the toner ejected from the conductive brush 31 to reach the fourth image forming unit. Therefore, there is no difference between Tm and Tf, which are both 4.0 sec in this example and Comparative Example 1. In other words, the waiting time during the image forming job can be shortened in comparison with Comparative Example 1, in which the number of times the discharge process is simply performed is small.

As described above, in this embodiment, when the count value is equal to or greater than the threshold value Xch when full-mono switching is performed, full-mono switching is performed after performing the discharge process in the full-color mode, which has a high collection efficiency of the discharged toner. As a result, it is possible to reduce the frequency of the discharge process and reduce the waiting time for image formation.

[Wait time measurement result]
Next, experimental results comparing the standby time between Comparative Example 1 and the present embodiment will be described. In this experiment, in an image forming job that alternately repeats the full-color mode and the monochrome mode, the waiting time associated with the ejection process was measured, and the waiting time was compared between Comparative Example 1 and the present embodiment.

As an image forming job, 33 pages of images with an average print rate of 15% are formed in full-color mode, and then 30 pages of images are formed with an average print rate of 7.5% (black print rate of 30%) in monochrome mode. . Subsequently, 33 pages of images with an average print rate of 15% are formed in the full-color mode, and then 20 pages of images with an average print rate of 7.5% (black print rate of 30%) are formed in the monochrome mode.

The image forming apparatus 10 used in this experiment has a process speed of 180 mm/sec and a throughput of 30 sheets per minute. GF-C081 (manufactured by Canon Inc., product name) was used as the transfer material P, and the plain paper mode was selected as the image forming mode. Table 2 is a table showing the number of times the discharge process is performed and the waiting time during the image forming job in Comparative Example 1 and the present embodiment.

As shown in Table 2, in Comparative Example 1, in which the ejection process is not performed before the full-mono switching, the number of times of performing the ejection process in the monochrome mode, which has a low discharge toner collection efficiency, is large, and the number of times the ejection process is performed in the entire image forming job is high. The number of times the discharge process was performed was five in total. As a result, the waiting time for the entire image forming job was 20 seconds.

In contrast, in this embodiment, the count value is referred to before performing the full-mono switching, and if the count value is equal to or greater than the threshold value Xch, the discharge process is performed in the full-color mode. As a result, the number of discharge processes performed in the full-color mode, in which the discharged toner collection efficiency is high, was increased, and the number of times of the discharge process performed in the entire image forming job was three in total. As a result, the waiting time for the entire image forming job in this embodiment was 12 seconds, which was shorter than that in Comparative Example 1.

As described above, in this embodiment, the count value is referred to before performing the full-mono switching, and when the count value is equal to or greater than the threshold value Xch, the discharge process is performed in the full-color mode. With this configuration, it is possible to reduce the number of times the discharge process is performed during an image forming job and reduce the waiting time.

In this embodiment, the threshold value Xch of the count value for determining whether or not to perform the discharge process before performing the full mono switching is set to 100, but it is not limited to this. The threshold value Xch may be a value smaller than the threshold value Xfc in the full-color mode, and may be the same value as the threshold value Xmc in the monochrome mode, for example.

<Modification>
Also, in this embodiment, the configuration in which only the conductive brush 31 is provided as the charging member for charging the transfer residual toner has been described, but the present invention is not limited to this. For example, as a modification of this embodiment, as shown in FIG. 7, in order to obtain higher charging performance, a charging power supply 52 with a positive polarity is provided downstream of the conductive brush 31 with respect to the moving direction of the intermediate transfer belt 20 . A charging roller 32 (conductive member) to which a voltage is applied may be provided. As the charging roller 32, it is possible to use, for example, a nickel-plated steel bar covered with a solid elastic body made of EPDM rubber in which carbon is dispersed.

The detection means 72 is for detecting the current flowing through the charging roller 32. By controlling the output voltage from the charging power supply 52 so that the current value detected by the detection means 72 is constant, the conductive brush 31 is The transfer residual toner that has passed through is uniformly charged to a positive polarity. By providing the charging roller 32 in this way, it is possible to further improve the charging performance of charging the transfer residual toner.

In the configuration shown in FIG. 7, voltages are applied to the conductive brush 31 and the charging roller 32 from separate charging power sources 51 and 52, respectively.

In addition, in the modification shown in FIG. 7, when the discharge process is performed, the negative toner may be applied not only to the conductive brush 31 but also to the charging roller 32 . With such a configuration, the negative toner discharged from the conductive brush 31 is prevented from adhering to the charging roller 32, and the negative toner adhering to the charging roller is transferred onto the intermediate transfer belt 20. It is possible to exhale to

In this embodiment, a predetermined threshold stored in advance in the controller 110 is used as the threshold Xch, but it is not limited to this. For example, the threshold value Xch may be set based on image formation reservation information (hereinafter simply referred to as reservation information) acquired by the controller 110 instead of a fixed value. More specifically, for example, the number of images to be formed in the monochrome mode after switching to full mono and the average print rate may be acquired in advance as reservation information for image formation, and the threshold value Xch may be set based on the acquired reservation information. . By setting in this way, it is possible to perform the discharge process at a more appropriate timing, and it is possible to further improve the effect of reducing the waiting time under each image forming condition.

In this embodiment, when switching from the full-color mode to the monochrome mode, if the count value is equal to or greater than the threshold value Xch, the discharge process is performed in the full-color mode from which more image forming units collect the discharged toner. . However, the present invention is not limited to this, and if the time required for switching from the monochrome mode to the full-color mode (switching to the full-color mode) is extremely short, the ejection process may be performed entirely in the full-color mode. That is, when the count value exceeds Xmc during image formation in the monochrome mode, after performing the discharge process in the full-color mode by switching to the full-mono mode, the full-mono mode may be switched again to restart the image formation in the monochrome mode. In this way, by performing the discharge process in the full-color mode in which the discharged toner collection efficiency is high, it is possible to shorten the waiting time of the entire image forming job.

2 Photosensitive Drum 20 Intermediate Transfer Belt 24 Secondary Transfer Roller 31 Conductive Brush 110 Controller

Claims (9)

a first image carrier carrying a black toner image and a second image carrier carrying a toner image of a color different from that of the first image carrier; An endless intermediate transfer member onto which the toner image carried on at least one of the image carrier and the second image carrier is primarily transferred, and the toner image primarily transferred on the intermediate transfer member is transferred onto a transfer material. a secondary transfer member for performing secondary transfer, and a downstream side of a secondary transfer portion, which is a contact portion between the intermediate transfer member and the secondary transfer member, with respect to the movement direction of the intermediate transfer member; , the first image carrier and the second image carrier are provided upstream of a position where the intermediate transfer member contacts the intermediate transfer member, forming a charging portion in contact with the intermediate transfer member; a charging member that charges the toner remaining on the body at the charging portion; a charging power supply that applies a voltage to the charging member; a control means capable of executing an image forming operation by selecting a first mode in which only the first image bearing member is brought into contact with the intermediate transfer member and a second mode in which only the first image bearing member is brought into contact with the intermediate transfer member; In an image forming apparatus equipped with
The control means is
After the toner attached to the charging member is moved from the charging member to the intermediate transfer member by applying a voltage having the same polarity as the normal charging polarity of the toner from the charging power supply to the charging member, A discharging step of moving the toner transferred to the intermediate transfer member to at least one of the first image carrier and the second image carrier contacting the intermediate transfer member,
In the first mode, when the amount of toner adhering to the charging member exceeds a first threshold value, the first image carrier and the second image carrier are brought into contact with the intermediate transfer member. Execute the discharging step in a state of contact,
When switching from the first mode to the second mode and the amount of toner adhering to the charging member is equal to or greater than a second threshold smaller than the first threshold, the first switching to the second mode after executing the discharging step in the mode of
In the second mode, when the amount of toner adhering to the charging member is smaller than the first threshold value and exceeds a third threshold value equal to or greater than the second threshold value, the 1. An image forming apparatus according to claim 1, wherein said discharging step is performed while one image bearing member is in contact with said intermediate transfer member . a transfer power supply that applies a voltage to the secondary transfer member;
While the toner image is secondarily transferred from the intermediate transfer member to the transfer material in the secondary transfer portion, the control means controls the transfer power source to transfer the toner image to the secondary transfer member with a polarity opposite to the normal charging polarity of the toner. 2. The image according to claim 1, wherein a voltage having the same polarity as the normal charging polarity of the toner is applied from the transfer power source to the secondary transfer member while the voltage is applied and the discharging step is performed. forming device. The control means calculates an average printing rate of the image formed on the transfer material, and calculates an integrated value obtained by adding a count for each page of the transfer material set based on the average printing rate. 3. The image forming apparatus according to claim 1, wherein the amount of toner is predicted. 4. The control means executes the discharge operation in the first mode when determining that the integrated value is equal to or greater than the first threshold value in the first mode. The image forming apparatus according to . When the control means switches from the first mode to the second mode,
5. The image according to claim 3 , wherein when the integrated value is equal to or greater than the second threshold, switching to the second mode is performed after executing the ejection step in the first mode. forming device. In the second mode, the controller controls that the amount of toner adhering to the charging member is smaller than the first threshold value and exceeds a third threshold value equal to or greater than the second threshold value. when the first image bearing member is in contact with the intermediate transfer member, performing the discharging step,
6. The discharging operation in the second mode is performed when it is determined that the integrated value is equal to or greater than the third threshold value in the second mode. 2. The image forming apparatus according to item 1. 7. The image forming apparatus according to claim 3 , wherein said control means subtracts a predetermined value from said integrated value after executing said discharge step. 8. The image forming apparatus according to claim 1 , wherein the charging member is a brush member. Toner that has passed through the charging section is downstream of the charging section and upstream of the first image carrier and the second image carrier with respect to the moving direction of the intermediate transfer member. 9. The image forming apparatus according to claim 1 , further comprising a conductive member for charging.

Images