Patent Grant
12164238
The present disclosure relates to an image forming apparatus, such as a laser printer, a copying machine, and a facsimile machine, that obtains a recorded image by transferring a toner image electrophotographically formed on an image bearing member to a recording material.
An electrophotographic system has been known as an image recording system used in image forming apparatuses such as a printer and a copying machine. In the electrophotographic system, a toner image is formed by forming an electrostatic latent image on a photosensitive drum (hereinafter, may be referred to as a drum) with a laser beam using an electrophotographic process, and developing the electrostatic latent image with a charged color material (hereinafter, referred to as toner). The toner image is then transferred to a recording material and fixed for image formation. To reduce the size of an image forming apparatus, a cleaner-less system has recently been discussed. The cleaner-less system refers to a system for removing and collecting toner remaining on the surface of a drum after a transfer process by cleaning the toner at the same time with development using a developing unit, and reusing the collected toner.
With the foregoing cleaner-less system, the absence of a cleaning unit that usually is disposed on the photosensitive drum can cause an image defect since paper dust adheres to the photosensitive drum in the process of transfer to the recording material.
In view of this, Japanese Patent Application Laid-Open No. 2003-271030 discusses a configuration where a fixed brush is disposed downstream of a transfer portion and upstream of a charging portion in the direction of rotation of the photosensitive drum to collect paper dust adhering to the photosensitive drum in the transfer process.
However, the configuration discussed in Japanese Patent Application Laid-Open No. 2003-271030 has raised the following issue. In a state where the toner is new and not sufficiently charged by friction at the developing portion, a lot of fog toner can be developed on non-image portions of the drum as an initial characteristic of toner. A high proportion of such a fog toner has polarity opposite to normal polarity. As the number of printed sheets increases after the start of using new toner, and the charging of the toner stabilizes, the ratio of toner of normal polarity in the fog toner increases.
Since the cleaner-less system discussed in Japanese Patent Application Laid-Open No. 2003-271030 does not use a cleaner, some of the toner remaining on the drum surface after the transfer process accumulates on the brush. It has been found that the toner accumulated on the brush also changes in polarity with the foregoing change in the polarity of the fog toner at the initial stage where the use of the new toner is started.
As the polarity of the toner accumulated on the brush changes, the electrostatic sensitivity of toner discharge from the brush due to variations in the drum potential changes. As a result, an image defect can occur because of unintended transfer of toner from the brush to the drum.
The present disclosure is directed to an image forming apparatus including a brush in contact with a photosensitive drum, where an image defect resulting from toner accumulated on the brush is prevented.
According to an aspect of the present disclosure, an image forming apparatus includes an image bearing member that is rotatable, a charging member configured to charge a surface of the image bearing member at a charging portion opposed to the surface of the image bearing member, a developing member configured to supply toner charged to normal polarity to the surface of the image bearing member, a transfer member configured to come into contact with the image bearing member to form a transfer portion, and sandwich and convey a recording material and transfer the toner supplied to the image bearing member to the recording material at the transfer portion, a transfer voltage application unit configured to apply a transfer voltage having polarity opposite to the normal polarity to the transfer member, a brush configured to come into contact with the surface of the image bearing member to form a brush portion downstream of the transfer portion and upstream of the charging portion in a direction of rotation of the image bearing member, a brush voltage application unit configured to apply a brush voltage of the normal polarity to the brush, a storage unit configured to store information about use of the toner, and a control unit configured to control the transfer voltage application unit and the brush voltage application unit, wherein, after the toner supplied to the surface of the image bearing member is transferred to the recording material at the transfer portion, the developing member is configured to collect toner remaining on the surface of the image bearing member, and wherein, in a case where, in a state where a leading edge of the recording material in a conveyance direction of the recording material or a trailing edge of the recording material in the conveyance direction is sandwiched at the transfer portion, an area of the image bearing member in the conveyance direction where the recording material is sandwiched in a direction perpendicular to the conveyance direction at the transfer portion is a first area, and an area of the image bearing member in the conveyance direction where the recording material is not sandwiched in the direction perpendicular to the conveyance direction at the transfer portion is a second area, the control unit is configured to perform control so that a first potential difference formed between a surface potential formed on the second area and the brush voltage, which is based on first information stored in the storage unit, in a case where the second area reaches the brush portion and a second potential difference formed between the surface potential formed on the second area and the brush voltage, which is based on second information stored in the storage unit and different from the first information, in the case where the second area reaches the brush portion are different.
Further features of the present disclosure will become apparent from the following description of exemplary embodiments with reference to the attached drawings.
Exemplary embodiments of the present disclosure will be described in detail below with reference to the drawings. Dimensions, materials, shapes, and relative arrangement of components described in the exemplary embodiments are subject to appropriate changes depending on the configurations and various conditions of apparatuses to which the exemplary embodiments are applied. In other words, the following exemplary embodiments are not intended to limit the scope of the present disclosure.
1. Image Forming Apparatus
The image forming apparatus 100 according to the present exemplary embodiment is a monochrome laser beam printer using a cleaner-less contact charging system.
The image forming apparatus 100 according to the present exemplary embodiment includes a cylindrical photosensitive member serving as an image bearing member, i.e., a photosensitive drum 1. A charging roller 2 serving as a charging unit and a developing device 3 serving as a developing unit are disposed near the photosensitive drum 1. In
The photosensitive drum 1 according to the present exemplary embodiment is a negatively charged, organic photosensitive member. This photosensitive drum 1 includes a photosensitive layer on an aluminum drum-shaped base, and is driven to rotate at a predetermined process speed in the direction of the arrow in the diagram (clockwise) by a driving motor (driving unit) 110 (
The charging roller 2 that is a charging member is brought into contact with the photosensitive drum 1 with a predetermined pressure contact force, and driven to rotate on the photosensitive drum 1 while forming a charging portion. A charging voltage power supply 120 (
In the present exemplary embodiment, the exposure device 4 serving as the exposure unit is a laser scanner device. The exposure device 4 outputs laser light corresponding to image information input from an external apparatus such as a host computer, and scans and exposes the surface of the photosensitive drum 1 with the laser light. This exposure forms an electrostatic latent image (electrostatic image) on the surface of the photosensitive drum 1 based on the image information. In the present exemplary embodiment, the exposure by the exposure device 4 reduces the dark portion potential Vd formed on the surface of the photosensitive drum 1 by the uniform charging processing into a light portion potential V1 in absolute value. Here, the position where the photosensitive drum 1 is exposed by the exposure device 4 in the direction of rotation of the photosensitive drum 1 is referred to as an exposure portion (exposure position). The exposure device 4 is not limited to a laser scanner device. For example, a light-emitting diode (LED) array including a plurality of LEDs arranged along the longitudinal direction of the photosensitive drum 1 may be used.
In the present exemplary embodiment, a contact developing system is used as the developing system. The developing device 3 includes a developing roller 31 serving as a developing member and a developer bearing member, a toner supply roller 32 serving as a developer supply unit, a developer accommodation chamber (developer container, developer accommodation unit) 33 accommodating toner, and a developing blade 34.
Toner supplied from the developer accommodation chamber 33 to the developing roller 31 by the toner supply roller 32 passes through a blade nip that is a contact portion between the developing roller 31 and the developing blade 34, and is thereby charged to a predetermined polarity. At a developing portion, the toner borne on the developing roller 31 moves from the developing roller 31 to the photosensitive drum 1 depending on the electrostatic image. Here, the developing portion refers to the contact portion between the developing roller 31 and the photosensitive drum 1 in the direction of rotation of the photosensitive drum 1. In the present exemplary embodiment, the developing roller 31 and the photosensitive drum 1 are constantly in contact with each other. In the present exemplary embodiment, the developing roller 31 is driven to rotate counterclockwise so that the photosensitive drum 1 and the developing roller 31 move in a forward direction at the developing portion. As in the present exemplary embodiment, the driving motor 110 serving as the driving unit to drive the developing roller 31 may be a main motor 110 common with the driving unit of the photosensitive drum 1. Alternatively, the photosensitive drum 1 and the developing roller 31 may be rotated by respective different driving motors such as a photosensitive drum driving unit and a developing roller driving unit. During development, a predetermined developing voltage is applied to the developing roller 31 by a developing voltage power supply 140 (
In the following description, a potential or applied voltage of negative polarity having a large absolute value (for example, −1350 V with respect to −800 V) will be referred to as a high potential. A potential or applied voltage of negative polarity having a small absolute value (for example, −400 V with respect to −800 V) will be referred to as a low potential. The reason is that the toner with negative chargeability is assumed as a reference in the present exemplary embodiment.
In the present exemplary embodiment, voltages are expressed in terms of potential differences from a ground potential (0 V). The developing voltage Vdc=−400 V is thus interpreted as having a potential difference of −400 V from the ground potential because of the developing voltage applied to the core of the developing roller 31. The same applies to the charging voltage and the transfer voltage.
In the present exemplary embodiment, the toner charged with the same polarity as the charging polarity of the photosensitive drum 1 (in the present exemplary embodiment, negative polarity) adheres to exposed surfaces (image portions) that are image forming portions of the photosensitive drum 1 where the potential is reduced in absolute value by the exposure after the uniform charging processing. Such a developing system is referred to as a reversal developing system. In the present exemplary embodiment, the normal polarity that is the charging polarity of the toner during development is negative. While a one-component nonmagnetic contact development method is used in the present exemplary embodiment, the present disclosure is not limited thereto. A two-component nonmagnetic contact development method, a noncontact development method, or a magnetic development method may be used. The two-component nonmagnetic contact development method refers to a method in which a two-component developer including nonmagnetic toner and a magnetic carrier is used as the developer, and the developer (magnetic brush) borne on the developer bearing member is brought into contact with the photosensitive drum 1 for development. The noncontact development method refers to a method in which development is performed by causing toner to discharge from a developer bearing member opposed to a photosensitive member in a contactless manner to the photosensitive member. The magnetic development method refers to a method in which development is performed by magnetically bearing magnetic toner on a developer bearing member that includes a built-in magnet serving as a magnetic field generation unit and is opposed to a photosensitive member in a contact or contactless manner. In the present exemplary embodiment, toner having an average median particle diameter of 6 μm and a negative polarity as its normal charging polarity is used.
The transfer roller 5 serving as a transfer member suitably includes an elastic member made of sponge rubber such as polyurethane rubber, ethylene propylene diene monomer (EPDM) rubber, and nitrile butadiene rubber (NBR). The transfer roller 5 is pressed onto the photosensitive drum 1 to form a transfer portion where the photosensitive drum 1 and the transfer roller 5 are pressed against each other. During transfer, a transfer voltage power supply 160 (
The toner image is electrostatically transferred from the photosensitive drum 1 to a recording material (hereinafter, may be referred to as a sheet) S by the action of an electric field formed between the transfer roller 5 and the photosensitive drum 1.
A recording material S stored in a cassette 6 is fed by a feed unit 7 in synchronization with the timing when the toner image formed on the photosensitive drum 1 reaches the transfer portion. The recording material S is passed between a registration roller pair 8 and conveyed to the transfer portion. The toner image formed on the photosensitive drum 1 is transferred to the recording material S by the transfer roller 5 to which the predetermined transfer voltage is applied by the transfer voltage power supply 160 serving as the transfer voltage application unit.
After the transfer of the toner image, the recording material S is conveyed to a fixing device 9. The fixing device 9 is a film-heating fixing device including a not-illustrated fixing heater, a fixing film 91 including a not-illustrated built-in thermistor for measuring the temperature of the fixing heater, and a pressure roller 92 to be pressed against the fixing film 91. The recording material S is heated and pressed to fix the toner image, and is discharged outside the image forming apparatus 100 through a discharge roller pair 12.
In the present exemplary embodiment, a brush 10 (brush member) serving as a paper dust removal member is disposed in contact with the photosensitive drum 1 downstream of the transfer portion. The brush 10 removes paper dust transferred to the photosensitive drum 1 when the recording material S passes the transfer portion, from the photosensitive drum 1.
In the present exemplary embodiment, a pre-exposure device 13 serving as a pre-charging exposure unit is disposed to uniformize the potential of the photosensitive drum 1 after transfer downstream of the contact portion between the photosensitive drum 1 and the brush 10 and upstream of the charging portion in the direction of rotation of photosensitive drum 1. In the present exemplary embodiment, a not-illustrated LED disposed on a side surface of the main body is operated as the pre-exposure device 13, so that the photosensitive drum 1 is irradiated in a direction parallel to the main scanning direction of the photosensitive drum 1. A light guide serving as a light guide member for reducing irradiation nonuniformity in the main scanning direction may also be used.
Transfer residual toner remaining on the photosensitive drum 1 without being transferred to the recording material S passes the contact portion with the brush 10. After the potential of the photosensitive drum 1 is uniformized by the pre-exposure device 13, the charging roller 2 charges the transfer residual toner with the negative polarity again by using a discharge at the charging portion. As the photosensitive drum 1 rotates, the transfer residual toner charged to the negative polarity again by the charging roller 2 reaches the developing portion. The transfer residual toner having reached the developing portion moves to the surface of the developing roller 31 and is collected into the developer container 33.
2. Control Unit
Next, the control unit 200 will be described.
For example, the control unit 200 controls the applied voltages and exposure amounts of the charging voltage power supply 120, the developing voltage power supply 140, the exposure device 4, the transfer voltage power supply 160, the pre-exposure device 13, and a brush power supply 130. The control unit 200 also control the main motor (driving unit) 110. The image forming apparatus 100 forms an image on a recording material S based on an electrical image signal input to the controller 202 from the host apparatus. Examples of the host apparatus include an image reader, a personal computer, a facsimile, and a smartphone. The image forming operation and other control according to the present exemplary embodiment will be described below.
3. Brush Configuration
The image forming apparatus 100 includes the brush 10 that comes into contact with the surface of the photosensitive drum 1 at a brush portion. In the first exemplary embodiment, the brush 10 collects paper dust adhering to the surface of the photosensitive drum 1. The brush 10 comes into contact with the surface of the photosensitive drum 1 to form a contact portion downstream of the transfer portion and upstream of the charging portion in the direction of rotation of the photosensitive drum 1.
As illustrated in
The brush 10 is disposed with the longitudinal direction thereof parallel to the direction of the rotation axis of the photosensitive drum 1. Aside from nylon®, the threads 11a can be made of rayon, acrylic, and polyester materials. While conductive threads are used as the threads 11a in the first exemplary embodiment, insulating threads may be used. The threads 11a may be any thread-like articles and not limited to ones formed by twisting fibers.
As illustrated in
The dimension of the brush 10 in the longitudinal direction (in the direction parallel to the rotation axis of the photosensitive drum 1) is set so that the brush 10 comes into contact with the entire image forming area (area where a toner image can be formed) of the photosensitive drum 1 in the direction of the rotation axis of the photosensitive drum 1. The dimension of the brush 10 in the transverse direction (in a direction parallel to the circumferential direction or the direction of rotation of the photosensitive drum 1) is set as appropriate based on the life of the image forming apparatus 100 or the process cartridge.
The brush 10 is fixed at a constant position with respect to the photosensitive drum 1, and slides on the surface of the photosensitive drum 1 as the photosensitive drum 1 moves (rotates). The brush 10 catches (collects) adhering substances such as paper dust transferred from the recording material S to the photosensitive drum 1 at the transfer portion, and thereby reduces the amount of paper dust moving to the charging portion and the developing portion downstream of the brush 10 in the moving direction (direction of rotation) of the photosensitive drum 1.
In the first exemplary embodiment, the length L1 of the threads 11a of the brush 10 in the natural state is 4.8 mm. The amount of inroad of the brush 10 with respect to the photosensitive drum 1 is 1.5 mm (L2=3.3 mm). The brush 10 has a transverse length L3 of 5 mm, and a longitudinal length of 230 mm. The threads 11a have a fineness (thickness) of 2 deniers (expressing the thickness of a thread of 9000 m weighing 2 g), and a density of 240 kF/inch2 (kF/inch2 is a unit of brush density, indicating the number of filaments per square inch). The threads 11a are almost uniformly arranged from the bottom of the base fabric 11b to the tips that are the contact portion with the surface of the photosensitive drum 1. The transverse length of the brush 10 is just an example and not limited to the foregoing. The greater the transverse length of the brush 10, the longer period the brush 10 can collect paper dust for. The longitudinal length of the brush 10 is just an example and not limited to the foregoing. For example, the longitudinal length of the brush 10 can be set based on the maximum sheet-passing width of the image forming apparatus 100. Moreover, the fineness of the threads 11a of the brush 10 is just an example and not limited to the foregoing. The fineness of the threads 11a can be determined in consideration of the passability of paper dust. The brush 10 with a too small fineness has low capability of holding paper dust, and paper dust is likely to pass through. Paper dust having passed through the brush 10 can interfere with the charging of the photosensitive drum 1 by the charging roller 2 and cause an image defect. On the other hand, if the threads 11a of the brush 10 have a too large fineness, toner and fine paper dust are unable to be collected. This can make the amount of adhering toner uneven in the longitudinal direction of the charging roller 2 and cause an image defect due to uneven image density and insufficient charging at areas where paper dust adheres. The density of the threads 11a of the brush 10 is just an example and not limited to the foregoing. The density of the threads 11a can be set in consideration of the toner passability and paper dust collectability. If the density of the threads 11a of the brush 10 is too high, the toner can get stuck due to low toner passability, and the stuck toner can scatter to stain the interior of the image forming apparatus 100. If the density of the threads 11a of the brush 10 is too low, sufficient paper dust collection performance cannot be provided. In view of the paper dust collection performance, the fineness and density of the threads 11a are desirably 1 to 6 deniers and 150 to 350 kF/inch2, respectively. In view of long life, the transverse length L3 of the brush 10 is desirably 3 mm or more.
The brush power supply 130 serving as a brush voltage application unit is connected to the brush 10. During image formation, the brush power supply 130 applies a direct-current voltage of negative polarity to the brush 10 as a brush voltage.
4. Image Output Operation
The image forming apparatus 100 performs a series of operations to form an image on one or more recording materials S based on an instruction to start an image output operation (job) from an external apparatus (not illustrated) such as a personal computer. A job typically includes a pre-rotation step, an image formation step (printing step), a sheet interval step in the case of forming images on a plurality of recording materials S, and a post-rotation step. The image formation step includes forming an electrostatic image on the photosensitive drum 1, developing the electrostatic image (forming a toner image), transferring the toner image, and fixing the toner image. The period where the image forming step is performed is referred to as an image formation period. In the image formation period, i.e., in the period where the image formation step is performed, the operations such as the formation of the electrostatic image, the formation of the toner image, the transfer of the toner image, and the fixing of the toner image are performed at respective different timings. The pre-rotation step is a step of performing preparatory operations before the image formation step. The sheet interval step is a step performed between the image formation step on a first recording material S and the image formation step on a second recording material S subsequent to the first recording material S in continuously performing the image formation operations on a plurality of recording materials S (during continuous image formation). The post-rotation step is a step of performing rearranging operations (preparatory operations) after the image formation step. The periods other than the image formation period, i.e., the periods including the pre-rotation step, the sheet interval step, and the post-rotation step will be referred to as a non-image formation period. A preliminary rotation step of performing preparatory operations upon power-on of the image forming apparatus 100 or upon recovery from a sleep state is also included in the non-image formation period.
5. Mode of Control in Present Exemplary Embodiment
The control unit 200 is a control unit that controls operation of the image forming apparatus 100 in a centralized manner. The control unit 200 controls the transmission and reception of various electrical information signals and driving timing, and performs the predetermined image formation sequence. Various components of the image forming apparatus 100 are connected to the control unit 200. For example, as far as the present exemplary embodiment is concerned, the charging voltage power supply 120, the developing voltage power supply 140, the transfer voltage power supply 160, and the brush power supply 130 are connected to the control unit 200.
Next, to facilitate understanding of the issues to be described below and the control according to the present exemplary embodiment, basic control of various voltages, the surface potential formed on the photosensitive drum 1, and the transfer voltage will be described.
In the present exemplary embodiment, to uniformly charge the surface of the photosensitive drum 1, a charging voltage of −1350 V is applied to the charging roller 2. The photosensitive drum 1 is thereby charged to a non-image portion potential or dark portion potential Vd of −800 V. Next, the dark portion potential Vd formed by the uniform charging processing is reduced into an image portion potential or light portion potential V1 in absolute value by exposure by the exposure device 4. In the present exemplary embodiment, the light portion potential V1 is −100 V. Next, in the present exemplary embodiment, a developing voltage Vdc of −400 V is applied to the developing roller 31 to develop portions having the light portion potential V1. Moreover, in the present exemplary embodiment, a brush voltage of −400 V is applied to the brush 10.
Next, transfer control during a print operation will be described. If a print job is input, the photosensitive drum 1 and the developing roller 31 initially start to be driven to rotate, and the foregoing charging voltage and developing voltage are applied.
After the rotation speeds and the surface potential formed on the photosensitive drum 1 stabilize, the transfer voltage power supply 160 applies a voltage of positive polarity to the transfer roller 5. Here, the output voltage value from the transfer voltage power supply 160 is adjusted and sampled so that the value of the current flowing through the transfer roller 5 detected by a not-illustrated current detection circuit converges to a target current value. A resistance detection voltage value VO during non-sheet passing is thereby calculated. The control unit 200 then switches to constant voltage control in synchronization with timing when the leading edge of a recording material S (referred to as a recording material leading edge) enters the transfer portion in the conveyance direction of the recording material S. This constant voltage control includes applying a voltage (recording material leading edge voltage) the value of which is determined by calculation processing of multiplying the resistance detection voltage value V0 by a predetermined coefficient.
Then, when the recording material leading edge passes a certain distance from the transfer portion, the control unit 200 switches to constant current control. In the present exemplary embodiment, the target current value of the constant current control during sheet passing is 15 μA. In this constant current control interval, a resistance detection voltage value V1 during sheet passing is calculated. Next, the control unit 200 switches to constant voltage control a predetermined time before the trailing edge of the recording material S (referred to as a recording material trailing edge) enters the transfer portion in the conveyance direction of the recording material S. This control voltage control includes applying a voltage (recording material trailing edge voltage) the value of which is determined by multiplying the resistance detection voltage value V1 by a predetermined coefficient. At timing when the recording material trailing edge passes a predetermined distance from the transfer portion, the recording material trailing voltage is switched to a sheet interval voltage. In the case of a continuous sheet-passing job, the recording material leading edge voltage is applied again in synchronization with the subsequent recording material leading edge, and the foregoing control is repeated. After the transfer operation of the last image of the job is ended by such operations, the post-rotation operation is performed and stopped.
In the present exemplary embodiment, a sheet-to-sheet distance is 70 mm, which is shorter than the circumferential length of the photosensitive drum 1.
6. Mechanism for Discharging Toner from Brush
Next, to facilitate understanding of the control according to the present exemplary embodiment, a toner transfer operation from the brush 10 to the surface of the photosensitive drum 1, or a mechanism for discharging toner from the brush 10, will be described.
Initially, toner accumulation on the brush 10 will be described. There are two types of toner accumulated on the brush 10. One is fog toner, and the other is transfer residual toner. The fog toner is a part of toner coating the developing roller 31 that is transferred to non-image potential portions (portions having the dark portion potential Vd) formed on the surface of the photosensitive drum 1. The transfer residual toner is toner remaining on the surface of the photosensitive drum 1 after the developed toner in image potential portions (portions having the light portion potential V1) is transferred to a recording material S at the transfer portion. The two types of toner vary in the amount collected by the brush 10 and in a polarity ratio depending on the following factors: an electrostatic factor due to a potential difference between the surface potential formed on the photosensitive drum 1 and the brush voltage, and a physical factor due to being held at the gaps between the threads 11a and by the contact pressure of the threads 11a.
For example, the present exemplary embodiment employs an image forming apparatus 100 where the brush voltage is set to be sufficiently lower than the non-image portion potential Vd and higher than the image portion potential V1. As illustrated in
As a result, two types of toner, toner charged to positive polarity and toner charged to negative polarity, always accumulate on the brush 10, whereas the ratio depends on the polarities of the fog toner and the transfer residual toner.
Next, the tendency of the fog toner at the developing portion according to the present exemplary embodiment will be described. To examine the tendency of the fog toner at the developing portion of the image forming apparatus 100 according to the present exemplary embodiment, a fog toner concentration (%) on the surface of the photosensitive drum 1 was measured in the following manner.
Initially, the image forming apparatus 100 according to the present exemplary embodiment was activated in the same manner as with a print operation. Desired latent image settings were made by setting the charging voltage and the developing voltage to the above-described conditions. The rotational driving of the photosensitive drum 1 was then stopped. After the rotation driving of the photosensitive drum 1 was stopped, a polyester tape (manufactured by NICHIBAN Co., Ltd., No. 5511) was attached to the surface of the photosensitive drum 1 between the developing portion and the transfer portion in the direction of rotation of the photosensitive drum 1. The attached tape was peeled off to sample the fog toner on the surface of the photosensitive drum 1. The fog toner on the surface of the photosensitive drum 1 was sampled a plurality of times with different latent image settings, where a back contrast Vback, or a difference between the surface potential of the photosensitive drum 1 at the developing portion and the developing voltage, was set as appropriate from 50 V to 500 V in steps of 50 V. The strips of tape with the sampled fog toner from the surface of the photosensitive drum 1 were attached to Xerox Vitality Multipurpose Paper (Letter size, 20 lbs.). A degree of whiteness D1 (%) of the areas where the strips were attached and a degree of whiteness D2 (%) of the areas where the strips were not attached were measured using a fogging measuring instrument (product name: REFLECTOMETER MODEL TC-6DS, manufactured by Tokyo Denshoku Co., Ltd.). From the measurements, “D2(%)−D1(%)” was calculated as a fog toner concentration (%).
In such a manner, the fog toner concentration (%) on the surface of the photosensitive drum 1 was measured when the image forming apparatus 100 according to the present exemplary embodiment was new, i.e., when the toner was new, after printing of a total of 30 sheets, and after printing of a total of 100 sheets.
As illustrated in
As described above, the closer to the initial state the image forming apparatus 100 or the toner is, the more likely reversal fogging occurs and the less likely normal fogging occurs. It can be seen that the longer the image forming apparatus 100, i.e., the toner, is used, the less likely reversal fogging occurs and the more likely normal fogging occurs.
Transfer residual toner is considered to have a similar tendency to that of the fog toner in terms of the change in polarity. Specifically, if the same transfer voltage is applied, transfer residual toner is more likely to be reversal and includes a higher proportion of toner of positive polarity when the condition of the toner is closer to the new condition. Transfer residual toner becomes less likely to be reversal as the cumulative number of printed sheets increases. In other words, as the cumulative number of printed sheets increases, toner of negative polarity becomes more likely to reside.
The polarity of the toner accumulated on the brush 10 described above also changes with the foregoing change in the polarity of the fog toner and transfer residual toner in the initial stage when the image forming apparatus 100 is new. Specifically, if the condition of the image forming apparatus 100 is closer to the new condition, the ratio of toner of positive polarity accumulated on the brush 10 is higher. As the cumulative number of printed sheets increases, the ratio of toner of negative polarity increases.
Next, the transfer of the toner accumulated on the brush 10 to the surface of the photosensitive drum 1 (toner discharge) will be described. In the present exemplary embodiment, toner discharge caused by a change in the surface potential of the photosensitive drum 1 when the trailing edge of a recording material S passes the transfer portion will be described with reference to
Initially, nip areas when the trailing edge of a recording material S is passing the transfer portion (transfer nip portion) according to the present exemplary embodiment will be described with reference to
In the present exemplary embodiment, whether the recording material S is interposed in the conveyance direction of the recording material S is determined depending on whether the recording material S is present in a longitudinal direction perpendicular to the conveyance direction of the recording material S within the transfer nip surface.
The second nip area is divided into a gap wall portion D formed at the end of the recording material S as illustrated in
Next, a relationship between the surface potential of the photosensitive drum 1 after transfer and the brush voltage when the recording material S passes the transfer portion will be described with reference to
In the recording material trailing edge voltage interval, the constant voltage control produces a difference in the surface potential of the photosensitive drum 1 before and after recording material trailing edge passes the transfer portion. The reason is that the resistance of the transfer portion varies depending on the presence or absence of the recording material S. The contact portion after the passing of the recording material S passes a current from the photosensitive drum 1 to the transfer roller 5. The surface potential of the photosensitive drum 1 in the contact portion therefore drops. The surface potential after transfer on the surface of the photosensitive drum 1 corresponding to the contact portion will be denoted by Vb.
The brush voltage applied to the brush 10 will be denoted by Vc.
In light of the foregoing, toner discharge from the brush 10 due to a change in the surface potential of the photosensitive drum 1 will be described.
Toner of positive polarity accumulated on the brush 10 is mostly discharged due to a potential difference VA (=Va−Vc) from the surface potential Va of the photosensitive drum 1 locally higher than the brush voltage Vc. The discharged toner of positive polarity then adheres to the charging roller 2 located downstream in the direction of rotation of the photosensitive drum 1, and the charging roller 2 temporarily rotates with the toner adhered thereon. As a result, a white streak-like image defect (hereinafter, lateral white streak) occurs at rotation periods of the charging roller 2. The greater the potential difference VA on the negative polarity side (the higher the surface potential Va on the negative polarity side), the more toner is discharged from the brush 10 and the worse the lateral white streak becomes. The more toner of positive polarity the toner accumulated on the brush 10 includes, the more toner is discharged to the surface of the photosensitive drum 1.
If the polarity ratio of the toner accumulated on the brush 10 remains constant, the transfer voltage can be uniquely controlled so that the photosensitive drum 1 has an optimum surface potential to not cause a lateral white streak or a leading edge discharge. On the other hand, if the polarity ratio of the toner accumulated on the brush 10 changes with the cumulative number of printed sheets because of the foregoing change in the polarity of fog toner, the optimum surface potential of the photosensitive drum 1 at which a lateral white streak or a leading edge discharge does not occur changes. The potential difference formed between the surface potential of the photosensitive drum 1 and the brush voltage therefore needs to be controlled accordingly.
7. Control and Effect of Present Exemplary Embodiment
In view of the foregoing issue, in the present exemplary embodiment, the following control is performed to deal with the electrostatic sensitivity of the toner discharge from the brush 10 that changes with a change in the potential of the photosensitive drum 1 due to the polarity change of the toner accumulated on the brush 10. Characteristically, the surface potential of the photosensitive drum 1 is controlled by switching the transfer voltage to be applied to the recording material trailing edge based on the cumulative number of sheets printed by the image forming apparatus 100 that is used as information about the user of toner.
Details of the present exemplary embodiment will be described below with reference to
8. Effect
Next, a result of a sheet passing test conducted to examine the effect of the present exemplary embodiment will be described. The sheet passing test was performed under the following condition. In an environment of 23° C. in temperature and 50% in relative humidity, the following test was conducted using Xerox Vitality Multipurpose Paper (Letter size, 20 lbs.) as recording materials S. A two-sheet intermittent print job of leaving the entire first sheet blank and printing a 50%-density halftone image on the second sheet was repeated to a total of 200 sheets, and the second sheet of each job was checked for a lateral white streak and a leading edge discharge. Here, the transfer voltage at the recording material trailing edge according to the present exemplary embodiment was set to approximately 1750 V up to the 100th sheet, and approximately 1200 V at and after the 101st sheet as illustrated in
Table 1 illustrates the result of the foregoing sheet passing test conducted on a first comparative example where the transfer voltage at the recording material trailing edge was fixed at 1750 V, a second comparative example where the transfer voltage was fixed at 1200 V, and the present exemplary embodiment.
The result of Table 1 shows that in the first comparative example, a leading edge discharge occurred when the number of printed sheets is 101 to 200. In the second comparative example, a lateral white streak occurred before the number of printed sheets reaches 100 sheets after the condition of the image forming apparatus 100 was new. By contrast, neither of the image defects occurred in the present exemplary embodiment.
In the present exemplary embodiment, the recording material trailing edge voltage is switched from that in
As described above, the polarity of the toner accumulated on the brush 10 changes and the range of the lateral white streak and leading edge discharge OK potentials shifts depending on the cumulative number of printed sheets. The shift in the range of the lateral white streak and leading edge discharge OK potentials is unable to be dealt with by the constant transfer voltage setting as in the comparative examples. In the present exemplary embodiment, the occurrence of the lateral white streak and the leading edge discharge can be prevented by switching the recording material trailing edge voltage based on the cumulative number of printed sheets.
The image forming apparatus 100 according to the first exemplary embodiment has the following configuration and characteristics.
The image forming apparatus 100 includes the rotatable photosensitive drum 1, the charging roller 2 that charges the surface of the photosensitive drum 1 at the charging portion opposed to the surface of the photosensitive drum 1, and the developing roller 31 that supplies toner charged to normal polarity to the surface of the photosensitive drum 1. The image forming apparatus 100 further includes the transfer roller 5 that comes into contact with the photosensitive drum 1 to form the transfer portion, and sandwiches and conveys a recording material S and transfers the toner supplied to the photosensitive drum 1 to the recording material S at the transfer portion, and the transfer voltage application unit (transfer voltage power supply) 160 that applies the transfer voltage having polarity opposite to the normal polarity to the transfer roller 5. The image forming apparatus 100 further includes the brush 10 that comes into contact with the surface of the photosensitive drum 1 to form the brush portion downstream of the transfer portion and upstream of the charging portion in the direction of rotation of the photosensitive drum 1, and the brush voltage application unit (brush power supply) 130 that applies the brush voltage of the normal polarity to the brush 10. The image forming apparatus 100 further includes the memory 154 that stores information about the use of the toner, and the control unit 200 that controls the transfer voltage application unit 160 and the brush voltage application unit 130. The developing roller 31 is configured to, after the toner supplied to the photosensitive drum 1 is transferred to the recording material S at the transfer portion, collect toner remaining on the surface of the photosensitive drum 1.
In a state where the leading edge of the recording material S in the conveyance direction of the recording material S or the trailing edge of the recording material S in the conveyance direction is sandwiched at the transfer portion, the area of the photosensitive drum 1 in the conveyance direction where the recording material S is sandwiched in the direction perpendicular to the conveyance direction at the transfer portion is referred to as the first area. The area of the photosensitive drum 1 in the conveyance direction where the recording material S is not sandwiched in the direction perpendicular to the conveyance direction is referred to as the second area. A potential difference formed between the surface potential formed on the second area and the brush voltage in a case where the second area reaches the brush portion, which is determined based on first information stored in the memory 154, is referred to as a first potential difference.
A potential difference formed between the surface potential formed on the second area and the brush voltage in the case where the second area reaches the brush portion, which is determined based on second information stored in the memory 154 and different from the first information, is referred to as a second potential difference. In such a case, the control unit 200 performs control so that the first potential difference and the second potential difference are different.
The second area includes the gap wall portion D formed at the leading edge of the recording material S or the trailing edge of the recording material S and the contact portion where the photosensitive drum 1 and the transfer roller 5 are in contact with each other. The surface potential of a first surface that is the surface of the photosensitive drum 1 forming the gap wall portion D when the first surface reaches the brush portion is denoted by Va. The surface potential of a second surface that is the surface of the photosensitive drum 1 forming the contact portion when the second surface reaches the brush portion is denoted by Vb, and the brush voltage is denoted by Vc. In such a case, a potential difference formed between the surface potential of the first surface at the brush portion and the brush voltage, i.e., Va-Vc, is denoted by VA. A potential difference formed between the surface potential of the second surface at the brush portion and the brush voltage, i.e., Vb-Vc, is denoted by VB. The control unit 200 may sequentially control switching of the transfer voltage or the brush voltage to control the potential differences VA and VB. Here, the control unit 200 desirably makes the potential difference VA greater and the potential difference VB smaller in using the second information than in using the first information. In the present exemplary embodiment, the control unit 200 controls the recording material trailing edge voltage including the first and second areas to set the potential differences VA and VB within a suitable range.
The second information is information about the toner that is used longer than the toner in the first information. In forming the first potential difference based on the second information, the control unit 200 performs control so that the surface potential formed on the second area has an absolute value smaller than that of the brush voltage. The control unit 200 performs control so that the transfer voltage in forming the second potential difference at the contact portion is lower than in forming the first potential difference at the contact portion. In the present exemplary embodiment, if the second area forms the transfer portion, the control unit 200 performs control so that a first transfer voltage applied based on the first information stored in the memory 154 and a second transfer voltage applied based on the second information stored in the memory 154 are different. Specifically, the control unit 200 performs control so that the first transfer voltage has an absolute value greater than that of the second transfer voltage.
In the present exemplary embodiment, the recording material trailing edge voltage is switched to deal with the toner discharge from the brush 10. It will be understood, however, that if the brush voltage is variable, similar effects can be obtained by switching the brush voltage to control the potential difference between the surface potential of the photosensitive drum 1 after transfer and the brush voltage. The control unit 200 may perform control 0 so that the brush voltage in forming the second potential difference is lower than in forming the first potential difference. The control unit 200 may perform control so that a first brush voltage applied based on the first information stored in the memory 154 and a second brush voltage applied based on the second information stored in the memory 154 are different. Specifically, the control unit 200 may perform control so that the first brush voltage is higher than the second brush voltage.
While the control performed when the trailing edge of the recording material S passes the transfer nip portion has been described in the present exemplary embodiment, it will be understood that similar effects can be obtained by performing control when the leading edge of the recording material S passes the transfer nip portion.
The foregoing configuration of the first exemplary embodiment can prevent an image defect resulting from toner accumulated on the brush 10.
While in the present exemplary embodiment the recording material trailing edge voltage is switched based on a predetermined cumulative number of printed sheets as the threshold, this is not restrictive. For example, the recording material trailing edge voltage may be continuously changed based on the cumulative number of printed sheets.
While in the present exemplary embodiment the recording material trailing edge voltage is switched based on the cumulative number of printed sheets, this is not restrictive. For example, the cumulative number of rotations of the developing roller 31 may be used. Since the change in the polarity of the fog toner is caused by a change in the charging state of the toner due to friction in the developer container 33, the cumulative number of rotations is more direct than the cumulative number of printed sheets and even desirable in terms of accuracy. The cumulative number of rotations of the developing roller 31 will be described in detail in a second exemplary embodiment.
Alternatively, information about the remaining level of toner in the developer container 33 accommodating the toner may be used.
Next, a second exemplary embodiment of the present disclosure will be described. A basic configuration and operation of an image forming apparatus according to the second exemplary embodiment are similar to those of the image forming apparatus 100 according to the first exemplary embodiment. Components of the image forming apparatus of the second exemplary embodiment having functions or configuration similar or corresponding to those of the image forming apparatus 100 of the first exemplary embodiment are thus denoted by the same reference numerals as with the image forming apparatus 100 of the first exemplary embodiment. A detailed description thereof will be omitted.
In the first exemplary embodiment, the switching control of the recording material trailing edge voltage based on the change in the polarity of fog toner from the new condition has been described. In the present exemplary embodiment, switching control of the recording material trailing edge voltage based on a change in the polarity of fog toner due to toner degradation will be described.
The toner in the developer accommodation chamber 33 degrades gradually due to mechanical damage from agitation and sliding friction against the developing blade 34. Specifically, the toner drops in chargeability because of omission or embedding of additives contributing to the toner chargeability, or deformation of the toner itself. Such toner degradation worsens as the cumulative number of rotations of the developing roller 31 increases from the new condition.
In the present exemplary embodiment, the use amount of the developing roller 31 is used as an index for determining the cumulative number of rotations of the developing roller 31. It will be understood that the cumulative number of printed sheets may be used as described in the first exemplary embodiment.
The use amount of the developing roller 31 is defined by the following Eq. 1:
The use amount of the developing roller 31=the cumulative number of rotations of the developing roller 31÷the total number of rotations of the developing roller 31 at which an image defect can occur×100 (%). (Eq. 1)
Here, the use amount of a new developing roller 31 is 0%, and the use amount of the developing roller 31 at which an image defect such as a blank dot and a vertical streak can occur is 100%.
Next, a sheet passing test conducted to examine the use amount of the developing roller 31 and a change in the characteristic of fog toner according to the present exemplary embodiment will be described. The sheet passing test was performed under the following condition. In an environment of 23° C. in temperature and 50% in relative humidity, 5000 sheets were passed by two-sheet intermittent printing of an image with a printing ratio of 4%, using Xerox Vitality Multipurpose Paper (Letter size, 20 lbs.) as recording materials S. The amount of toner in a new developer accommodation chamber 33 was 100 g. It was assumed that when the sheet passing test consumed 80 g of the toner, the use amount of the developing roller 31 was considered to reach 100%.
In the process where the use amount of the developing roller 31 reached 100% in the foregoing test, the fog toner concentration was measured by using the same measurement method as in the first exemplary embodiment.
As can be seen from the result of
The image forming apparatus 100 according to the second exemplary embodiment has the following configuration and characteristics.
The control unit 200 performs the following control in controlling a third potential difference formed between the surface potential formed on the second area and the brush voltage in a case where the second area reaches the brush portion, which is determined based on third information about the toner that is used longer than the toner in the second information. The information about the use of the toner may be use information about the developing roller 31. In forming the third potential difference, the control unit 200 performs control so that the surface potential formed on the second area has an absolute value smaller than that of the brush voltage. It is important to control the transfer voltage in forming the third potential difference at the contact portion to be higher than the transfer voltage in forming the second potential difference at the contact portion. The control unit 200 thus performs control so that a third transfer voltage applied based on the third information is higher than the second transfer voltage. Here, the control unit 200 may perform control so that a third brush voltage in forming the third potential difference is higher than the second brush voltage in forming the second potential difference.
In the present exemplary embodiment, the toner degradation is associated with the use amount of the developing roller 31. However, this is not restrictive. For example, the remaining level of the toner in the developer accommodation chamber 33 may be used. The reason is that as the amount of toner in the developer accommodation chamber 33 is smaller, the frequency of agitation of a single toner particle and the sliding friction against the developing blade 34 becomes relatively higher, which causes the degradation to progress. Examples of a unit for detecting the remaining toner level are broadly classified into the following two types. One is a hardware prediction unit that predicts the remaining level by detecting a change in the behavior of the toner in the developer accommodation chamber 33 using a change in the degree of light transmission. The other is a software prediction unit that predicts the remaining toner level based on the consumption predicted by integrating the number of pixel signals of image information.
In such a manner, the toner degradation can be associated with either the cumulative number of rotations of the developing roller 31 or the remaining toner level. It will be understood that both can be used in combination for improved accuracy.
The foregoing configuration of the second exemplary embodiment can prevent an image defect resulting from toner accumulated on the brush 10 near the end of its life.
Next, a third exemplary embodiment of the present disclosure will be described.
In the present exemplary embodiment, control of an image forming apparatus 300 of toner replenishment type where the developer accommodation chamber 33 is replenished with toner will be described. A system for replenishing toner from a toner container 21 outside the main body of the image forming apparatus 300 as illustrated in
In the second exemplary embodiment, the switching control of the recording material trailing edge voltage based on a change in the polarity of the fog toner due to toner degradation has been described. In the present exemplary embodiment, control to be performed after the remaining level of the toner in the developer accommodation chamber 33 is equal to or less than a predetermined level and the developer accommodation chamber 33 is then replenished with toner again will be described.
If the developer accommodation chamber 33 is replenished with a large amount of toner after a decrease in the remaining toner level, the ratio of new toner increases. In such a case, fog toner is expected to behave similarly to the case where the toner is in the new condition described in the first exemplary embodiment. Suppose, for example, the use amount of the developing roller 31 reaches 100% in the sheet passing test according to the second exemplary embodiment and then 80 g of toner is replenished.
As described above, in the toner replenishment system according to the present exemplary embodiment, when toner is detected to be replenished, the use amount of the developing roller 31 starts to be calculated from 0% after the replenishment and referred, aside from the use amount of the developing roller 31 calculated before the replenishment. The recording material trailing edge voltage can thus be appropriately switched based on a change in the polarity of the fog toner corresponding to the use amount of the developing roller 31 after the toner replenishment, and an image defect due to toner discharge from the brush 10 can be prevented.
As has been described above, according to an exemplary embodiment of the present disclosure, an image defect resulting from toner accumulated on a brush can be prevented.
Embodiments of the present disclosure can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described Embodiments and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described Embodiments, and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described Embodiments and/or controlling the one or more circuits to perform the functions of one or more of the above-described Embodiments. The computer may include one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read-only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc™(BD)), a flash memory device, a memory card, and the like.
While the present disclosure has been described with reference to exemplary embodiments, it is to be understood that the disclosure is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions.
This application claims the benefit of Japanese Patent Application No. 2022-045082, filed Mar. 22, 2022, which is hereby incorporated by reference herein in its entirety.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2022-045082 | Mar 2022 | JP | national |
| Number | Name | Date | Kind |
|---|---|---|---|
| 20080193153 | Yoshikawa | Aug 2008 | A1 |
| Number | Date | Country |
|---|---|---|
| 05313431 | Nov 1993 | JP |
| 2003271030 | Sep 2003 | JP |
| 2005017448 | Jan 2005 | JP |
| 2005189319 | Jul 2005 | JP |
| 2006171247 | Jun 2006 | JP |
| Number | Date | Country | |
|---|---|---|---|
| 20230305421 A1 | Sep 2023 | US |
