JP2018036298A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent image defects including dirt inside the body of an image forming apparatus and marking back associated thereto, and shorten a cleaning time for recovering a fogged toner to a developing device.SOLUTION: An image forming apparatus recovers a toner remaining on an image carrier after transfer to a developing device with a developing member, and comprises a control unit that executes, in a non-image formation period, a cleaning operation to electrostatically transfer a toner adhered to a charging member and charged to a polarity opposite to a normal polarity from the charging member to the image carrier, and electrostatically transfer the toner from the image carrier to the developing member to be recovered to the developing device. After transferring the toner charged to the polarity opposite to the normal polarity from the charging member to the image carrier in the cleaning operation, the control unit performs, with image exposure means, whole surface exposure on the entire area of the image carrier to which the toner is transferred.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus that uses an electrophotographic recording system such as a laser printer, a copying machine, or a facsimile.

  Conventionally, in an electrophotographic image forming apparatus, a photosensitive drum as an image carrier is uniformly charged and then exposed according to an image pattern to form an electrostatic latent image on the photosensitive drum. Thereafter, the electrostatic latent image on the photosensitive drum is developed with toner, and is visualized and transferred to a recording material such as paper. Then, the transfer residual toner remaining on the photosensitive drum is removed from the photosensitive drum and collected.

  As a means for removing the transfer residual toner from the photosensitive drum and collecting it, a cleaning device having a cleaning member such as a cleaning blade is widely used. The toner collected by the cleaning device becomes waste toner, but it is desirable that the waste toner does not come out from the viewpoint of environmental conservation and effective use of resources. Further, it is desirable that no cleaning device is provided in view of miniaturization of the device.

  Therefore, in recent years, the “cleaner-less” image forming apparatus that eliminates the cleaning device, removes the transfer residual toner on the photosensitive drum from the photosensitive drum by “development simultaneous cleaning” by the developing device, collects it in the developing device, and reuses it. Has been made.

  On the other hand, as a means for charging the photosensitive drum, in recent years, a contact charging method in which a charging member is brought into contact with the image carrier and a voltage is applied to the charging member to charge the image carrier has been increasingly adopted. . Compared to the corona charging method, the contact charging method has a smaller amount of ozone generated by the charging process and requires a lower voltage. Therefore, the contact charging method is preferable from the viewpoint of the environment and downsizing of the apparatus, and is widely used. .

  By the way, in an image forming apparatus adopting a cleaner-less method and a contact charging method, a part of toner charged to a polarity opposite to the charging polarity at the time of development adheres to the charging member, and the charging ability is accumulated by accumulating this toner. May deteriorate.

  Therefore, a configuration in which the toner is collected at the developing unit is disclosed. In Patent Document 1, in the cleaning operation during non-image formation, the positively charged toner attached to the charging member is transferred to the photosensitive drum, and the transferred toner is electrostatically transferred in the direction from the developing member toward the photosensitive drum. The developing unit is passed through in an energized state. Then, the charged polarity of the toner that has passed is reversed to negative polarity in the charging portion, and the toner is collected in the developing portion.

JP-A-2006-71296

  However, in the image forming apparatus employing the cleanerless system and the contact charging system as disclosed in Patent Document 1, fog toner is transferred from the developing member to the photosensitive drum during the cleaning operation of the charging member during non-image formation. There was a problem that it was transferred.

  Specifically, in order to transfer the positively charged toner attached to the charging member to the photosensitive drum, the charging application bias is switched from HIGH to LOW. At that time, the potential difference between the photosensitive drum and the charging member becomes equal to or higher than the discharge start voltage Vth, reverse discharge from the photosensitive drum to the charging member occurs, and the surface potential of the photosensitive drum after passing through the charging member decreases. At this time, the surface potential of the photosensitive drum becomes unstable due to reverse discharge, and a portion where the absolute value of the surface potential is locally large and a portion where the absolute value is small are generated. Therefore, even when the bias value applied to the developing member is made variable according to the decrease in the surface potential of the photosensitive drum, there are portions where the white background contrast (hereinafter referred to as Vback) in the developing portion is locally large and small. .

  Here, the fog curve on the photosensitive drum with respect to Vback is shown in FIG. As shown in FIG. 5, when Vback is about Δ100 to 300 V, the fogging amount (%) on the photosensitive drum is small although the reverse fogging region is applied. However, ground fog toner starts to be generated abruptly when Vback is equal to or less than Δ100 V, and a large amount of ground fog toner is transferred from the developing member onto the photosensitive drum when Δback is V0. Therefore, the background fog toner is generated in the portion where Vback is locally near Δ0 V due to the reverse discharge from the photosensitive drum to the charging member, and the background fog toner stains the transfer member, or the background fog toner is transferred to the transfer nip portion. It will scatter and stain the interior of the machine, eventually leading to image defects such as back dirt. Further, it takes a cleaning time to collect the ground fog toner again by the developing member, and as a result, the life of the main body of the image forming apparatus is shortened.

  An object of the present invention is to prevent image defects such as dirt in the main body of an image forming apparatus and back dirt accompanying therewith, and to shorten a cleaning time for developing and collecting fog toner.

  In order to achieve the above object, the present invention exposes a rotatable image carrier, a charging member that contacts the image carrier to charge the image carrier, and the charged image carrier. An image exposing means for forming an electrostatic image; a developing device including a developing member for supplying a toner charged with a normal polarity to the electrostatic image of the image bearing member to form a toner image; and A transfer member that electrostatically transfers a toner image to a transfer target, and the toner remaining on the image carrier after the transfer is collected by the developing device by the developing member; The toner charged to a polarity opposite to the normal polarity attached to the charging member is electrostatically transferred from the charging member to the image carrier, and electrostatically transferred from the image carrier to the developing member. Cleaning operation that is collected by the developing device during non-image formation A control unit that executes the control unit, wherein the control unit transfers toner charged to a polarity opposite to the normal polarity from the charging member to the image carrier in the cleaning operation; The entire area of the transferred area of the toner is exposed entirely by the image exposure means.

  According to the present invention, in the cleaning operation of the charging member, the entire surface area of the image carrier to which the toner having the polarity opposite to the normal polarity adheres is exposed, so that the surface potential of the unstable image carrier is obtained. And the toner can be collected by the developing device without transferring fog toner from the developing member to the image carrier. As a result, image defects such as dirt in the main body of the image forming apparatus and the back dirt accompanying therewith can be prevented, and further, the cleaning time for developing and collecting fog toner can be shortened.

1 is a diagram illustrating an image forming apparatus according to Embodiment 1. FIG. FIG. 3 is a diagram illustrating a schematic control mode of the image forming apparatus according to the first embodiment. The figure which shows the timing chart of the cleaning operation in Example 1. The figure which shows the timing chart of the cleaning operation in Example 2. The figure which shows the fog curve on the photosensitive drum with respect to the conventional Vback.

  Hereinafter, exemplary embodiments of the present invention will be described in detail with reference to the drawings. However, the dimensions, materials, shapes, and relative arrangements of the components described in the following embodiments should be appropriately changed according to the configuration of the apparatus to which the present invention is applied and various conditions. Therefore, unless specifically stated otherwise, the scope of the present invention is not intended to be limited thereto.

[Example 1]
<Image forming apparatus>
A schematic configuration of the image forming apparatus 100 will be described with reference to FIG. FIG. 1 is a schematic configuration diagram of the image forming apparatus 100.

  The image forming apparatus 100 of the present embodiment is an electrophotographic image forming apparatus that employs a cleanerless system and a contact charging system, and a laser beam printer is illustrated here.

  The image forming apparatus 100 includes a photosensitive drum 1 that is a drum-type (cylindrical) electrophotographic photosensitive member as a rotatable image carrier. When the image output operation is started, the photosensitive drum 1 is rotationally driven in the direction of arrow R1 in the figure by a drive motor (not shown). The surface of the rotating photosensitive drum 1 is uniformly charged to a predetermined potential having a predetermined polarity (negative polarity in this embodiment) by a charging roller 2 which is a roller-type charging member as a charging unit. The charging roller 2 is disposed in contact with the photosensitive drum 1 and is rotationally driven in the direction of arrow R2 in the figure by a drive motor. At this time, a predetermined charging voltage (charging bias), which is a negative DC voltage, is applied to the charging roller 2 from a charging power source E1 (FIG. 2) as charging voltage application means. A contact portion between the photosensitive drum 1 and the charging roller 2 is a charging nip a. Further, the position where the charging process is performed by the charging roller 2 on the photosensitive drum 1 in the rotation direction of the photosensitive drum 1 is a charging unit. The charging roller 2 is caused by a discharge generated at least one of the gaps formed between the charging roller 2 upstream and downstream of the charging nip a and the photosensitive drum 1 in the rotation direction of the photosensitive drum 1. Charge the surface. In the present embodiment, for the sake of easy understanding, the charging nip may be described as the charging portion a on the assumption that the surface of the photosensitive drum 1 is charged in the charging nip.

  The surface of the charged photosensitive drum 1 is scanned and exposed by a laser beam L modulated in accordance with image data by an exposure device (laser exposure unit) 3 as image exposure means (electrostatic image forming means). The exposure device 3 performs exposure in the sub-scanning direction (surface movement direction) while repeating exposure in the main scanning direction (rotation axis direction) of the photosensitive drum 1 by the laser beam L, thereby electrostatically forming on the photosensitive drum 1. A latent image is formed. In the rotation direction of the photosensitive drum 1, the exposure position by the exposure device 3 on the photosensitive drum 1 is the image exposure unit b.

  The electrostatic latent image formed on the photosensitive drum 1 is developed (visualized) as a toner image using toner as a developer by a developing device 4 as developing means. The developing device 4 includes a developing container 45 and a developing sleeve 41 as a developing member (developer carrier) that is rotatably supported by the developing container 45. The developing container 45 contains a black toner T which is a magnetic one-component developer as a developer. The toner T of this embodiment has a negative charging characteristic. That is, in this embodiment, the normal polarity of toner T (charging polarity during development) is negative. The developing sleeve 41 is disposed in an opening provided at a position facing the photosensitive drum 1 of the developing container 45 so that a part thereof is exposed to the outside. The developing sleeve 41 is provided with a conductive elastic rubber layer having a predetermined volume resistance around a hollow non-magnetic metal (such as aluminum) element tube. In the hollow portion of the developing sleeve 41, a magnet roller 43 as a magnetic field generating means is fixed and arranged.

  The toner T accommodated in the developing container 45 is stirred by the stirring member 44 and supplied to the surface of the developing sleeve 41 by the magnetic force of the magnet roller 43. The toner T supplied to the surface of the developing sleeve 41 passes through a portion facing the developing blade 42 as the developer regulating means as the developing sleeve 41 rotates, so that the toner T is uniformly thinned and frictionally charged. To be negatively charged. Thereafter, the toner on the developing sleeve 41 is transported to a developing position where it contacts the photosensitive drum 1 as the developing sleeve 41 rotates, and is transferred to the photosensitive drum 1 in accordance with the electrostatic latent image on the photosensitive drum 1 to be photosensitive. The electrostatic latent image on the drum 1 is developed. At this time, a predetermined developing voltage (developing bias) that is a negative DC voltage is applied to the developing sleeve 41 from a developing power supply E2 (FIG. 2) as a developing voltage applying means. In this embodiment, a toner image is formed by image portion exposure and reversal development. That is, the same polarity as the charged potential of the photosensitive drum 1 (in this embodiment, the exposed portion (image portion) on the photosensitive drum 1 where the absolute value of the potential is reduced by exposure after being uniformly charged) Negatively charged toner adheres.

  In the rotation direction of the photosensitive drum 1, a position facing (contacting) the developing sleeve 41 on the photosensitive drum 1 is a developing unit c.

  In this embodiment, the developing sleeve 41 is rotationally driven in the direction of arrow R3 in the figure by a drive motor (not shown) so that the moving direction of the photosensitive drum 1 and the developing sleeve 41 is the same in the developing portion c. The

  The toner image formed on the photosensitive drum 1 is sent to a transfer portion d that is a contact portion between the photosensitive drum 1 and a transfer roller 5 that is a roller-type transfer member as a transfer unit. Further, in synchronization with the toner image on the photosensitive drum 1, the recording material P such as a recording sheet, which is a transfer target, is conveyed from the storage unit 8 to the transfer unit d by the conveyance roller 9. Then, the toner image on the photosensitive drum 1 is transferred onto the recording material P that is nipped and conveyed between the photosensitive drum 1 and the transfer roller 5 by the action of the transfer roller 5 in the transfer portion d. At this time, the transfer roller 5 receives a predetermined transfer voltage (a positive voltage in this embodiment) from a transfer power source E3 (FIG. 2) as a transfer voltage application unit. Transfer bias) is applied. As a result, the toner image is electrostatically transferred from the photosensitive drum 1 to the recording material P by the action of an electric field formed between the transfer roller 5 and the photosensitive drum 1.

  The recording material P to which the toner image has been transferred is sent to a fixing device 7 as fixing means. In the fixing device 7, heat and pressure are applied to the recording material P, and the toner image transferred to the recording material P is fixed to the recording material P.

  On the other hand, transfer residual toner (residual toner) that is not transferred to the recording material P and remains on the photosensitive drum 1 is collected by the developing device 4 by simultaneous development cleaning. In other words, the developing device 4 has a function of supplying a negatively charged toner T to the electrostatic image on the photosensitive drum 1 in the developing unit c when a voltage is applied, and a transfer residue remaining on the photosensitive drum 1 after the transfer. It also has a function of collecting toner. Details of the simultaneous development cleaning will be described later.

  Here, the image forming apparatus 100 performs a series of image output operations (jobs) for forming an image on a single or a plurality of recording materials P, which is started by an instruction from an external device (not shown). A job generally includes an image forming process (printing process), a pre-rotating process, a paper-to-paper (inter-recording-material) process when images are formed on a plurality of recording materials P, and a post-rotating process. The image forming process is a period in which an electrostatic latent image is actually formed on the photosensitive drum 1, an electrostatic latent image is developed, a toner image is transferred, and a toner image is fixed. The timing of the image forming process varies depending on the position where each process such as development, transfer, and fixing is performed. The pre-rotation process is a period for performing a preparatory operation before the image forming process. The inter-sheet process is a period corresponding to the interval between the recording material P and the recording material P in the transfer portion d when the image forming process is continuously performed on the plurality of recording materials P. The post-rotation process is a period during which an organizing operation (preparation operation) after the image forming process is performed. The image forming process is when an image is formed, and a period other than the time when the image is formed (such as a pre-rotation process, a sheet-interval process, and a post-rotation process) is a non-image formation period. In this embodiment, a cleaning operation for discharging the toner adhering to the transfer roller 5 onto the photosensitive drum 1 is executed at a predetermined timing during non-image formation.

<Simultaneous development cleaning>
Details of the simultaneous development cleaning will be described. The image forming apparatus 100 according to the present exemplary embodiment includes a pre-exposure device 6 serving as a discharging unit that discharges the photosensitive drum 1 on the downstream side of the transfer unit d and the upstream side of the charging unit a in the rotation direction of the photosensitive drum 1. Yes. The pre-exposure device 6 performs photostatic discharge on the surface potential of the photosensitive drum 1 before entering the charging unit a in order to generate a stable discharge at the charging unit a. In the rotation direction of the photosensitive drum 1, the exposure position by the pre-exposure device 6 is the charge removal unit e. As the transfer residual toner, a toner charged to a positive polarity and a toner charged to a negative polarity but not having a sufficient charge are mixed. These toners can be negatively charged again by discharging the photosensitive drum 1 after transfer by the pre-exposure device 6 and generating a uniform discharge during the charging process.

  The toner charged negatively in the charging unit a is sent to the developing unit c as the photosensitive drum 1 rotates. The toner sent to the developing section c is transferred to the developing sleeve 41 in the non-image area (non-exposed area) due to the potential difference between the dark portion potential (Vd) on the surface of the photosensitive drum 1 and the developing bias (Vdc). Collected by the developing device 4. On the other hand, the image area (exposure area) is not transferred to the developing sleeve 41 due to a potential difference between the light portion potential (Vl) on the surface of the photosensitive drum 1 and the developing bias (Vdc), and is directly used as an image portion of the photosensitive drum 1. With rotation, it is sent to the transfer section d and transferred to the recording material P. Vdc is set to a potential between Vd and Vl.

<Control mode>
FIG. 2 is a block diagram illustrating a schematic control mode of a main part of the image forming apparatus 100. A control unit 150 serving as a control unit provided in the image forming apparatus 100 includes a CPU 151 that is a central element that performs arithmetic processing, a memory 152 such as a ROM and RAM that are storage elements, and the like. The RAM stores sensor detection results, calculation results, and the like, and the ROM stores control programs, data tables obtained in advance, and the like. The control unit 150 is a control unit that comprehensively controls the operation of the image forming apparatus 100, and controls transmission / reception of various electrical information signals, driving timing, and the like, and performs predetermined image forming sequence control and the like. Control. Each control object in the image forming apparatus 100 is connected to the control unit 150. For example, a charging power source E1, a developing power source E2, a transfer power source E3, a pre-exposure device 6 and the like are connected to the control unit 150. In particular, in relation to the present embodiment, the control unit 150 controls ON / OFF of various power sources E1, E2, and E3, output values, ON / OFF of the discharge light emitted by the pre-exposure device 6, and the like. Then, a cleaning operation of the transfer roller 5 described later is executed.

<Cleaning operation of the charging roller>
As described above, the transfer residual toner is negatively charged in the charging unit a, passes through the charging unit a, and is sent to the developing unit c. However, there are some toners that continue to adhere to the charging roller 2 without being sufficiently negative in the charging portion a. If image formation is continued with the toner attached to the charging roller 2, the charging performance may deteriorate due to the toner adhering to the charging roller 2. Therefore, it is desired that the toner attached to the charging roller 2 is attached to the photosensitive drum 1 and sent to the developing unit c and returned to the developing device 4 at a predetermined timing. However, as a result of studies by the present inventors, the following has been found regarding toner adhesion to the charging roller 2. That is, if toner adheres to the charging roller 2, when the photosensitive drum 1 is charged by the discharge from the charging roller 2, the toner adhering to the charging roller 2 increases due to the discharge phenomenon. Therefore, even if the charging polarity of the toner is reversed to the negative polarity by friction, the toner charged to the positive polarity may not be sufficiently reversed to the negative polarity.

  Therefore, in this embodiment, the control unit 150 executes a cleaning operation for reducing the toner adhering to the charging roller 2 as described below at a predetermined timing during non-image formation. That is, in the cleaning operation, the control unit 150 electrostatically transfers the positively charged toner attached to the charging roller 2 from the charging roller 2 to the photosensitive drum 1 while rotating the photosensitive drum 1. Further, the control unit 150 allows the transferred toner to pass through the developing unit c in a state of being electrostatically biased in the direction from the developing sleeve 41 toward the photosensitive drum 1. Further, the control unit 150 inverts the charged polarity of the toner that has passed through to negative polarity in the charging unit a. Then, the control unit 150 electrostatically transfers the toner, whose charging polarity is reversed, from the photosensitive drum 1 to the developing sleeve 41 in the developing unit c and causes the developing device 4 to collect the toner. This will be described in more detail below.

  In this embodiment, the positively charged toner attached to the charging roller 2 is electrostatically (electrically) attached to the photosensitive drum 1 while being charged positively. That is, by setting the voltage applied to the charging roller 2 to a voltage on the positive side with respect to the surface potential of the photosensitive drum 1, the toner is electrostatically attached from the charging roller 2 to the photosensitive drum 1 while being charged positively. Let The positively charged toner adhering to the photosensitive drum 1 is allowed to pass through the developing unit c without being collected by the developing unit 4 in the developing unit c due to the charged polarity. Thereafter, the area of the photosensitive drum 1 to which the positively charged toner is attached is charged by discharging from the charging roller 2, and the charged polarity of the positively charged toner is reversed to the negative polarity. At this time, in this embodiment, the region of the photosensitive drum 1 to which the positively charged toner is attached is subjected to a charge process by the charging roller 2 after being subjected to photostatic discharge by the pre-exposure device 6. Then, the negatively charged toner passes through the charging portion a, is electrostatically transferred to the developing sleeve 41 in the developing portion c, and is collected by the developing device 4.

  FIG. 3 is a timing chart of the cleaning operation of the charging roller 2 of this embodiment. The cleaning operation of the charging roller 2 is executed by the control unit 150 controlling the operation of each unit at the timing shown in FIG. In this embodiment, when the number of output images exceeds a predetermined threshold, the cleaning operation of the charging roller 2 is executed in the post rotation (during non-image formation).

Timing (a):
3A shows the timing when the toner image formed on the photosensitive drum 1 passes through the transfer portion d after the image exposure in the printing process is completed. At this timing, the transfer bias is changed from HIGH to LOW. By setting the transfer bias to LOW, charge inflow from the transfer roller 5 is eliminated, and a decrease in the surface potential of the photosensitive drum 1 after passing through the transfer portion d is suppressed. At the same time, the pre-exposure device 6 is turned OFF to eliminate the decrease in the surface potential of the photosensitive drum 1 due to the light neutralization before reaching the charging portion a. In this way, the surface potential of the photosensitive drum 1 is maintained by setting the transfer bias to LOW and turning off the pre-exposure device 6. Even after the printing process is finished, the charging bias and the developing bias remain HIGH. That is, a predetermined charging voltage is applied to the charging roller 2 to charge the photosensitive drum 1, and a predetermined developing voltage is applied to the developing sleeve 41 when the region of the photosensitive drum 1 subjected to the charging process passes through the developing portion c. Is applied.

Timing (b):
Next, charging is performed at a timing when the region of the photosensitive drum 1 that has passed through the transfer portion d with the transfer bias being LOW and passed through the charge removal portion e with the pre-exposure device 6 turned off reaches the charging portion a. The bias is changed from HIGH to LOW. As a result, the charging bias becomes a voltage higher than the surface potential of the photosensitive drum 1 on the positive side (for example, a voltage having the same polarity as the surface potential of the photosensitive drum 1 and a small absolute value). In this embodiment, the LOW charging bias applied to the charging roller 2 at this time is set so that the potential difference between the photosensitive drum 1 and the charging roller 2 in the charging portion a is equal to or higher than the discharge start voltage Vth. Therefore, reverse discharge from the photosensitive drum 1 to the charging roller 2 occurs, and the absolute value of the surface potential of the photosensitive drum 1 becomes small. That is, the absolute value of the surface potential of the photosensitive drum 1 is smaller than that immediately before reaching the charging unit a by passing through the charging unit a. Then, the positively charged toner attached to the charging roller 2 is electrostatically attached to the photosensitive drum 1 while being charged positively. A small amount of toner having no polarity on the charging roller 2 is negatively charged by the reverse discharge. That is, the voltage applied to the charging roller 2 is set to be higher than the surface potential when the region of the photosensitive drum 1 charged in step (a) reaches the charging unit a when the region passes through the charging unit a. Change to a higher voltage on the positive polarity side.

Timing (c):
Next, at the timing when the region of the photosensitive drum 1 to which the positively charged toner is attached at the timing (b) reaches the image exposure unit b, the entire region of the region to which the toner is attached is exposed. The whole surface exposure is to expose the entire length of the photosensitive drum 1 with the same amount of light as the image exposure during the printing process. As a result, the absolute value of the surface potential of the exposed area of the entire surface of the photosensitive drum 1 is uniformly reduced. That is, the absolute value of the surface potential of the photosensitive drum 1 is uniformly reduced by passing through the image exposure unit b than immediately before reaching the image exposure unit b. By this overall exposure, the surface potential of the photosensitive drum 1 that is unstable due to reverse discharge from the photosensitive drum 1 to the charging roller 2 can be stabilized between timings (b) to (c). At this time, the positively charged toner that is electrostatically attached to the photosensitive drum 1 is electrostatically attached to the photosensitive drum 1 even after passing through the image exposure portion b.

Timing (d):
Next, the developing bias is changed from HIGH to LOW at the timing when the area of the photosensitive drum 1 that has been exposed at the timing (c) reaches the developing portion c. That is, the absolute value of the surface potential of the photosensitive drum 1 is uniformly reduced by the overall exposure. Therefore, correspondingly, the developing bias is set to a voltage higher than the surface potential of the photosensitive drum 1 on the positive polarity side (for example, a voltage having the same polarity as the surface potential of the photosensitive drum 1 and a small absolute value). Set development bias to LOW. By setting the developing bias to LOW, most of the positively charged toner on the photosensitive drum 1 passes through the developing portion c while being electrostatically urged from the developing sleeve 41 toward the photosensitive drum 1. To do. At this time, a part of the positively charged toner on the photosensitive drum 1 may be collected by the developing device 4. That is, the voltage applied to the developing sleeve is more positive than the predetermined developing voltage when the photosensitive drum region in which the absolute value of the surface potential is uniformly reduced by the step (c) passes through the developing portion c. Change to a higher voltage on the side.

Timing (e):
Next, after the charging roller 2 is rotated by one turn or more in a state where the charging bias is LOW, the charging bias is returned to HIGH. By increasing the charging bias to a negative polarity side with respect to the surface potential of the photosensitive drum 1 (increasing the absolute value with the same polarity), a small amount of negative polarity remaining on the charging roller 2 by applying the LOW charging bias is obtained. The charged toner is electrostatically attached to the photosensitive drum 1. The timing for returning the charging bias from LOW to HIGH is preferably at least one turn after the charging roller 2 in order to enable cleaning of the entire circumference of the charging roller 2. The HIGH charging bias after being changed here is not limited to the voltage before being changed to LOW, and may be any voltage higher on the negative polarity side than the LOW charging bias. That is, the voltage applied to the charging roller 2 is the same as the region of the photosensitive drum 1 that has passed through the charging unit a while the voltage after the change in the step (b) is applied to the charging roller 2. (B) The voltage is changed to a higher voltage on the negative polarity side than the voltage after the change in the step.

Timing (f):
Next, at the timing when the region of the photosensitive drum 1 that has passed through the charging unit a reaches the image exposure unit b with the charging bias returned to HIGH, the exposure device 3 is turned off and the entire exposure on the photosensitive drum 1 is stopped. To do. Thereby, the surface potential of the photosensitive drum 1 after passing through the image exposure part b is maintained. That is, the photosensitive drum 1 remains at the surface potential after the charging process after the charging bias is returned to HIGH.

-Timing (g):
Next, the developing bias is returned to HIGH at the timing when the region of the photosensitive drum 1 that has passed through the charging portion a reaches the developing portion c in a state where the charging bias is returned to HIGH. The region of the photosensitive drum 1 that has passed through the charging unit a with the charging bias being LOW is a region in which the absolute value of the surface potential of the photosensitive drum 1 has decreased. Further, the region of the photosensitive drum 1 that has passed through the charging unit a with the charging bias returned to HIGH is a region of the charging potential of the photosensitive drum 1 during normal image formation in this embodiment. The timing for returning the developing bias from LOW to HIGH can be changed according to the timing for returning the charging bias from LOW to HIGH. Further, the HIGH development bias after the change is not limited to the voltage before the change to LOW, and may be a voltage higher on the negative side than the LOW development bias. That is, when the voltage applied to the developing sleeve 41 is applied to the charging roller 2 while the voltage after the change in the step (e) is applied to the charging roller 2, the region of the photosensitive drum 1 that has passed through the charging portion a passes through the developing portion c. (C) The voltage is changed to a higher voltage on the negative polarity side than the voltage after the change in the step.

・ Timing (h):
Next, the pre-exposure device 6 is turned on at the timing when the region of the photosensitive drum 1 that has passed through the charging unit a in the state where the charging bias is LOW reaches the neutralization unit e. That is, the region of the photosensitive drum 1 that has passed through the charging unit a with the charging bias being LOW is a region where toner charged positively from the charging roller 2 is attached, and this region enters the charging unit a again. Prior to this, the area is subjected to photostatic discharge by the pre-exposure device 6. By turning on the pre-exposure device 6 and reducing the absolute value of the surface potential of the photosensitive drum 1 entering the charging portion a, discharge from the charging roller 2 to the photosensitive drum 1 is promoted. Thereby, the charging polarity of the positively charged toner adhering to the photosensitive drum 1 is sufficiently reversed to the negative polarity. The negatively charged toner passes through the charging portion a while being electrostatically urged from the charging roller 2 toward the photosensitive drum 1, and then electrostatically is applied to the developing sleeve 41 in the developing portion c. It is transferred and collected by the developing device 4. That is, when the region of the photosensitive drum 1 that should pass through the charging portion a when the voltage after the change of the step (e) is applied to the charging roller 2 passes through the static elimination portion e, the pre-exposure device 6 The neutralization process of the photosensitive drum 1 is performed.

<Specific control>
In this embodiment, the outer diameter of the photosensitive drum 1 is 20 mm, the outer diameter of the charging roller 2 is 9 mm, and the peripheral speed (surface speed) of the photosensitive drum 1 is 160 mm / sec. Further, since the charging roller 2 is rotationally driven at a speed that is 1.2 times the peripheral speed (surface speed) of the photosensitive drum 1, the peripheral speed (surface speed) of the charging roller 2 is 192 mm / sec. Each bias at the time of image formation is a transfer bias of +1000 V (HIGH), a charging bias of −1400 V (HIGH), and a developing bias of −400 V (HIGH). At the time of image formation, the surface of the photosensitive drum 1 is charged to a charging potential of -800 V (dark portion potential Vd) by the charging bias. Further, the surface potential of the photosensitive drum 1 that has been subjected to photostatic elimination by the pre-exposure device 6 is about −100 V, which is equivalent to the surface potential of the exposure portion (bright portion potential Vl) by the exposure device 3.

  At the timing when the image exposure in the printing process is completed and the toner image formed on the photosensitive drum 1 passes through the transfer portion d, a transfer bias of about −300 V (LOW) is applied to the transfer roller 5, and the pre-exposure device 6 is also turned off (timing (a)). Therefore, the surface potential of the photosensitive drum 1 immediately before the charging portion a is about −800V.

  Next, the charging bias is changed from −1400 V (HIGH) to 0 V (LOW) at the timing when the region where the surface potential of the photosensitive drum 1 is −800 V reaches the charging portion a (timing (b)). When the charging bias is set to 0 V, the potential difference between the surface potential of the photosensitive drum 1 and the potential of the charging roller 2 at the charging unit a becomes equal to or higher than the discharge start voltage Vth. Therefore, reverse discharge from the photosensitive drum 1 to the charging roller 2 occurs, and the surface potential of the photosensitive drum 1 becomes about −600V. In this embodiment, the discharge start voltage Vth (absolute value) between the photosensitive drum 1 and the charging roller 2 is about 600V.

  Next, at the timing when the region where the surface potential of the photosensitive drum 1 becomes −600 V reaches the image exposure unit b, the exposure device 3 is turned on to expose the entire surface of the photosensitive drum 1 (timing (c)). As a result, the surface potential of the photosensitive drum 1 subjected to the entire surface exposure is uniformly -100V.

  Next, the developing bias is changed from −400 V (HIGH) to 0 V (LOW) at the timing when the region where the surface potential of the photosensitive drum 1 becomes −100 V reaches the developing unit c (timing (d)). As a result, most of the positively charged toner on the photosensitive drum 1 passes through the developing portion c while being electrostatically urged from the developing sleeve 41 toward the photosensitive drum 1.

  Next, after elapse of 300 msec, which is one or more rounds of the charging roller 2 after the charging bias is set to 0 V (LOW), the charging bias is returned from 0 V (LOW) to −1400 V (HIGH) (timing (e) ). When the charging bias is set to −1400 V (HIGH), the surface potential of the photosensitive drum 1 after passing through the charging portion a becomes −800 V.

  Next, at the timing when the region of the photosensitive drum 1 that has passed through the charging unit a reaches the image exposure unit b in a state where the charging bias is returned to −1400 V (HIGH), the exposure device 3 is turned off to turn on the photosensitive drum 1. Full exposure is stopped (timing (f)). Thereby, the surface potential of the photosensitive drum 1 after passing through the image exposure part b is kept at −800V.

  Next, the developing bias is returned from 0 V (LOW) to −400 V (HIGH) at the timing when the region where the surface potential of the photosensitive drum 1 is −800 V reaches the developing portion c (timing (g)).

  Next, the pre-exposure device 6 is turned on (timing (h) at a timing when a region where the surface potential of the photosensitive drum 1 is −600 V (a region where positively charged toner is attached) reaches the static elimination unit e. )). Then, the surface potential of the photosensitive drum 1 is lowered from −600 V before passing through the charge removal unit e to −100 V after passing through the charge removal unit e. As a result, the discharge amount from the charging roller 2 to the region where the surface potential of the photosensitive drum 1 is lowered to −100 V is increased, and the charge polarity of the positively charged toner attached to the region is sufficiently high. Inverted to negative polarity toner. The toner whose charging polarity is reversed to the negative polarity passes through the charging portion a, and thereafter electrostatically transfers to the developing sleeve 41 and is collected in the developing container 45 of the developing device 4.

<Operational effect of the present embodiment>
As described above, according to this embodiment, in the cleaning operation of the charging roller 2, the entire surface of the photosensitive drum 1 is exposed at the timing when the region of the photosensitive drum 1 to which the positive toner is attached reaches the image exposure unit. As a result, the surface potential of the photosensitive drum 1 which is unstable can be stabilized, and can be collected in the developing device 4 without transferring fog toner onto the photosensitive drum 1 in the developing unit. As a result, image defects such as dirt in the main body of the image forming apparatus and the back dirt accompanying therewith can be prevented, and further, the cleaning time for developing and collecting fog toner can be shortened.

  In this embodiment, the period in which the charging bias is LOW is set to 300 msec, which is a time of one or more turns of the charging roller 2 (less than one turn of the photosensitive drum 1), but is not limited thereto. It is also possible to rotate the charging roller 2 a plurality of times while the charging bias is LOW. It is also possible to repeat the charging bias state of LOW and HIGH state. In this case, the developing bias may be repeatedly performed in a LOW state and a HIGH state correspondingly. Further, the timings (a) to (h) shown in FIG. 3 are not necessarily in this order. For example, the timing at which the transfer bias is set to LOW and the timing at which the pre-exposure is turned off need not be the same, and either may be first.

[Example 2]
An image forming apparatus according to Embodiment 2 will be described. In the configuration of the image forming apparatus 100 applied in the present embodiment, the same members as those in the first embodiment are denoted by the same reference numerals, and description thereof is omitted.

<Features of this embodiment>
FIG. 4 is a timing chart of the cleaning operation of the charging roller 2 of this embodiment. The image forming apparatus 100 according to the present embodiment uniformly reduces the amount of light of the entire surface exposure during the cleaning operation as compared to the time of the printing operation, and the surface potential of the photosensitive drum 1 after the entire surface exposure is an absolute value than the HIGH of the developing bias. By making the potential high, the developing bias during the cleaning operation is not switched to HIGH during the printing operation.

<Specific control>
At the timing when the image exposure (HIGH) in the printing process is completed (OFF) and the toner image formed on the photosensitive drum 1 passes through the transfer portion d, the transfer roller 5 has a transfer bias of about −300 V (LOW). The pre-exposure device 6 is also turned off (timing (a)). Therefore, the surface potential of the photosensitive drum 1 immediately before the charging portion a is about −800V. Here, the amount of image exposure (HIGH) during the printing process in this embodiment is set to an exposure intensity of 0.5 μJ / cm ² on the photosensitive drum 1.

  Next, the charging bias is changed from −1400 V (HIGH) to 0 V (LOW) at the timing when the region where the surface potential of the photosensitive drum 1 is −800 V reaches the charging portion a (timing (b)). When the charging bias is set to 0V, reverse discharge from the photosensitive drum 1 to the charging roller 2 occurs as in the first embodiment, and the surface potential of the photosensitive drum 1 becomes about −600V.

Next, the exposure apparatus 3 is switched to LOW at the timing when the region where the surface potential of the photosensitive drum 1 becomes −600 V reaches the image exposure unit b (timing (c)). Here, the light amount of image exposure (LOW) during the cleaning operation in this embodiment is smaller than the light amount at the time of image formation (during the printing process), and the exposure intensity on the photosensitive drum 1 is 0.1 μJ / cm ² . . As a result, the surface potential of the photosensitive drum 1 subjected to the entire surface exposure is uniformly −500V.

  In this embodiment, the developing bias is kept at −400 V (HIGH) at the timing when the region where the surface potential of the photosensitive drum 1 becomes −500 V reaches the developing portion c (timing (d)). Even in this state, most of the positively charged toner on the photosensitive drum 1 passes through the developing portion c while being electrostatically urged from the developing sleeve 41 toward the photosensitive drum 1. .

  Next, after elapse of 300 msec, which is one or more rounds of the charging roller 2 after the charging bias is set to 0 V (LOW), the charging bias is returned from 0 V (LOW) to −1400 V (HIGH) (timing (e) ). When the charging bias is set to −1400 V (HIGH), the surface potential of the photosensitive drum 1 after passing through the charging portion a becomes −800 V.

  Next, at the timing when the region of the photosensitive drum 1 that has passed through the charging unit a reaches the image exposure unit b in a state where the charging bias is returned to −1400 V (HIGH), the exposure device 3 is turned off to turn on the photosensitive drum 1. Full exposure is stopped (timing (f)). Thereby, the surface potential of the photosensitive drum 1 after passing through the image exposure part b is kept at −800V.

  The developing bias remains at −400 V (HIGH) (timing (g)) at the timing when the region where the surface potential of the photosensitive drum 1 is −800 V reaches the developing portion c.

  Next, the pre-exposure device 6 is turned on (timing (h) at a timing when a region where the surface potential of the photosensitive drum 1 is −600 V (a region where positively charged toner is attached) reaches the static elimination unit e. )). Then, the surface potential of the photosensitive drum 1 is lowered from −600 V before passing through the charge removal unit e to −100 V after passing through the charge removal unit e. As a result, the discharge amount from the charging roller 2 to the region where the surface potential of the photosensitive drum 1 is lowered to −100 V is increased, and the charge polarity of the positively charged toner attached to the region is sufficiently high. Inverted to negative polarity toner. The toner whose charging polarity is reversed to the negative polarity passes through the charging portion a, and thereafter electrostatically transfers to the developing sleeve 41 and is collected in the developing container 45 of the developing device 4.

<Operational effect of the present embodiment>
As described above, according to the present embodiment, in the cleaning operation of the charging roller 2, the light amount of the entire surface exposure during the cleaning operation is uniformly reduced as compared with the printing process, and the surface of the photosensitive drum 1 after the entire surface exposure is performed. By controlling the potential to an absolute value higher than the developing bias HIGH and not switching the developing bias to the bias during the printing operation during the cleaning operation, the unstable surface potential of the photosensitive drum 1 is stabilized. On the other hand, the generation of fog toner due to a steep potential change at the time of developing bias switching can be suppressed, and the cleaning time for developing and collecting the fog toner can be shortened.

[Other Examples]
As mentioned above, although this invention was demonstrated according to the specific Example, this invention is not limited to the above-mentioned Example.

  For example, in the above-described embodiments, the case where the present invention is applied to a DC charging type image forming apparatus has been described as an example. However, a vibration in which a DC voltage (DC component) and an AC voltage (AC component) are superimposed as a charging voltage is described. The present invention can also be applied to an AC charging type image forming apparatus using voltage. Also in this case, in the cleaning operation of the charging member, the same effect as in the above-described embodiment can be obtained if the DC component (DC bias) of the charging voltage has the same potential relationship as in the above-described embodiment.

  In the above-described embodiments, only the DC component has been described for the development voltage. However, the development voltage may be an oscillating voltage in which a DC voltage (DC component) and an AC voltage (AC component) are superimposed. Also in this case, in the cleaning operation of the charging member, the same effect as that of the above-described embodiment can be obtained if the DC component (DC bias) of the developing voltage has the same potential relationship as that of the above-described embodiment.

  In the above-described embodiments, the charging member is described as being driven to rotate with a peripheral speed difference with respect to the image carrier. This makes it difficult for dirt such as positively charged toner to adhere to the charging member as described above, but the charging member may be driven to rotate with respect to the rotation of the image carrier.

  In the above-described embodiments, the charging member is described as a roller-shaped member, but the present invention is not limited to this. For example, other types of rotating members such as an endless belt-shaped charging member wound around a plurality of support rollers (for example, one of the plurality of support rollers abutting the image carrier via the belt) It can be used suitably.

  In the above-described embodiments, the cleaning operation of the charging member has been described as being performed in the post-rotation process at the time of non-image formation. However, the present invention is not limited to this. It can be executed at any timing. For example, in the above-described embodiment, when the number of output images in a certain job exceeds a predetermined threshold value, the charging member cleaning operation is executed in the post-rotation process after all image formation for the job is completed. On the other hand, when the number of output images exceeds a predetermined threshold during the job, the charging member cleaning operation can be executed by extending the interval between sheets.

  In the above-described embodiment, information on the number of accumulated image outputs is used as an index for estimating the degree of adhesion (accumulation) of dirt such as positively charged toner on the charging member. However, the present invention is not limited to this. It is not a thing. As the index, any information that correlates with the usage amount of the charging member, such as the number of rotations of the charging member, the rotation time, and the application time of the charging voltage can be suitably used, and a threshold value corresponding to each can be set. .

  In the above-described embodiments, toner that is a magnetic one-component developer as a developer is used, but a non-magnetic one-component developer may be used.

  In the above-described embodiments, an image forming apparatus having one image forming station having a photosensitive drum and a charging unit and a developing unit as process units acting on the photosensitive drum is used. It is not limited to. The present invention can also be applied to a full-color image forming apparatus having a plurality of image forming stations.

  In the above-described embodiment, the printer is exemplified as the image forming apparatus, but the present invention is not limited to this. For example, the image forming apparatus may be another image forming apparatus such as a copying machine or a facsimile machine, or another image forming apparatus such as a multi-function machine combining these functions. The same effect can be obtained by applying the present invention to these image forming apparatuses.

DESCRIPTION OF SYMBOLS 1 ... Photosensitive drum 2 ... Charging roller 3 ... Exposure apparatus 4 ... Developing apparatus 5 ... Transfer roller 6 ... Pre-exposure apparatus 41 ... Developing sleeve 100 ... Image forming apparatus 150 ... Control part

Claims (5)

A rotatable image carrier;
A charging member that contacts the image carrier and charges the image carrier;
Image exposure means for exposing the charged image carrier to form an electrostatic image;
A developing device including a developing member for supplying a toner charged with a normal polarity to the electrostatic image of the image carrier to form a toner image;
A transfer member for electrostatically transferring a toner image from the image carrier to a transfer target,
In the image forming apparatus for collecting the toner remaining on the image carrier after the transfer to the developing device by the developing member,
The toner charged to a polarity opposite to the normal polarity attached to the charging member is electrostatically transferred from the charging member to the image carrier, and electrostatically transferred from the image carrier to the developing member. A control unit that executes a cleaning operation to be collected by the developing device during non-image formation;
In the cleaning operation, the control unit transfers toner charged to a polarity opposite to the normal polarity from the charging member to the image carrier, and then covers the entire area of the image carrier on which the toner has been transferred. An image forming apparatus wherein the entire surface is exposed by the image exposure means.   In the cleaning operation, the controller is configured to cover the entire area of the image carrier on which the toner has been transferred at a timing when the toner transferred area of the image carrier reaches the exposure position by the image exposure unit. 2. The image forming apparatus according to claim 1, wherein the entire surface is exposed by an image exposure unit.   3. The image forming apparatus according to claim 1, wherein a light amount of the entire surface exposure by the image exposure unit in the cleaning operation is smaller than a light amount at the time of image formation.   The image forming apparatus according to claim 3, wherein the voltage applied to the developing member is not switched in the cleaning operation. A rotatable image carrier;
A charging member that contacts the image carrier and is applied with a voltage to charge the image carrier with a charging unit;
A charging power source for applying a voltage to the charging member;
Image exposure means for forming an electrostatic image on the charged image carrier;
A developing device including a developing member that forms a toner image by supplying a toner charged with a normal polarity to the electrostatic image of the image carrier in a developing unit to which a voltage is applied;
A developing power source for applying a voltage to the developing member;
A transfer member that electrostatically transfers a toner image from the image carrier to the transfer target at a transfer portion when a voltage is applied;
A transfer power source for applying a voltage to the transfer member;
In the image forming apparatus for collecting the toner remaining on the image carrier after the transfer to the developing device by the developing member,
A control unit that executes a cleaning operation for reducing toner adhering to the charging member during non-image formation;
In the cleaning operation, the control unit electrostatically transfers toner charged to a polarity opposite to the normal polarity attached to the charging member from the charging member to the image carrier while rotating the image carrier. The entire area of the image carrier on which the toner has been transferred is exposed by the image exposure means, and the transferred toner is electrostatically biased in the direction from the developing member toward the image carrier. In the state, the developing unit is passed, the charged polarity of the passed toner is reversed to the normal polarity in the charging unit, and the toner having the reversed charging polarity is transferred from the image carrier to the developing member in the developing unit. An image forming apparatus, wherein the image forming apparatus is electrostatically transferred to and collected by the developing device.

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