JP3605650B2 - Image forming device - Google Patents

Description

[0001]
TECHNICAL FIELD OF THE INVENTION
The present invention relates to an image forming apparatus, and more specifically, develops an electrostatic latent image formed on an image carrier having a charged surface with toner, and transfers a toner image obtained by the development to a recording medium. And an image forming apparatus for forming a visible image.
[0002]
[Prior art]
Conventionally, an image forming apparatus using a transfer roller has an image area larger than the size of a sheet, so that the toner of the image carrier is directly transferred to the transfer roller or when the sheet is jammed. However, there has been a problem that the transfer roller is significantly contaminated, thereby soiling the subsequent paper, or a transfer bias at the time of transfer is substantially insufficient to cause transfer failure.
[0003]
As a means for avoiding such a problem, a method of mechanically cleaning the surface of the transfer roller with a blade has been proposed. However, since a conductive foam sponge is generally used as a material of the transfer roller, the transfer is performed. Since the toner adhered to the roller surface cannot be sufficiently cleaned, a cleaning method using electrostatic force is preferable.
[0004]
As a cleaning method using electrostatic force, for example, Japanese Patent Application Laid-Open No. 8-248788 discloses that after the last sheet of a series of image forming operations has passed through a transfer position where an image carrier and a transfer unit are in contact, toner is transferred to the transfer unit. A method of applying a transfer bias of the same polarity and applying a transfer bias having a polarity opposite to that of the toner to the transfer unit before the first sheet of the series of image forming operations passes through the transfer unit, thereby cleaning the surface of the transfer roller. It has been disclosed.
[0005]
However, in the above-described method, a bias is applied to the transfer roller before the paper arrives, but it takes at least one turn of the transfer roller to clean the entire transfer roller. In addition, since sufficient cleaning performance cannot be obtained when the image carrier is not charged, in addition to the time for applying the bias to the transfer roller, the image carrier reaches the transfer unit after being charged by the charging unit. There was a problem that time was required until.
[0006]
By the way, charging by a corotron has been generally used in the past, but since ozone generated by corotron discharge has recently been closely related to environmental issues, charging rollers and the like which generate a very small amount of ozone as compared with a corotron have been used. Contact charging means has been put to practical use as an alternative technology.
[0007]
However, when a contact charging means such as a charging roller is used, if the image carrier has been charged to a polarity opposite to the original charging polarity in the previous image forming process or the like, it is not possible to charge sufficiently by one charging. However, the original charging potential cannot be secured. Therefore, when a transfer bias having a polarity opposite to that of the toner is applied to the transfer unit before the first sheet of a series of image forming operations passes, as in the above-described disclosure example, the bias applied to the transfer unit is the original bias of the image carrier. Since the charge polarity is opposite to that of the image carrier, a history due to the transfer bias may occur in the charge of the image carrier. This history is generated as a density difference particularly when a halftone image is printed.
[0008]
In order to prevent such an adverse effect of the transfer bias on the charging of the image carrier, a transfer bias having a polarity opposite to that of the toner for cleaning is applied to the transfer unit, and then the image carrier is applied twice or more by the contact charging unit. It is necessary.
[0009]
For this reason, in addition to the time for applying the bias to the transfer roller and the time from when the image carrier is charged by the charging unit to when the image carrier reaches the transfer unit, a time for the image carrier to make one or more rounds is required. As a result, the processing time for cleaning before the first sheet of the series of image forming operations passes is long.
[0010]
By the way, in a developing system having a large amount of toner of opposite polarity charged to a positive potential on the developing roller, the opposite polarity is caused by a potential difference between a developing roller to which a developing bias is not applied and a photosensitive drum charged to a negative potential during non-development. The toner may fly to the photosensitive drum.
[0011]
For this reason, the transfer roller that is in contact with the photosensitive drum may be stained with toner, and the back surface of the sheet may be stained during transfer. This can be avoided by applying a positive bias voltage to the transfer roller before the transfer to the paper, so that the back surface of the paper can be prevented from being stained. Therefore, the processing time required for cleaning before the sheet first passes becomes longer by the time for applying the bias voltage.
[0012]
As described above, conventionally, it is difficult to shorten the processing time related to cleaning before the first sheet of a series of image forming operations passes and to improve the processing efficiency of the image forming process while maintaining high image quality. Was.
[0013]
[Problems to be solved by the invention]
SUMMARY An advantage of some aspects of the invention is to provide an image forming apparatus capable of efficiently preventing toner from adhering to a transfer roller while ensuring high image quality. I do.
[0014]
[Means for Solving the Problems]
In order to achieve the above object, the image forming apparatus according to claim 1, wherein the image forming apparatus is arranged near the image carrier without contact with the image carrier and formed on the image carrier having a charged surface. Developing means for developing the electrostatic latent image with toner to form a toner image; and transferring the toner image to a recording medium disposed in contact with the image carrier and conveyed to a contact portion with the image carrier. Transfer means and not before the transfer processing by the transfer means, but only after the transfer processing by the transfer means , The transfer means Before Apply a voltage of the same polarity as the charging polarity of the toner And then apply a voltage of the opposite polarity to the charging polarity of the toner. At the same time, when development is being performed, a developing bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing unit, and when development is not being performed, the potential difference between the developing unit and the image carrier is calculated. Control means for reducing a potential difference at which toner flies from the developing means to the image carrier.
[0015]
In the image forming apparatus according to the second aspect, in the image forming apparatus according to the first aspect, the control unit applies only a DC voltage component of a developing bias voltage to the developing unit, so that the developing unit and the developing unit are connected to each other. It is characterized in that the potential difference from the image carrier is reduced.
[0016]
In the image forming apparatus according to a third aspect, in the image forming apparatus according to the first aspect, the control unit prohibits application of a DC voltage component of a charging bias voltage of a charging unit that charges a surface of the image carrier. Thus, a potential difference between the developing unit and the image carrier is reduced.
[0017]
Further, in the image forming apparatus according to the fourth aspect, in the image forming apparatus according to the first aspect, the control unit exposes the entire surface of the image carrier to reduce a potential difference between the developing unit and the image carrier. It is characterized in that it is made smaller.
[0018]
In the image forming apparatus according to the first aspect, the developing unit is disposed in the vicinity of the image carrier in a non-contact manner with the image carrier, and the electrostatic unit formed on the image carrier having a charged surface. The latent image is developed with toner to form a toner image. Then, the transfer means arranged in contact with the image carrier transfers the toner image to the recording medium conveyed to the contact portion with the image carrier. Thereby, a visible image is formed on the recording medium.
[0019]
In such an image forming apparatus, the control unit does not start the transfer process by the transfer unit but after the transfer process by the transfer unit (for example, after the end of a series of visible image forming processes on a plurality of recording media). Only, transfer means To Voltage of the same polarity as the charging polarity of the toner And then apply a voltage of the opposite polarity to the charging polarity of the toner. I do.
[0020]
For example, when positive and negative voltages are sequentially applied to remove negatively charged toner, a negative voltage is first applied to clean toner (mainly negative toner) attached to the transfer unit. Thereafter, a positive voltage is applied to remove the positive toner adhered during the cleaning. Specifically, when a transfer roller is used as the transfer unit, a negative voltage is applied for a time period during which the transfer roller rotates several times (two to four times), and a positive voltage is applied for a time period during which the transfer roller rotates one time. Just do it. Thus, the surface of the transfer unit can be cleaned.
[0021]
Further, such cleaning is performed not before the start of the transfer process but after the end of the transfer process. Therefore, only the time required for transporting the recording medium is sufficient before the start of the transfer process, so that the time before the start of the transfer process can be shortened, and from when the printing instruction is given to when the image formation on the first recording medium is completed and output. (So-called FPOT, first printout time) can be shortened.
[0022]
Even if the post-processing after the completion of the transfer processing becomes long, the influence on the user is small, and it does not cause a serious problem.
[0023]
Further, the control means includes: When development is being performed, a developing bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing unit, When development is not taking place Is The potential difference between the developing means and the image carrier is made smaller than the potential difference at which toner flies from the developing means to the image carrier.
[0024]
Specifically, the control means may reduce the potential difference between the developing means and the image carrier by applying only the DC voltage component of the developing bias voltage to the developing means. . That is, in general, when development is not performed, a potential difference occurs between the developing unit to which the developing bias voltage is not applied and the charged image carrier, but only the DC voltage component of the developing bias voltage is applied to the developing unit as described above. When the voltage is applied, the potential of the developing unit approaches the potential of the charged image carrier, so that the potential difference between the developing unit and the image carrier can be made smaller than the potential difference at which toner flies from the developing unit.
[0025]
Further, as described in claim 3, the control means inhibits the application of the DC voltage component of the charging bias voltage of the charging means for charging the surface of the image carrier, thereby reducing the potential difference between the developing means and the image carrier. It may be smaller. That is, a charging bias voltage in which a DC voltage component and an AC voltage component are superimposed is applied to the charging unit, and the image bearing member charged by the charging unit and the developing unit to which the developing bias voltage is not applied are charged. Although a potential difference occurs during non-development, if the application of the DC voltage component of the charging bias voltage is prohibited as described above, the charge level of the image carrier decreases, and the potential of the image carrier approaches the potential of the developing unit. The potential difference between the developing means and the image carrier can be made smaller than the potential difference at which toner flies from the developing means.
[0026]
Further, as described in claim 4, the control means may reduce the potential difference between the developing means and the image carrier by exposing the entire surface of the image carrier. That is, in general, when development is not performed, a potential difference occurs between the developing unit to which the developing bias voltage is not applied and the charged image carrier. However, if the entire surface of the image carrier is exposed as described above, Since the charge level is reduced over the entire surface of the image forming device and the potential of the image carrier approaches the potential of the developing device, the potential difference between the developing device and the image carrier can be made smaller than the potential difference at which toner flies from the developing device. it can.
[0027]
As described above, by making the potential difference between the developing means and the image carrier smaller than the potential difference at which the toner flies from the developing means to the image carrier, the toner flies from the developing means to the image carrier. Thus, toner contamination on the surface of the image carrier can be prevented. Along with this, it is possible to prevent the transfer unit in contact with the image carrier from soiling the recording medium at the time of transferring stains with toner.
[0028]
That is, without applying a bias voltage for preventing toner contamination to the transfer unit before the transfer process, it is possible to prevent the toner from adhering to the transfer unit without affecting the formed image.
[0029]
As described above, the control means controls the transfer To Voltage of the same polarity as the charging polarity of the toner And then apply a voltage of the opposite polarity to the charging polarity of the toner. In addition, since the potential difference between the developing means and the image carrier when the development is not performed is smaller than the potential difference at which the toner flies from the developing means, there is no need to perform cleaning before the image forming operation, and the transfer process is started. The previous time can be reduced, and toner contamination of the transfer unit can be prevented while ensuring high image quality.
[0030]
BEST MODE FOR CARRYING OUT THE INVENTION
Hereinafter, various embodiments of the invention will be described in order.
[0031]
[First Embodiment]
First, a first embodiment corresponding to the first and second aspects of the present invention will be described.
[0032]
[Schematic configuration of laser beam printer]
FIG. 1 is a schematic configuration diagram of a laser beam printer 1 as an example of an image forming apparatus, and FIG. 2 is an enlarged view of the vicinity of a photosensitive drum 8.
[0033]
As shown in FIG. 1, a laser beam printer 1 is provided with a laser scanning device 2, and the laser scanning device 2 modulates and outputs a laser beam in accordance with digital image data of an image to be formed. A semiconductor laser (not shown) is provided. The laser beam emitted from this semiconductor laser enters the polygon mirror 4 and is deflected according to the rotation of the polygon mirror 4. After the deflected laser beam passes through the fθ lens 5, its traveling direction is changed by mirrors 6 and 7, and the laser beam is output from the laser scanning device 2. The optical path of the laser beam is indicated by a broken line 23 in FIG. The digital image data of the image to be formed may be digital image data input to the laser beam printer 1 from an external information processing device (a personal computer or the like), or may be stored in a memory (not shown) built in the laser beam printer 1. Digital image data may be used.
[0034]
A photosensitive drum 8 that rotates at a constant speed in the direction of arrow A is disposed on the side of the laser scanning device 2, and the laser beam output from the laser scanning device 2 is exposed to a predetermined light on the photosensitive drum 8. The position 9 is repeatedly scanned in the axial direction of the photosensitive drum 8 (that is, the main scanning direction).
[0035]
As shown in FIGS. 1 and 2, in the vicinity of the surface of the photosensitive drum 8, a charging roller 10 for uniformly charging the surface of the photosensitive drum 8 along an arrow A direction, a developing device for performing a developing process 11, a developing roller 12, a transfer roller 14 for transferring a toner image to a recording sheet 22, a discharging needle 17 for discharging the recording sheet 22, and a cleaning device 21 for removing residual toner from the surface of the photosensitive drum 8 are arranged in this order. I have.
[0036]
The photosensitive drum 8, the charging roller 10, the developing roller 12 and the transfer roller 14 are respectively driven to rotate by motors (not shown), and the charging roller 10, the developing roller 12 and the transfer roller 14 are rotated counterclockwise at a constant speed in FIG. Rotate.
[0037]
The charging roller 10 is connected to a charging bias voltage applying device 46 including an AC power supply 46B, a DC power supply 46C, and a control unit 46A for adjusting and controlling power supply from these power supplies. A charging bias voltage obtained by superimposing a negative DC bias voltage from the DC power supply 46C on an AC voltage from the AC power supply 46B is applied to the charging roller 10.
[0038]
Similarly, the developing roller 12 is connected to a developing bias voltage applying device 44 including an AC power supply 44B, a DC power supply 44C, and a control unit 44A for adjusting and controlling power supply from these power supplies. 44A applies to the developing roller 12 a developing bias voltage in which a negative DC bias voltage from the DC power supply 44C is superimposed on an AC voltage from the AC power supply 44B.
[0039]
The transfer roller 14 is connected to a transfer bias voltage applying device 42 including a DC power supply 42B and a control unit 42A for adjusting and controlling the power supply from the DC power supply 42B. The polarity of the DC bias voltage from the power supply 42 </ b> B is appropriately switched between positive and negative, and the DC bias voltage is applied to the transfer roller 14.
[0040]
In the present embodiment, an OPC (organic photoconductor) is used for the photosensitive drum 8, and the surface of the photosensitive drum 8 is charged to a negative potential by the application of the charging bias voltage by the charging roller 10. You.
[0041]
By irradiating the photosensitive drum 8 having a charged surface with laser light modulated in accordance with digital image data of an image to be formed, an exposure position 9 of the photosensitive drum 8 corresponds to an image to be formed. A formed electrostatic latent image is formed. The potential of the portion where the electrostatic latent image is formed is relatively higher than the potential of the non-formed portion.
[0042]
The portion where the electrostatic latent image is formed reaches the developing position 25 by the developing device 11 by the rotation of the photosensitive drum 8. The developing device 11 is arranged in contact with the photosensitive drum 8 to supply toner to the developing roller 12 for magnetically raising the toner to develop an electrostatic latent image and toner in the cartridge to the developing roller 12. And a toner supply device 13 for performing the operation. The developing bias voltage is applied to the developing roller 12 by the developing bias voltage applying device 44, and a potential difference is generated between the portion of the photosensitive drum 8 where the electrostatic latent image is formed and the surface of the developing roller 12. Due to this potential difference, the toner charged to the negative potential flies from the developing roller 12 to the electrostatic latent image forming portion on the photosensitive drum 8, and a toner image corresponding to the electrostatic latent image is formed on the photosensitive drum 8. You.
[0043]
The portion where the toner image is formed reaches the transfer position 26 by the transfer roller 14 by the rotation of the photosensitive drum 8. The recording paper 22 is conveyed to the transfer position 26 at the same time as the toner image reaches the transfer position 26. When the recording sheet 22 is conveyed to the transfer position 26, a transfer bias voltage is applied to the transfer roller 14 by the transfer bias voltage applying device 42, and the toner image on the photosensitive drum 8 is transferred onto the recording sheet 22. .
[0044]
A leading edge detection sensor 33 that detects the leading edge of the recording paper 22 is provided near the upstream side of the transfer position 26 in the paper transport direction, and the leading edge of the recording paper 22 reaches the vicinity of the transfer position 26 by the leading edge detection sensor 33. When this is detected, the application of the transfer bias voltage to the transfer roller 14 is started.
[0045]
Further, a rear end detection sensor 34 for detecting the rear end of the recording sheet 22 is provided near the downstream side of the transfer position 26 in the sheet conveyance direction, and the rear end detection sensor 34 transfers the rear end of the recording sheet 22. When the passage of the position 26 is detected, the application of the transfer bias voltage to the transfer roller 14 is stopped. The front end detection sensor 33 and the rear end detection sensor 34 can be configured by, for example, an optical sensor including a pair of a light emitting element and a light receiving element.
[0046]
Next, the transport path of the recording paper 22 will be briefly described. Below the laser beam printer 1, a cassette tray 15 in which recording paper 22 is stored is detachably installed. The recording paper 22 stored in the uppermost layer of the cassette tray 15 is sent out by a half-moon roller 16 to a transport path 24 shown by a dashed line in FIG. 1 and is transported upward by transport rollers 31 and 32. It should be noted that other paper conveying means such as a retard roller may be used instead of the half-moon roller 16.
[0047]
During the image forming process, the recording paper 22 is sent out from the cassette tray 15 to the transport path 24 by the half-moon roller 16 in synchronization with the rotation of the photoconductor drum 8, and is transferred between the photoconductor drum 8 and the transfer roller 14 at a desired timing. (Transfer position). A positive DC bias voltage is applied to the transfer roller 14 only at the time of the passage, whereby the toner image on the photosensitive drum 8 is electrostatically attracted toward the transfer roller 14 and is transferred onto the recording paper 22 passing through the transfer position. The toner image is transferred.
[0048]
The recording paper 22 to which the toner image has been transferred is neutralized from the back surface of the recording paper 22 by the static elimination needle 17 arranged downstream of the transfer roller 14, and is separated from the surface of the photosensitive drum 8. After the peeled recording paper 22 is transported on the transport path 24, it is transported to a fixing device 20 including a pair of a heat roller 18 and a pressure roller 19. In the fixing device 20, the recording paper 22 passes between the heat roller 18 and the pressure roller 19 which are nipped with a predetermined width. At this time, the side of the recording paper 22 on which the toner image is transferred is the heat roller side 18, and the pressure roller 19 presses the recording paper 22 against the heat roller 18 to enable efficient heat transfer. The heat roller 18 is controlled at a constant high temperature. In this state, the toner image on the recording paper 22 is heat-fixed to the paper surface.
[0049]
The toner image not transferred to the recording paper 22 is removed from the drum surface by the cleaning device 21. The photosensitive drum 8 is charged again by the charging roller 10, and the process proceeds to the next cycle.
[0050]
[Overview of control by control device]
By the way, the laser beam printer 1 is provided with a control device 40 including a microcomputer, and the control device 40 controls various operations in the laser beam printer 1 as described below, for example.
[0051]
(1) Drive control of the transport rollers 31 and 32 for transporting the recording paper 22
(2) Detection that the leading end of the recording paper 22 has reached the vicinity of the transfer position 26 based on the detection signal from the leading end detection sensor 33
(3) Detection that the rear end of the recording paper 22 has passed the transfer position 26 based on the detection signal from the rear end detection sensor 34
(4) Control of motors that drive each of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 and detection of the rotation amount of each motor
(5) Control of the operation of applying the charging bias voltage by the charging bias voltage applying device 46, the operation of applying the developing bias voltage by the developing bias voltage applying device 44, and the operation of applying the transfer bias voltage by the transfer bias voltage applying device 42 (ie, Control of the control units 46A, 44A, 42A)
(6) Control of laser beam output by laser scanning device 2
That is, the control device 40 controls the charging of the photosensitive drum 8, the formation of an electrostatic latent image by the laser beam from the laser scanning device 2, the development by the developing device 11, and the transfer of the toner image onto the recording paper 22 by the transfer roller 14. A series of image forming operations are synchronized. Further, control is performed such that the recording paper 22 is conveyed to the transfer position 26 and a transfer bias is applied to the transfer roller 14 at the transfer start timing, and the transfer end timing (when the rear end of the recording paper 22 passes the transfer position 26) Is controlled so that the application of the transfer bias to the transfer roller 14 is stopped.
[0052]
Further, when the last recording paper 22 of the series of image forming operations has passed the transfer position 26, the control device 40 applies a transfer bias (−500 V) having the same polarity as the normally charged toner (= minus charged toner) to the transfer roller 14. Is applied for a period of two rotations of the transfer roller 14, and then a transfer bias having a polarity opposite to that of the normally charged toner is applied for a period of one rotation of the transfer roller 14. As a result, the normally charged toner and the opposite polarity toner attached to the transfer roller 14 are removed, and the surface of the transfer roller 14 is cleaned (details will be described later).
[0053]
[Operation of First Embodiment]
Next, as an operation in the first embodiment, when the laser beam printer 1 receives digital image data of an image to be printed and print request information such as the number of prints and the paper size from an external information processing device, A control routine relating to the printing process executed by the control unit 40 will be described with reference to the flowcharts of FIGS. 4 and 5 and a time chart of FIG. In the time chart of FIG. 3 and the later-described time charts of FIGS. 6 and 9, ON / OFF of the application operation of various bias voltages (or the entire exposure operation in FIG. 9) is represented in time series.
[0054]
In step 100 in FIG. 4, each of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is driven to rotate, and is driven to rotate stably at a constant angular velocity. Also, a counter N for counting the number of prints is initialized to “0”.
[0055]
In the next step 102, the charging bias voltage in which the negative DC voltage is superimposed on the AC voltage is started to be applied to the charging roller 10 by the charging bias voltage applying device 46 (time t in FIG. 3). ₁ ). Thus, the surface of the photosensitive drum 8 is charged to a negative potential.
[0056]
Then, it waits until the charged position on the photosensitive drum 8 reaches the developing position 25 by the developing roller 12 by the rotation of the photosensitive drum 8 (waits for the time T1 in FIG. 3). When it is detected from the rotation angle of the photosensitive drum 8 that the charged position has reached the developing position 25, the process proceeds to step 106, where the developing bias voltage applying device 44 causes a predetermined developing bias voltage (AC voltage + negative). Start applying only the negative DC voltage to the developing device 11 (time t in FIG. 3). ₂ ).
[0057]
As a result, the potential of the developing roller 12 becomes a predetermined negative potential, so that the potential difference between the charged photosensitive drum 8 and the developing roller 12 becomes small, and the toner adhered to the surface of the developing roller 12, in particular, the positively charged reverse polarity It is possible to prevent the toner from flying to the photosensitive drum 8.
[0058]
In the next step 108, a subroutine of the bias control process 1 in FIG. 5 is executed as various bias control processes during the image forming process. First, it is determined whether or not the application timing of the developing bias voltage according to the timing when the recording paper 22 reaches the vicinity of the transfer position 26 (hereinafter, referred to as a paper passing start timing) is determined by the photosensitive drum 8 or the transport roller 31. , 32 (Step 202).
[0059]
The charged position of the photosensitive drum 8 is irradiated with a laser beam from the laser scanning device 2 (that is, a laser beam modulated in accordance with digital image data of an image to be formed) to form an electrostatic latent image. I do.
[0060]
After the formation of the electrostatic latent image, when it is time to start applying the developing bias voltage according to the sheet passing start time (Yes at step 202), the developing bias voltage applying device 44 causes the developing bias voltage applying device 44 to execute a predetermined developing bias voltage (AC voltage). The application of the AC voltage of (+ negative DC voltage) to the developing device 11 is started (step 204, time t in FIG. 3). ₃ ). As a result, a predetermined developing bias voltage (AC voltage + negative DC voltage) is applied to the developing device 11, and the developing device 11 develops the electrostatic latent image as usual, thereby forming a toner image. Is done.
[0061]
Next, it is determined whether or not the leading edge of the recording paper 22 has reached the vicinity of the transfer position 26 (that is, the paper passing start timing has been reached) based on the presence or absence of the detection signal from the leading edge detection sensor 33 (step 206). . Then, when the leading end of the recording paper 22 reaches the vicinity of the transfer position 26 and a detection signal is output from the leading end detection sensor 33 (Yes in Step 206), the transfer bias voltage applying device 42 performs predetermined transfer. Start applying a bias voltage (positive DC voltage) to the transfer roller 14 (step 208, time t in FIG. 3). ₄ ). As a result, when the recording paper 22 is passing through the transfer position 26, the toner image is transferred from the photosensitive drum 8 to the recording paper 22 by the transfer roller 14, and a toner image is formed on the recording paper 22. After that, the recording paper 22 is conveyed to the fixing device 20, and the toner image is fixed on the recording paper 22 by the fixing process by the fixing device 20, and a target image is formed on the recording paper 22.
[0062]
Then, it is determined whether or not the application timing of the developing bias voltage according to the timing when the rear end of the recording paper 22 passes the transfer position 26 (hereinafter referred to as the paper passing end timing) is determined by the photosensitive drum 8 or the conveyance roller. A determination is made based on the number of rotations of each motor that drives 31 and 32 (step 210). Then, when the application start timing of the developing bias voltage according to the paper passing end timing is reached (Yes at step 210), the application of the AC voltage of the predetermined developing bias voltage (AC voltage + negative DC voltage) is stopped. Stop (step 212, time t in FIG. 3) ₅ ). However, at this time, since the negative DC bias voltage is continuously applied to the developing device 11, the potential difference between the photosensitive drum 8 charged to the negative potential and the developing roller 12 having the negative potential remains small, and It is possible to prevent the toner attached to the surface of the roller 12, in particular, the positively-charged opposite-polarity toner from flying to the photosensitive drum 8.
[0063]
Further, it is determined whether or not the rear end of the recording paper 22 has passed the transfer position 26 (that is, the paper passing end time has come) based on the presence or absence of the detection signal from the rear end detection sensor 34 (step 214). Then, when the rear end of the recording paper 22 passes through the transfer position 26 and a detection signal is output from the rear end detection sensor 34 (Yes at Step 214), the transfer bias voltage (AC voltage + positive DC voltage) ) Is stopped (step 216, time t in FIG. 3). ₆ ), And increment the counter N by one (step 218).
[0064]
Here, it is determined whether or not the counter N has reached the total number of prints (step 220). If the counter N has not reached the total number of prints, it is considered that all print processing has not been completed, and the flow proceeds to step 202. Return, and execute the unprocessed print processing.
[0065]
Even when the processing is re-executed from step 202, since the negative DC voltage of the developing bias voltage is applied to the developing device 11 as shown in FIG. Is kept small, and it is possible to prevent the toner attached to the surface of the developing roller 12, in particular, the positively charged opposite polarity toner from flying to the photosensitive drum 8.
[0066]
In this manner, images are formed (printed) one by one on the recording paper 22. When the printing of all the sheets is completed, the counter N reaches the total number of printed sheets, and the process returns from the subroutine of FIG.
[0067]
After the completion of all the printing processes as described above, in steps 110 and 112 following the main routine in FIG. 4, the transfer roller 14 is charged with the normally charged toner (−charged toner) for the time required for the transfer roller 14 to make two rotations. A polarity (negative) bias is applied (time t in FIG. 3). ₇ ~ T ₈ ). As a result, the normally charged toner attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
[0068]
However, at this time, since the photosensitive drum 8 is also negatively charged, the potential difference between the transfer roller 14 and the photosensitive drum 8 is set to a potential difference sufficient for the toner to fly from the transfer roller 14 to the photosensitive drum 8. A negative bias voltage to the transfer roller 14 is set. The time for the transfer roller 14 to make two rotations can be detected from the number of rotations of the drive motor of the transfer roller 14.
[0069]
Then, in the next steps 114 and 116, a (positive) bias having a polarity opposite to that of the normally-charged toner is applied to the transfer roller 14 for a time during which the transfer roller 14 makes one rotation (time t in FIG. 3). ₈ ~ T ₉ ). Thereby, the opposite polarity toner (+ charged toner) attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
[0070]
Through the processing of steps 110 to 116, both the normally charged toner and the opposite polarity toner attached to the transfer roller 14 are removed, and the surface of the transfer roller 14 is cleaned.
[0071]
Then, as a post-process, the application of the charging bias is stopped in step 118 (time t in FIG. 3). ₁₀ Then, after a lapse of time T1, the application of the negative DC voltage of the developing bias voltage (AC voltage + negative DC voltage) is stopped in step 122 (time t in FIG. 3). ₁₁ ). Further, in step 124, the rotation driving of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is stopped, and the control routine is ended.
[0072]
According to the first embodiment described above, a bias of both positive and negative polarities is applied to the transfer roller 14 not after the start of the transfer process by the transfer roller 14 but after the end of the transfer process, so that the normal Both the charged toner and the opposite polarity toner are removed. Since the transfer roller 14 is cleaned after the completion of the transfer process in this manner, only the time required for transporting the recording medium is required before the start of the transfer process, so that the time before the start of the transfer process can be reduced, and the FPOT can be reduced.
[0073]
Further, since a negative DC voltage is continuously applied to the developing roller 12 from immediately after the start of the printing operation to immediately before the end of the printing operation, the potential of the developing roller 12 is maintained at a predetermined negative potential, and the charged photosensitive drum 8 and the developing roller 12 , The toner attached to the surface of the developing roller 12, in particular, the positively charged reverse polarity toner, can be prevented from flying to the photosensitive drum 8. Accordingly, it is possible to prevent the transfer roller 14 from being stained with the toner attached to the photosensitive drum 8 and the recording paper 22 conveyed to the transfer position 26 from being stained with the toner.
[0074]
[Second embodiment]
Next, a second embodiment corresponding to the first and third aspects of the present invention will be described. In the second embodiment, the configuration of the laser beam printer 1 is the same as that of the first embodiment, and the description of the device configuration will be omitted.
[0075]
Next, as an operation in the second embodiment, when the laser beam printer 1 receives digital image data of an image to be printed and print request information such as the number of prints and the paper size from an external information processing device, A control routine relating to the print processing executed by the control unit 40 will be described with reference to flowcharts of FIGS. 7 and 8 and a time chart of FIG.
[0076]
In step 130 in FIG. 7, each of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is driven to rotate, and is driven to rotate stably at a constant angular velocity. Also, a counter N for counting the number of prints is initialized to “0”.
[0077]
In the next step 132, only the AC voltage of the predetermined charging bias voltage (AC voltage + negative DC voltage) is started to be applied to the charging roller 10 (time t in FIG. 6). ₁ ). In the next step 134, a subroutine of the bias control process 2 in FIG. 8 is executed as various bias control processes during the image forming process.
[0078]
It is determined whether or not the application timing of the charging bias voltage according to the paper passing start time has come based on the rotation speeds of the respective motors that drive the photosensitive drum 8 and the transport rollers 31 and 32 (step 232). Then, when it is time to start applying the charging bias voltage according to the paper passing start time (Yes at step 232), the DC voltage of the predetermined charging bias voltage (AC voltage + negative DC voltage) is charged to the charging roller. 10 (step 234, time t in FIG. 6). ₂ ). As a result, a predetermined charging bias voltage (AC voltage + negative DC voltage) is applied to the charging roller 10 together with the AC voltage started in step 132, and the photosensitive drum by the charging roller 10 is operated as usual. 8 is performed.
[0079]
The charged position of the photosensitive drum 8 is irradiated with a laser beam from the laser scanning device 2 (that is, a laser beam modulated in accordance with digital image data of an image to be formed) to form an electrostatic latent image. I do.
[0080]
After the formation of the electrostatic latent image, when it is time to start applying the developing bias voltage according to the paper passing start time (Yes at step 236), a predetermined developing bias voltage (AC voltage + negative DC voltage) is developed. The application to the device 11 is started (step 238, time t in FIG. 6). ₃ ). As a result, a predetermined developing bias voltage (AC voltage + negative DC voltage) is applied to the developing device 11, and the developing device 11 develops the electrostatic latent image as usual, thereby forming a toner image. Is done.
[0081]
Next, it is determined whether or not the leading edge of the recording paper 22 has reached the vicinity of the transfer position 26 (that is, the paper passing start timing has been reached) based on the presence or absence of the detection signal from the leading edge detection sensor 33 (step 240). . When the leading end of the recording paper 22 reaches the vicinity of the transfer position 26 and a detection signal is output from the leading end detection sensor 33 (Yes at Step 240), a predetermined transfer bias voltage (AC voltage + positive (DC voltage) is applied to the transfer roller 14 (step 242, time t in FIG. 6). ₄ ). As a result, when the recording paper 22 is passing through the transfer position 26, the toner image is transferred from the photosensitive drum 8 to the recording paper 22 by the transfer roller 14, and a toner image is formed on the recording paper 22.
[0082]
After that, the recording paper 22 is conveyed to the fixing device 20, and the toner image is fixed on the recording paper 22 by the fixing process by the fixing device 20, and a target image is formed on the recording paper 22.
[0083]
Then, when the application end time of the charging bias voltage according to the paper end time comes (when affirmed in step 244), the application of the DC voltage of the charging bias voltage (AC voltage + negative DC voltage) is stopped. (Step 246, time t in FIG. 6) ₅ ).
[0084]
Then, when the application end time of the developing bias voltage according to the paper passing end time comes (Yes at Step 248), the application of the developing bias voltage (AC voltage + Negative DC voltage) is stopped (Step 250, FIG. Time t of 6 ₆ ).
[0085]
At this time, since the application of the DC voltage of the charging bias voltage has been stopped, the surface potential of the photosensitive drum 8 is substantially “0”, and when the application of the developing bias voltage is stopped in step 250, the surface of the developing roller 12 is stopped. The potential is also almost “0”. For this reason, the potential difference between the developing roller 12 and the photosensitive drum 8 remains small, and the toner attached to the surface of the developing roller 12, in particular, the positively charged reverse polarity toner is prevented from flying to the photosensitive drum 8. Can be.
[0086]
Further, it is determined whether or not the rear end of the recording paper 22 has passed the transfer position 26 (that is, the paper passing end time has come) based on the presence or absence of the detection signal from the rear end detection sensor 34 (step 252). Then, when the rear end of the recording paper 22 passes through the transfer position 26 and a detection signal is output from the rear end detection sensor 34 (Yes at Step 252), the transfer bias voltage (AC voltage + positive DC voltage) ) Is stopped (step 254, time t in FIG. 6). ₇ ), The counter N is incremented by one (step 256).
[0087]
Here, it is determined whether or not the counter N has reached the total number of prints (step 258). If the counter N has not reached the total number of prints, it is considered that all print processing has not been completed, and the process proceeds to step 232. Return, and execute the unprocessed print processing.
[0088]
Even when the process is executed again from step 232, no developing bias is applied as shown in FIG. period (T in FIG. 6) ₆ ~ T ₉ Corresponding to) period (T in FIG. 6) _Five ~ T ₈ In (2), since the application of the negative DC voltage of the charging bias voltage is stopped, both the surface potential of the photosensitive drum 8 and the surface potential of the developing roller 12 become substantially “0”. For this reason, the potential difference between the developing roller 12 and the photosensitive drum 8 remains small, and the toner attached to the surface of the developing roller 12, in particular, the positively charged reverse polarity toner is prevented from flying to the photosensitive drum 8. Can be.
[0089]
In this way, images are formed (printed) one by one on the recording paper 22. When the printing of all the sheets is completed, the counter N reaches the total number of printed sheets, and the process returns from the subroutine of FIG.
[0090]
After the completion of all the printing processes as described above, in steps 136 and 138 following the main routine of FIG. 7, the transfer roller 14 is charged with the normally charged toner (−charged toner) for the time required for the transfer roller 14 to make two rotations. A polarity (negative) bias is applied (time t in FIG. 6). ₁₀ ~ T ₁₁ ). As a result, the normally charged toner attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
Then, in the next steps 140 and 142, a bias (positive) having a polarity opposite to that of the normally-charged toner is applied to the transfer roller 14 for a time during which the transfer roller 14 makes one rotation (time t in FIG. 6). ₁₁ ~ T ₁₂ ). Thereby, the opposite polarity toner (+ charged toner) attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
[0091]
Through the processing in steps 136 to 142, both the normally charged toner and the opposite polarity toner attached to the transfer roller 14 are removed, and the surface of the transfer roller 14 is cleaned.
[0092]
Then, as a post-process, application of the AC voltage of the charging bias is stopped in step 144 (time t in FIG. 6). _Thirteen In step 146), the rotational drive of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is stopped, and the control routine ends.
[0093]
According to the second embodiment described above, no developing bias is applied. period (T in FIG. 6) ₆ ~ T ₉ Corresponding to) period (T in FIG. 6) _Five ~ T ₈ In (2), since the application of the negative DC voltage of the charging bias voltage is stopped, both the surface potential of the photosensitive drum 8 and the surface potential of the developing roller 12 become substantially “0”, and the photosensitive drum 8 and the developing roller 12 , And the toner attached to the surface of the developing roller 12, in particular, the positively charged reverse-polarity toner, can be prevented from flying to the photosensitive drum 8. Accordingly, it is possible to prevent the transfer roller 14 from being stained with the toner attached to the photosensitive drum 8 and the recording paper 22 conveyed to the transfer position 26 from being stained with the toner.
[0094]
[Third embodiment]
Next, a third embodiment corresponding to the inventions described in claims 1 and 4 will be described. In the third embodiment, the configuration of the laser beam printer 1 is the same as that of the first embodiment, and a description of the device configuration will be omitted.
[0095]
Next, as an operation of the third embodiment, when the laser beam printer 1 receives digital image data of an image to be printed and print request information such as the number of prints and the paper size from an external information processing device, A control routine relating to the print processing executed by the control unit 40 will be described with reference to flowcharts of FIGS. 10 and 11 and a time chart of FIG.
[0096]
In step 160 in FIG. 10, each of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is driven to rotate, and is driven to rotate stably at a constant angular velocity. Also, a counter N for counting the number of prints is initialized to “0”.
[0097]
In the next step 162, application of a charging bias voltage in which a negative DC voltage is superimposed on an AC voltage to the charging roller 10 is started (time t in FIG. 9). ₁ ). Thus, the surface of the photosensitive drum 8 is charged to a negative potential.
[0098]
Then, it waits until the charged position on the photosensitive drum 8 reaches the exposure position of the laser beam from the laser scanning device 2 by the rotation of the photosensitive drum 8 (waits for the time T2 in FIG. 9). When it is detected from the rotation angle of the photoconductor drum 8 that the charged position has reached the exposure position, the process proceeds to step 166, where the laser scanning device 2 irradiates a laser beam to start overall exposure (FIG. 9). Time t ₂ ).
[0099]
Since the entire surface of the surface of the photosensitive drum 8 charged to the negative potential is exposed, the potential level on the surface approaches “0”, so that the developing roller 12 and the photosensitive drum 8 to which the developing bias is not applied are applied. , And the toner attached to the surface of the developing roller 12, in particular, the positively charged reverse-polarity toner, can be prevented from flying to the photosensitive drum 8.
[0100]
In the next step 168, a subroutine of the bias control process 3 in FIG. 11 is executed as various bias control processes during the image forming process.
[0101]
It is determined whether or not the exposure start time according to the paper passing start time has been reached based on the rotation speed of each motor for driving the photosensitive drum 8 and the transport rollers 31 and 32 (step 262). Then, when the exposure start timing according to the paper passing start timing is reached (Yes at step 262), the entire surface exposure is stopped in order to switch to the exposure using laser light modulated according to the image signal of the image to be formed. (Step 264, time t in FIG. 9) ₃ ). After stopping the entire surface exposure, the photosensitive drum 8 is irradiated with a laser beam modulated according to digital image data of an image to be formed to form an electrostatic latent image.
[0102]
After the formation of the electrostatic latent image, when it is time to start applying the developing bias voltage according to the paper passing start time (Yes at step 266), the predetermined developing bias voltage (AC voltage + negative DC voltage) is developed. The application to the device 11 is started (step 268, time t in FIG. 9). ₄ ). As a result, a predetermined developing bias voltage (AC voltage + negative DC voltage) is applied to the developing device 11, and the developing device 11 develops the electrostatic latent image as usual, thereby forming a toner image. Is done.
[0103]
Next, it is determined whether or not the leading edge of the recording sheet 22 has reached the vicinity of the transfer position 26 (that is, the sheet passing start time has come) based on the presence or absence of the detection signal from the leading edge detection sensor 33 (step 270). . When the leading edge of the recording paper 22 reaches the vicinity of the transfer position 26 and a detection signal is output from the leading edge detection sensor 33 (Yes at Step 270), a predetermined transfer bias voltage (AC voltage + positive DC voltage) is applied to the transfer roller 14 (step 272, time t in FIG. 9). ₅ ). As a result, when the recording paper 22 is passing through the transfer position 26, the toner image is transferred from the photosensitive drum 8 to the recording paper 22 by the transfer roller 14, and a toner image is formed on the recording paper 22.
[0104]
After that, the recording paper 22 is conveyed to the fixing device 20, and the toner image is fixed on the recording paper 22 by the fixing process by the fixing device 20, and a target image is formed on the recording paper 22.
[0105]
Then, when the exposure end time according to the paper passing end time comes (Yes in step 274), the entire exposure of the photosensitive drum 8 is restarted (step 276, time t in FIG. 9). ₆ ).
[0106]
Then, when the application end time of the developing bias voltage according to the paper passing end time comes (Yes at Step 278), the application of the developing bias voltage (AC voltage + Negative DC voltage) is stopped (Step 280, FIG. 9 time t ₇ ).
[0107]
Further, it is determined whether or not the rear end of the recording paper 22 has passed the transfer position 26 (that is, the paper passing end time has come) based on the presence or absence of the detection signal from the rear end detection sensor 34 (step 282). Then, when the rear end of the recording paper 22 passes through the transfer position 26 and a detection signal is output from the rear end detection sensor 34 (Yes at Step 282), the transfer bias voltage (AC voltage + positive DC voltage) ) Is stopped (step 284, time t in FIG. 9). ₈ ), And increment the counter N by one (step 286).
[0108]
Here, it is determined whether or not the counter N has reached the total number of prints (step 288). If the counter N has not reached the total number of prints, it is considered that all print processing has not been completed, and the process proceeds to step 262. Return, and execute the unprocessed print processing.
[0109]
Even when the process is executed again from step 262, no developing bias is applied as shown in FIG. period (Figure 9 T ₇ ~ T _Ten Corresponding to) period (T in FIG. 9) ₆ ~ T ₉ In (2), since the photosensitive drum 8 is entirely exposed, the surface potential of the photosensitive drum 8 that has been entirely exposed approaches “0”, and the potential difference between the developing roller 12 to which the developing bias is not applied and the photosensitive drum 8 is As a result, it is possible to prevent the toner attached to the surface of the developing roller 12, in particular, the positively charged opposite polarity toner from flying to the photosensitive drum 8.
[0110]
In this manner, images are formed (printed) one by one on the recording paper 22. When the printing of all the sheets is completed, the counter N reaches the total number of printed sheets, and the process returns from the subroutine of FIG.
[0111]
After the entire print processing is completed as described above, the entire surface exposure is stopped in step 170 following the main routine in FIG. 10 (time t in FIG. 9). _Thirteen ). The stop timing of the overall exposure may actually be in the middle of applying a bias of the same polarity to the transfer roller 14 in step 172 described later, as shown in FIG.
[0112]
In the next steps 172 and 174, a (negative) bias having the same polarity as that of the normally charged toner (-charged toner) is applied to the transfer roller 14 for the time that the transfer roller 14 makes two rotations (time t in FIG. 9). ₁₂ ~ T ₁₄ ). As a result, the normally charged toner attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
[0113]
Then, in the next steps 176 and 178, a bias (positive) having a polarity opposite to that of the normally charged toner is applied to the transfer roller 14 for a time during which the transfer roller 14 makes one rotation (time t in FIG. 9). ₁₄ ~ T _Fifteen ). Thereby, the opposite polarity toner (+ charged toner) attached to the transfer roller 14 flies from the transfer roller 14 to the photosensitive drum 8.
[0114]
Through the processing of steps 172 to 178, both the normally charged toner and the opposite polarity toner attached to the transfer roller 14 are removed, and the surface of the transfer roller 14 is cleaned.
[0115]
Then, as a post-process, the application of the charging bias is stopped in step 180 (time t in FIG. 9). ₁₆ In step 182, the rotation of the photosensitive drum 8, the charging roller 10, the developing roller 12, and the transfer roller 14 is stopped, and the control routine is terminated.
[0116]
According to the third embodiment described above, no developing bias is applied. period (T in FIG. 9) ₇ ~ T _Ten Corresponding to) period (T in FIG. 9) ₆ ~ T ₉ In (2), since the entire surface of the photosensitive drum 8 is exposed, the surface potential of the photosensitive drum 8 approaches "0", and the potential difference between the developing roller 12 to which the developing bias is not applied and the photosensitive drum 8 becomes small. It is possible to prevent the toner attached to the surface of the roller 12, in particular, the positively charged opposite polarity toner from flying to the photosensitive drum 8. Accordingly, it is possible to prevent the transfer roller 14 from being stained with the toner attached to the photosensitive drum 8 and the recording paper 22 conveyed to the transfer position 26 from being stained with the toner.
[0117]
In the first to third embodiments, after the transfer process in the image forming process is completed, a positive DC bias voltage is applied for the time that the transfer roller 14 makes two turns, and a negative DC bias voltage is applied for the time that the transfer roller 14 makes one turn. Although the example in which the bias voltage is applied is described above, the application time of each DC bias voltage is an example, and is applied only for a time sufficient to remove from the transfer roller 14 in accordance with the amount of the normally charged toner and the amount of the opposite polarity toner. Just do it.
[0118]
Further, the example in which the photosensitive drum is used as the image carrier has been described, but the present invention is applicable to a case in which another image carrier such as a photosensitive belt is used. The transfer unit may be a transfer belt other than the transfer roller, and the recording medium on which an image is formed may be another recording medium such as an OHP sheet other than the paper.
[0119]
In the above description, an example is shown in which a bias voltage having the same polarity as the charging polarity (−) of the toner and a bias voltage having the opposite polarity are sequentially applied to the transfer roller. However, the opposite polarity toner hardly adheres to the transfer roller. In this case, the cleaning effect of the transfer roller can be sufficiently obtained only by applying only the bias voltage having the same polarity as the charge polarity (-) of the toner to the transfer roller.
[0120]
【The invention's effect】
As described above, claims 1 to 1 4 According to the invention described in (1), the control means is provided only after the transfer process is completed. , The transfer means To Voltage of the same polarity as the charging polarity of the toner And then apply a voltage of the opposite polarity to the charging polarity of the toner. In addition, since the potential difference between the developing means and the image carrier when the development is not performed is smaller than the potential difference at which the toner flies from the developing means, there is no need to perform cleaning before the image forming operation, and the transfer process is started. The previous time can be reduced, and toner contamination of the transfer unit can be prevented while ensuring high image quality.
[Brief description of the drawings]
FIG. 1 is a sectional view of a main part of a laser beam printer according to an embodiment of the present invention.
FIG. 2 is an enlarged view around a photosensitive drum.
FIG. 3 is a timing chart showing application timings of a charging bias, a developing bias, and a transfer bias in the first embodiment.
FIG. 4 is a flowchart illustrating a control routine according to the first embodiment.
FIG. 5 is a flowchart showing a subroutine of bias control processing 1;
FIG. 6 is a timing chart showing application timings of a charging bias, a developing bias, and a transfer bias in a second embodiment.
FIG. 7 is a flowchart illustrating a control routine according to a second embodiment.
FIG. 8 is a flowchart showing a subroutine of bias control processing 2;
FIG. 9 is a timing chart showing the application timing of a charging bias, a developing bias, and a transfer bias, and the overall exposure timing in the third embodiment.
FIG. 10 is a flowchart illustrating a control routine according to a third embodiment.
FIG. 11 is a flowchart showing a subroutine of bias control processing 3;
[Explanation of symbols]
1 laser beam printer (image forming device)
8 Photoconductor drum (image carrier)
10 Charging roller
11 Developing device
12 Developing roller
14 Transfer roller
22 Recording paper (recording medium)
40 control device
42 Transfer bias voltage applying device
44 Development bias voltage application device
46 Charging bias voltage applying device

Claims (4)

Developing means for developing a toner image by developing an electrostatic latent image formed on the image carrier whose surface is charged with the image carrier in a non-contact manner in the vicinity of the image carrier, and forming a toner image;
Transfer means for transferring the toner image to a recording medium arranged in contact with the image carrier and conveyed to a contact portion with the image carrier;
The not pre-transfer process start by the transfer means, the only after transfer process is completed by the transfer means, after the application of the same polarity as the charging polarity of the voltage of the previous SL toner to the transfer unit, the toner charging polarity and opposite polarity A voltage is applied , and a developing bias voltage obtained by superimposing a DC voltage on an AC voltage is applied to the developing unit when the developing is being performed, and a potential difference between the developing unit and the image carrier when the developing is not being performed. Control means for reducing the potential difference at which toner flies from the developing means to the image carrier,
An image forming apparatus having: 2. The image forming apparatus according to claim 1, wherein the control unit reduces a potential difference between the developing unit and the image carrier by applying only a DC voltage component of a developing bias voltage to the developing unit. . The control unit reduces the potential difference between the developing unit and the image carrier by inhibiting the application of a DC voltage component of a charging bias voltage of a charging unit that charges the surface of the image carrier. The image forming apparatus according to claim 1. The image forming apparatus according to claim 1, wherein the control unit reduces the potential difference between the developing unit and the image carrier by exposing the entire surface of the image carrier.

Images