Abstract:
An image forming apparatus includes an image bearing member and a charging device for charging the image bearing member. An electrostatic image forming device selectively removes charges on the image bearing member charged by the charging device to form an electrostatic image. A developing device develops the electrostatic image with toner as a toner image. The toner image on the image bearing member is transferred by a transfer device onto a transfer material. The charging device temporarily collects residual toner after transferring the toner image. A return bias applying device applies a return bias for returning the toner collected in the charging device to the image bearing member in a nonimage forming period. The length of a return bias applying time period of the return bias applying device is variably controlled by a controller.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus such as a copying machine, a printer and the like, utilizing an electrophotographic system or an electrostatic recording system. 
     2. Related Background Art 
     Image forming apparatuses utilizing an electrophotographic system are well-known. In an image forming process of such an apparatus, after a surface of a photosensitive drum as a latent image bearing member is uniformly charged by a charging device, an electrostatic latent image is formed on the photosensitive drum by a laser or an LED. Then, the electrostatic latent image is developed with toner as developer to form a toner image which is in turn transferred onto a recording material as a recording medium such as paper. 
     In color image formation, the above-mentioned process is repeated for magenta, cyan, yellow and black colors, and color toner images superimposed on the recording material is fixed to the recording material by heat to form a color image. During this process, after the transferring, residual toner remaining on the photosensitive drum is removed by an exclusive cleaning apparatus. 
     By the way, recently, a reduction in cost and compactness of the apparatus have been requested. To this end, a so-called cleaner-less image forming apparatus in which such a cleaning apparatus is not provided around the latent image bearing member has been proposed. 
     In such a cleaner-less image forming apparatus, after the transferring, in order to remove the residual toner remaining on the photosensitive drum (referred to as “transfer-residual toner” hereinafter), for example, there has been proposed a method in which, after the transferring, a small amount of toner remaining on the photosensitive drum is once received by the charging device as charging means of contact type to change the electrostatic property and then the toner is returned to the photosensitive drum again, and, thereafter, the toner is collected by a developing apparatus also acting as collecting means to use the toner again. By this method, the residual toner on the surface of the photosensitive drum is removed and collected in a sheet-to-sheet interval during the print job or within a predetermined time period after the print job. Such a method is described in the Applicant&#39;s U.S. Pat. No. 6,215,967. 
     However, in such an image forming apparatus, during continuous printing of images having high density, an amount of transfer-residual toner is increased, with the result that the process for returning the toner to the photosensitive drum may not completely follow the process for receiving the residual toner once by the charging device. In such a case, within the charging device, the toner is mixed with a ferrite carrier as a low resistance carrier forming a dielectric brush. If the amount of toner is greatly increased in the charging device, the surface of the photosensitive drum cannot be uniformly charged with a predetermined potential, with the result that desired image density may not be achieved. 
     Further, even when images having the same density are printed, in the charging device which has once been used, efficiency of toner discharging process will be worsened, in comparison with a new charging device. 
     In addition, even when only images having low density are printed, in dependence upon the environment, a small amount of toner may be gradually accumulated in the charging device, thereby worsening the charging ability. 
     SUMMARY OF THE INVENTION 
     An object of the present invention is to provide an image forming apparatus in which collection and discharging of toner can be effected without worsening a charging ability of a charging device. 
     Another object of the present invention is to provide an image forming apparatus in which a discharging time from charging means can be altered. 
     A further object of the present invention is to provide an image forming apparatus comprising an image bearing member, charging means for charging the image bearing member, electrostatic image forming means for selectively removing charges on the image bearing member charged by the charging means to form an electrostatic image with toner as a toner image, transfer means for transferring the toner image on the image bearing member onto a transfer material (the charging means temporarily collecting residual toner after the transferring), return bias applying means for applying a return bias for returning the toner collected in the charging means to the image bearing member in a nonimage forming period, and control means for controlling a return bias applying time period of the return bias applying means. 
     The other objects and features of the present invention will be apparent from the following detailed explanation referring to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a sectional view showing a schematic construction of a color image forming apparatus as an example of an image forming apparatus according to an embodiment of the present invention; 
     FIG. 2 is a block diagram showing a construction of a digital image processing portion in the embodiment of the present invention; 
     FIG. 3 is a block diagram showing a construction of an LED driving portion in the embodiment of the present invention; 
     FIG. 4 is a sectional view showing a construction of charging means provided in the color image forming apparatus of FIG. 1; 
     FIG. 5 is a block diagram showing a construction of a video signal count portion in the embodiment of the present invention; 
     FIG. 6 is a table for determining a residual developer removing time in the embodiment of the present invention; and 
     FIG. 7 is a flowchart for explaining residual developer removing control in the embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present invention will now be fully explained in connection with embodiments thereof with reference to the accompanying drawings. 
     FIG. 1 is a sectional view showing a schematic construction of a color image forming apparatus as an example of an image forming apparatus according to an embodiment of the present invention. 
     Such a color image forming apparatus includes a color reader portion  1  and a color printer portion  2 . 
     First of all, the color reader portion  1  will be described. 
     In the color reader portion  1 , as shown in FIG. 1, an original resting glass (platen)  301  is located at an upper portion, and an original feeding apparatus (DF)  302  is provided above the original resting glass. Incidentally, in place of the original feeding apparatus  302 , a mirror surface pressing plate may be mounted. Within a first carriage  314 , there are mounted light sources  303  and  304  as halogen lamps, reflection hoods  305  and  306  for collecting lights from the light sources  303  and  304  onto the original, and a reflection mirror  307  for reflecting reflection light or projection light from the original. Further, within a second carriage  315 , there are mounted mirrors  308  and  309  for collecting the reflection light from the mirror  307  onto a CCD  101 . 
     The color reader portion  1  further comprises a digital image processing portion (referred to as “image processing portion” hereinafter)  312  including a substrate  311  on which the CCD  101  is mounted, elements shown in FIG. 2 other than the CCD  101 , and a binary value converting portion  201  and delay portions  202 ,  203  and  204  as shown in FIG. 3, and an interface (I/F) portion  313  to other elements such as an IPU. 
     Incidentally, the first carriage  314  and the second carriage  315  are mechanically shifted at speeds of V and V/2, respectively by driving means  316  in a direction perpendicular to an electrical scanning direction (main scanning direction) of the CCD  101 , thereby scanning (sub-scanning) the entire surface of the original. 
     FIG. 2 is a block diagram showing the image processing portion  312  in detail. 
     In the image processing portion  312 , as shown in FIG. 2, the lights from the light sources  303  and  304  are reflected by the original rested on the original resting glass  301 , and the reflected lights are directed to the CCD  101 , where the lights are converted into electric signals. Incidentally, in the case of a color sensor, the CCD  101  may be constituted so that R (red), G (green) and B (blue) color filters are arranged in-line CCD in the order of R, G and B or may be constituted so that the R filter, G filter and B filter are arranged on three CCDs, respectively or may be constituted so that the filters are designed as on-chip or the filters are provided independently of the CCDs. 
     The electric signals (analogue image signals) are inputted to the image processing portion  312 , sample hold (S/H) is effected in a clamp &amp; Amp. &amp; S/H &amp; A/D portion  102 , so that dark levels of the analogue image signals are clamped to reference potentials and amplified to predetermined amounts (processing order is not necessarily the above-mentioned order) and A/D-converted thereby to be converted into R, G and B 8-bit digital signals, for example. 
     Then, shading correction and black correction of the R, G and B signals are effected in a shading portion  103 . In the case where the CCD  101  is three-line CCD, since reading positions between lines are different regarding a binding process, then, in a binding &amp; MTF correction &amp; original detecting portion  104 , delay amounts of respective line are adjusted in accordance with reading speeds to correct signal timing so that the reading positions of three lines become equal. Further, regarding MTF correction, since MTF readings are changed in accordance with the reading speeds and variable power rates, which changes are corrected. Further, a size of the original on the original resting glass is ascertained by original detection. 
     The digital signals the reading position timings of which are corrected are inputted to an input masking portion  105 , where a spectral property of the CCD  101  and spectral properties of the light sources  303 ,  304  and the reflection hoods  305 ,  306  are corrected. Output from the input masking portion  105  is inputted to a selector  106  which can be switched to an external I/F signal. 
     The signals inputted to the undercolor removing portion  115 , where undercolor is removed, are then inputted to a black character judging portion  116  for judging whether characters on the original are black, where a black character signal is formed from the original. Further, in the color space compression &amp; undercolor removal &amp; LOG conversion portion  107  into which the output of the selector  106  was inputted, by color space compression, it is judged whether the read image signals are within a range in which the image signals can be reproduced by the printer. If the signals are within such a range, the signals are not processed; whereas, if the signals are not within such a range, the signals are corrected so that the signals fall within the range in which the image signals can be reproduced by the printer. Then, the undercolor removing process is performed, and, by LOG conversion, the R, G and B signals are converted into C (cyan), M (magenta) and Y (yellow) signals. 
     In order to correct the signal formed in the black character judging portion  116  and the timing, the output signals from the color space compression &amp; undercolor removal &amp; LOG conversion portion  107  are inputted to a delay portion  108 , where the timing is adjusted. The two kinds of signals are inputted to a moire removing portion  109 , where moire is removed. Then, the signals are inputted to a variable power processing portion  110 , where variable power processing is effected in the main scanning direction. 
     In an UCR &amp; masking &amp; black character reflecting portion  111 , regarding the signals processed in the variable power processing portion  110 , C, M, Y and K (black) signals are formed from the C, M and Y signals by UCR processing, and the signals are corrected to signals suitable for the output of the printer by masking processing, and the judging signal formed in the black character judging portion  116  is fed-back to the C, M, Y and K signals. 
     The signals processed in the UCR &amp; masking &amp; black character reflecting portion  111  are inputted to a γ correction portion  112 , where density is adjusted. Then, in a filter portion  113 , smoothing or edge processing is effected. 
     The signals processed in this way are inputted to a binary converting portion  201  shown in FIG. 3, where 8-bit multi-value signals are converted into binary signals. Incidentally, this converting method may be a deza method, an error diffusing method or an improved error diffusion method. 
     Next, the color printer portion  2  will be explained. 
     As shown in FIG. 1, the color printer portion  2  comprises a Y image forming portion  317 , an M image forming portion  318 , a C image forming portion  319  and a K image forming portion  320 , and these portions include photosensitive drums  342 ,  343 ,  344  and  345  as latent image bearing members, chargers  321 ,  324 ,  327  and  330  as charging means, LED portions  210 ,  211 ,  212  and  213  developing devices  322 ,  325 ,  328  and  331  also acting as collecting means, and auxiliary chargers  360 ,  361 ,  362  and  363  respectively. Further, the chargers  321 ,  324 ,  327  and  330  have charging sleeves  370 ,  371 ,  372  and  373  respectively, and the developing devices  322 ,  325 ,  328  and  331  have developing sleeves  354 ,  355 ,  356  and  357 , respectively. 
     Incidentally, since constructions of the M image forming portion  318 , C image forming portion  319  and K image forming portion  320  are the same as that of the Y image forming portion  317 , an explanation thereof will be omitted. 
     The Y image forming portion  317  has the photosensitive drum  342  around which the charger  321 , LED portion  210 , developing device  322  and auxiliary charger  360  are arranged. 
     In operation, first of all, the surface of the photosensitive drum  342  is charged by the auxiliary charger  360  and the charger  321 . As shown in FIG. 4, in the charger  321 , by rotating the charging sleeve  370  as a rotary member in a direction opposite to a rotational direction of the photosensitive drum  342 , a dielectric brush is formed from ferrite carrier  502  as low resistance carrier contained in a container (not shown), by which the surface of the photosensitive drum  342  is uniformly charged, thereby preparing for formation of the latent image. 
     Then, the latent image is formed on the surface of the photosensitive drum  342  by light from the LED array  210  and then is developed by the developing device  322  to form a toner image. 
     Incidentally in the developing device  322 , development is effected by applying developing bias between the photosensitive drum  342  and the developing sleeve  354 . 
     A transfer charger  323  urged against the photosensitive drum  342  with the interposition of a transfer belt  333  as transfer means below the developing device  322  effects discharging from the back side of the transfer belt  333 , with the result that the toner image on the photosensitive drum  342  is transferred onto a recording paper on the transfer belt  333 . 
     After the transferring, toner  503  remaining on the photosensitive drum  342  is once received by the charger  321  to change the electrostatic property of the photosensitive drum. Thereafter, the toner is returned to the photosensitive drum  342  again and then is collected by the developing device  322 . 
     Next, a sequence for forming the image on the recording paper will be explained. 
     Recording papers contained in a cassette  340  or  341  are fed one by one onto the moving transfer belt  333  by a pick-up roller  338  or  339  and sheet feeding rollers  336 ,  337 . The transfer belt  333  is shifted by a transfer belt roller  348  disposed below the Y image forming portion  317 , M image forming portion  318 , C image forming portion  319  and K image forming portion  320 . 
     A leading end of the recording paper fed to the transfer belt  333  is detected by a sheet leading end sensor  347 . A detection signal from the sheet leading end sensor is sent from the color printer portion  2  to the color reader portion  1  and is used as a subscanning synchronous signal when the video signal is sent from the color reader portion  1  to the color printer portion  2 . 
     Thereafter, the recording paper is conveyed by the transfer belt  333 , and the toner images are successively formed on the recording paper in the image forming portions  317  to  320  in the order of Y, M, C and K. 
     After the recording paper is passed through the K image forming portion  320 , electricity is removed from the recording paper by an electricity removing charger  349  in order to facilitate the separation of the recording paper from the transfer belt  333 . Then, the recording paper is separated from the transfer belt  333 . In this case, a peeling charger  350  disposed adjacent to the electricity removing charger  349  prevent distortion of the image due to peel discharging generated when the recording paper is separated from the transfer belt  333 . 
     The separated recording paper is charged by prefixing chargers  351  and  352  to promote a toner attracting force thereby to prevent the image distortion. Thereafter, the toner images are thermally fixed to the recording paper by a fixing device  334 . Then, the recording paper is discharged onto a sheet discharge tray  335 . Further, electricity is removed from the transfer belt  333  by inner and outer electricity removing chargers  353 . 
     Next, the LED image recording will be explained with reference to FIG.  3 . 
     In FIG. 3, the signals from the image processing portion are binarized in the binary converting portion  201  and then are sent to video signal counting portions  220  to  223  as image information detecting means. In the video signal counting portions  220  to  223 , the total numbers of the LEDs being illuminated are counted for the respective color images. 
     Thereafter, the binarized image signal are inputted to delay portions  202 ,  203 ,  204  and  205  where the signals are delayed in accordance with respective image forming positions and the sheet leading end sensor  347 , respectively. Then, the signals are sent to LED drivers  206 ,  207 ,  208  and  209 . The LED driving portion  206 ,  207 ,  208  and  209  serve to form or generate signals for driving LED portions  210 ,  211 ,  212  and  213 . 
     Next, control for interrupting the print job and for controlling length of return bias applying time for returning the toner from the charger to the photosensitive drum, which is a characteristic portion of the present invention, will be explained. 
     In the illustrated embodiment, the return bias applying time period is determined in accordance with an endurance level value sought from an image density total value from the initial condition of the image bearing member and from a count total value, and an image density total value between predetermined number count values obtained by number measuring means (not shown). 
     According to the illustrated embodiment, first of all, in the image formation of each image during the print job, the image density is detected. Here, as the image density for each color image, the total number of illuminated LEDs counted in the video count portion  220 ,  221 ,  222  or  223  as video counting means shown in FIG.  3 . 
     FIG. 5 shows the video signal count portion  220  in detail. Incidentally, the video signal count portions  221  to  223  have the same constructions as that of the video signal count portion  220 . 
     In the video signal count portion  220 , first of all, the image signal  700  sent from the binary converting portion  201  is counted by 29-bit counters  701  to  708  in parallel for each 8-bit as an image signal for one image, and counted results are added to each other by a 32-bit adder  709 , thereby obtaining the total number of illuminated LEDs as 32-bit data. 
     Such processing is effected for each image formation to seek the total number of illuminated LEDs (referred to as “video count” hereinafter), and a value obtained by successively adding the total numbers from the initial condition of the photosensitive drum  342  upon installation of the apparatus is regarded as V sum . Further, a value (calculated in terms of A4 sheet size) obtained by successively adding the image formation numbers from the initial condition of the photosensitive drum  342  upon installation of the apparatus is regarded as N sum . 
     When it is assumed that the sheet number corresponding to the service life of the drum is D and the video count value for A4 size solid print is V A4 , the endurance level value E is sought from the following equation: 
     
       
           E =( N/D )×( V   sum   /V   A4 ). 
       
     
     Then, by using the endurance level value E and a video count total value V 25  for previous 25 sheets, from a table shown in FIG. 6, a time period T1 for removing the residual toner. As shown in FIG. 6, since the higher the printed image density the greater the value V 25  and the longer the endurance time the greater the endurance level value E, the time period for removing the residual toner by returning the toner from the charge to the photosensitive drum becomes longer. 
     Further, when the total image formation number becomes 500 (sheets) from the previous removal of residual toner during the interruption of the job, a time period T2 for removing the residual toner is set to 60 seconds. 
     On the basis of T1 and T2, a time period T for removing the residual toner is sought from the following equation: 
     
       
           T=T   1   +T   2 . 
       
     
     Next, a method for removing the residual toner during the interruption of the job will be explained with reference to FIG.  7 . 
     In a step S 1 , if the fact that the sheet reaches a registration roller position is detected, in a step S 2 , the time period T for removing the residual toner is sought in the above-mentioned manner. Then, in a step S 3 , it is judged whether T=0, i.e., whether the removal of the residual toner is executed or not. If T=0, in a step S 5 , the sheet is fed from the registration rollers at a predetermined timing; whereas, if T≠0 (T is not 0), while the feeding of the sheet is being stopped at the registration roller position, in a step S 4 , the residual toner removing operation is carried out. In this operation, DC of −700 V and AC bias having a rectangular wave of 1.1 kV pp , 1 kHz and 50% duty are applied to the charger associated with the rotating photosensitive drum and the developing sleeve is driven, and DC of −550 V and AC bias having a rectangular wave of 1 kV pp , 2.2 kHz and 60% duty are applied to the developing device and the developing sleeve is driven. As a result, the charger once receives the toner on the photosensitive drum to change the electrostatic property and then returns the toner to the photosensitive drum again, and, thereafter, the developing device collects the toner. After this operation is performed by the time period T, the video count total value V 25  for previous 25 sheets and the image formation number count value for seeking the value T2 are reset. 
     In a step S 5 , the feeding of the sheet waiting at the registration roller position is started and the image formation is effected. 
     Incidentally, in the illustrated embodiment, while an example that the toner discharged from the charger is collected in the developing device was explained, in this case, by stopping the operation of the developing device, the residual toner may be transferred to the transfer belt and then may be collected by a transfer belt cleaner. 
     Further, in the illustrated embodiment, while an example that the image density is sought by using the total number of illuminated LEDs was explained, also in an apparatus in which a latent image is formed by a laser, similar control can be performed by using video count. 
     Further, as a method for seeking the image density, a potential sensor may be provided around the photosensitive drum to measure the potential of the photosensitive drum. 
     As mentioned above, in the image forming apparatus in which the transfer-residual toner on the latent image bearing member can be removed by the action of the charging means, the transfer-residual developer on the latent image bearing member can be removed and collected without worsening the charging ability of the charging means. 
     While the present invention was explained in connection with the specific embodiments, the present invention is not limited to such embodiments, but various alterations and modifications can be made within the scope of the invention.