JP5070191B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to a method for polishing a surface of a photoreceptor in a tandem type color image forming apparatus in which a plurality of image forming units including a photoreceptor are arranged in parallel along an intermediate transfer body.

  In recent years, an a-Si photosensitive drum has been widely used as an image carrier of an image forming apparatus using an electrophotographic process. The a-Si photoconductor drum has high hardness and excellent durability, and its characteristics as a photoconductor are hardly deteriorated even after long-term use. In addition, the image carrier is excellent in environmental safety.

  In such an image forming apparatus using an a-Si photosensitive drum, when the photosensitive drum is charged using a charging device, ozone is generated by discharge from the charging device. The components in the air are decomposed by the ozone, and ion products such as NOx and SOx are generated. Since this ion product is water-soluble, it adheres to the photosensitive drum and enters into a roughness structure of about 0.1 μm on the surface of the photosensitive drum, so that it can be removed by a cleaning system used in a general-purpose machine. Can not. In a high humidity environment, the ion product takes in moisture from the atmosphere and the resistance of the surface of the photosensitive drum is lowered. Therefore, the lateral flow of the potential occurs at the edge of the electrostatic latent image formed on the surface of the photosensitive drum. Occurs, resulting in an image drift phenomenon.

  In particular, when an image with a low printing rate (toner consumption is small) is output, the amount of ion products adhering to the surface of the photoreceptor increases, and the above-described image flow phenomenon occurs in a high humidity environment. In addition, the adherence increases the frictional resistance of the surface of the photoconductor, resulting in a decrease in transfer (primary transfer) efficiency from the photoconductor to the intermediate transfer member, image deterioration due to cleaning blade streaks, and winding up of the cleaning blade end. appear. Furthermore, since the driving load of the photosensitive member increases, it causes a jitter.

  In view of this, a method for reducing the image flow by suppressing a decrease in resistance on the surface of the photosensitive drum with a simple configuration has been proposed. Patent Documents 1 and 2 disclose a toner (polishing toner) mixed with an abrasive and a polishing member. There is disclosed a method of removing ozone products without using a heater or the like by polishing a photoconductor by the interaction of (rubbing roller and cleaning blade).

In the polishing system as described above, the surface of the photosensitive member is polished using the residual toner on the photosensitive member even during a normal printing operation, but at the time of non-printing (when the apparatus is started up or in a standby mode after printing). The photosensitive member refresh mode is used in which the toner is printed or developed, and the toner is actively supplied to the polishing means in the photosensitive unit without being transferred to the recording medium and used for polishing the surface of the photosensitive member.
Japanese Patent Laid-Open No. 11-3014 JP-A-11-133462 JP 2004-53917 A

  In a tandem type color image forming apparatus equipped with a plurality of photoconductors, even if the printing rate is low or single color printing is continued, toner is discharged onto the photoconductor for each color and polishing of each photoconductor is performed. However, it is necessary to always keep the frictional resistance of the surface of the photoreceptor below a certain level. Here, even when an image with the same printing rate is printed, the amount of deposits on the surface of the photoreceptor located upstream in the moving direction of the intermediate transfer member is larger than that on the photoreceptor located downstream, and the surface friction resistance value Rises.

  In other words, the toner developed on the upstream photoconductor is sequentially transferred onto the intermediate transfer member, so that the amount of toner on the intermediate transfer member increases toward the downstream side. Therefore, even if an image having the same printing rate is printed, the amount of toner supplied to the photoconductor is larger than that of the upstream photoconductor due to the reverse transfer phenomenon to the downstream photoconductor, and the downstream photoconductor surface is polished. This is because the effect becomes high. This phenomenon becomes remarkable particularly at a low printing rate of 2% or less.

  For this reason, it is necessary to increase the number of executions of the photoconductor refresh mode to sufficiently polish the upstream photoconductor. However, since the normal printing operation needs to be stopped during execution of the photoconductor refresh mode, the image forming efficiency is improved. There is a problem that the toner consumption is reduced except for the printing operation. Further, as a method of supplying toner to the upstream photoconductor, Patent Document 3 discloses an image in which the residual toner on the intermediate transfer member is reversely transferred onto the image carrier (photosensitive member), collected by the cleaning means, and reused. Although a forming apparatus is disclosed, in Patent Document 3, since the reverse transfer is performed only to the most upstream recovery agent-dedicated photoconductor, toner is supplied to all other photoconductors to suppress an increase in surface frictional resistance. I couldn't.

  In view of the above problems, the present invention efficiently removes deposits on the surface of the photoconductor and suppresses an increase in surface frictional resistance by efficiently supplying the toner to which the abrasive is added to the photoconductor on the upstream side. An object of the present invention is to provide a tandem type color image forming apparatus.

  To achieve the above object, the present invention provides an image carrier, a developing device that forms a toner image corresponding to an electrostatic latent image by attaching a toner containing an abrasive to the surface of the image carrier, and the image carrier. A plurality of image forming units that are pressed against the surface of the body with a predetermined pressure and that polish the surface of the image carrier, and an endless shape in which the developed toner images are sequentially stacked on the image carrier An intermediate transfer member, a primary transfer means for laminating a toner image developed on the image carrier on the intermediate transfer member, and a toner image laminated on the intermediate transfer member at a time onto a recording medium. One or more of the primary transfer during the rotation of the intermediate transfer body one or more times after the end of the single color printing or after the end of the final printing in the continuous color printing. Same polarity as toner It is characterized in that by applying a transfer reverse bias supplying the residual toner on the intermediate transfer body on one or more of the image bearing member.

  In the image forming apparatus having the above-described configuration, the surface potential of the image carrier to which the residual toner on the intermediate transfer member is supplied is approximately zero.

  According to the present invention, in the image forming apparatus having the above-described configuration, an intermediate transfer body cleaning unit that removes residual toner on the intermediate transfer body is provided, and the intermediate transfer body cleaning is performed while the intermediate transfer body is rotated at least once. The bias applied to the means is turned OFF or a bias having the same polarity as that of the toner is applied.

  According to the present invention, in the image forming apparatus having the above-described configuration, a black polishing image is formed on any one of the image forming units after completion of single color printing or after final printing in continuous color printing. The intermediate transfer member is rotated at least once in a state where the bias applied to the secondary transfer means is turned OFF or the transfer reverse bias having the same polarity as that of the toner is applied.

  Further, according to the present invention, in the image forming apparatus configured as described above, among the toners of a plurality of colors used in each image forming unit, the black toner contains more abrasive than the other color toners.

  According to the present invention, in the image forming apparatus configured as described above, a transfer reverse bias is applied only to the primary transfer unit corresponding to a low print unit having an average printing rate per predetermined number of sheets or less among the image forming units. It is characterized by that.

  According to the present invention, in the image forming apparatus configured as described above, a transfer reverse bias is applied only to the primary transfer unit corresponding to a low print unit having an average printing rate per predetermined number of sheets or less among the image forming units. At the same time, the polishing image is formed by a predetermined amount of toner together with the amount of toner per printed sheet used in the low printing unit.

  According to the first configuration of the present invention, since the residual toner on the intermediate transfer member can be supplied to the upstream image carrier having a small amount of toner after the end of the normal printing operation, the surface of the image carrier by the polishing member It is possible to suppress the increase in the coefficient of friction on the surface of the image carrier by promoting the polishing effect. Further, since the frequency of execution of the refresh mode in which the toner is forcibly discharged from the developing device and the surface of the image carrier is polished can be reduced, wasteful toner consumption is suppressed and the image forming efficiency is improved.

  Further, according to the second configuration of the present invention, in the image forming apparatus having the first configuration, the surface potential of the image carrier to which the residual toner on the intermediate transfer body is supplied is set to about 0, whereby the primary The residual toner on the intermediate transfer member can be smoothly moved onto the image carrier without excessively increasing the reverse transfer bias applied to the transfer means.

  According to the third configuration of the present invention, in the image forming apparatus having the first or second configuration, the bias applied to the intermediate transfer member cleaning unit is turned off while the intermediate transfer member is rotated at least once. Alternatively, by applying a bias having the same polarity as the toner, it is possible to prevent the toner from adhering to the intermediate transfer member cleaning means and to increase the efficiency of supplying the toner onto the image carrier.

  According to the fourth configuration of the present invention, in the image forming apparatus having any one of the first to third configurations, a black polishing image is formed on the intermediate transfer member after the normal printing operation is completed. Transfer and turn off the bias applied to the secondary transfer means, or rotate the intermediate transfer body one or more times with the reverse transfer bias of the same polarity as the toner applied, so that only images with a low printing rate Can output a sufficient amount of toner to the image carrier on the upstream side, thereby suppressing a reduction in polishing efficiency.

  Further, according to the fifth configuration of the present invention, in the image forming apparatus having the fourth configuration, among the plurality of color toners used in each image forming unit, the black toner has a larger amount than the other color toners. By adding an abrasive, it is possible to further enhance the polishing effect of the image carrier without changing the color of the toner.

  According to the sixth configuration of the present invention, in the image forming apparatus having any one of the first to fifth configurations, the transfer reverse bias is applied only to the primary transfer unit corresponding to the low print unit among the image forming units. By applying the toner, the residual toner on the intermediate transfer member can be preferentially fed to the image carrier with a small amount of toner supply, so the toner supply amount to each image carrier is averaged, and all image carriers are The body can be polished without excess or deficiency.

  According to the seventh configuration of the present invention, in the image forming apparatus having the fourth or fifth configuration, the transfer reverse bias is applied only to the primary transfer unit corresponding to the low print unit among the image forming units. At the same time, the minimum amount of black toner used can ensure a sufficient polishing effect by forming a polishing image with a predetermined amount of toner combined with the amount of toner per printed sheet used in the low printing unit. The amount can be suppressed.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings. FIG. 1 is a schematic diagram showing the configuration of a tandem color image forming apparatus of the present invention. In the main body of the image forming apparatus 100, four image forming units Pa, Pb, Pc, and Pd are sequentially arranged from the upstream side in the transport direction (the right side in FIG. 1). These image forming units Pa to Pd are provided corresponding to images of four different colors (yellow, cyan, magenta, and black), and yellow, cyan, magenta, and the like by respective steps of charging, exposure, development, and transfer. And a black image are sequentially formed.

  The image forming units Pa to Pd are provided with photosensitive drums 1a, 1b, 1c and 1d which carry visible images (toner images) of the respective colors, and are formed on these photosensitive drums 1a to 1d. The toner images thus transferred are sequentially transferred (primary transfer) onto an intermediate transfer belt 8 that moves adjacent to each image forming unit while rotating clockwise in FIG. 1 by a driving means (not shown). The image is transferred onto the paper P at a time (secondary transfer) by the next transfer roller 9 and further fixed on the paper P by the fixing unit 7 and then discharged from the apparatus main body. An image forming process for each of the photosensitive drums 1a to 1d is executed while rotating the photosensitive drums 1a to 1d counterclockwise in FIG.

  The paper P onto which the toner image is transferred is housed in a paper cassette 16 at the lower part of the apparatus, and is conveyed to the secondary transfer roller 9 via the paper feed roller 12a and the registration roller pair 12b. A sheet made of a dielectric resin is used for the intermediate transfer belt 8, and a belt in which both ends thereof are overlapped and joined to form an endless shape, or a belt without a seam (seamless) is used. A belt cleaning device 19 for removing the toner remaining on the surface of the intermediate transfer belt 8 is disposed upstream of the image forming unit Pa.

  Next, the image forming units Pa to Pd will be described. There are charging devices 2a, 2b, 2c and 2d for charging the photosensitive drums 1a to 1d and image information on the photosensitive drums 1a to 1d around and below the photosensitive drums 1a to 1d, which are rotatably arranged. The exposure unit 4 for exposing the toner, the developing devices 3a, 3b, 3c and 3d for forming toner images on the photosensitive drums 1a to 1d, and the developer (toner) remaining on the photosensitive drums 1a to 1d are removed. Cleaning devices 5a, 5b, 5c and 5d are provided.

  When an image formation start is input by the user, first, the surfaces of the photosensitive drums 1a to 1d are uniformly charged by the charging devices 2a to 2d, and then light is irradiated by the exposure unit 4 to each of the photosensitive drums 1a to 1d. An electrostatic latent image corresponding to the image signal is formed on the top. Each of the developing devices 3a to 3d includes a developing roller (developer carrying member) disposed opposite to the photosensitive drums 1a to 1d, and toners of each color of yellow, cyan, magenta, and black are supplied by a replenishing device (not shown). A predetermined amount is filled. This toner is supplied onto the photosensitive drums 1a to 1d by the developing rollers of the developing devices 3a to 3d, and electrostatically adheres to the electrostatic latent image formed by exposure from the exposure unit 4. A toner image is formed.

  After an electric field is applied to the intermediate transfer belt 8 at a predetermined transfer voltage, yellow, cyan, magenta and black toner images on the photosensitive drums 1a to 1d are transferred onto the intermediate transfer belt 8 by the transfer rollers 6a to 6d. Primary transcription. These four color images are formed with a predetermined positional relationship predetermined for forming a predetermined full-color image. Thereafter, the toner remaining on the surfaces of the photosensitive drums 1a to 1d is removed by the cleaning devices 5a to 5d in preparation for the subsequent formation of a new electrostatic latent image.

  The intermediate transfer belt 8 is stretched over a driven roller 10, a drive roller 11, and a tension roller 20, and the intermediate transfer belt 8 starts to rotate clockwise as the drive roller 11 is rotated by a drive motor (not shown). Then, the sheet P is conveyed from the registration roller 12b to the secondary transfer roller 9 provided adjacent to the intermediate transfer belt 8 at a predetermined timing, and the sheet P is conveyed at a nip portion (secondary transfer nip portion) with the intermediate transfer belt 8. A full color image is secondarily transferred onto P. The paper P on which the toner image is transferred is conveyed to the fixing unit 7.

  The sheet P conveyed to the fixing unit 7 is heated and pressurized when passing through the nip portion (fixing nip portion) of the pair of fixing rollers 13 to fix the toner image on the surface of the sheet P, and a predetermined full-color image is formed. It is formed. The paper P on which the full-color image is formed is distributed in the transport direction by the branching section 14 that branches in a plurality of directions. When an image is formed on only one side of the paper P, it is discharged as it is onto the discharge tray 17 by the discharge roller 15.

  On the other hand, when forming images on both sides of the paper P, a part of the paper P that has passed through the fixing unit 7 is once projected from the discharge roller 15 to the outside of the apparatus. Thereafter, the paper P is distributed to the paper transport path 18 by the branching section 14 by rotating the discharge roller 15 in the reverse direction, and is transported again to the secondary transfer roller 9 with the image surface reversed. Then, after the next image formed on the intermediate transfer belt 8 is transferred to the surface of the paper P on which the image is not formed by the secondary transfer roller 9 and conveyed to the fixing unit 7 to fix the toner image, It is discharged to the discharge tray 17.

  FIG. 2 is an enlarged view of the vicinity of the image forming unit Pa in FIG. Since the image forming units Pb to Pd have basically the same configuration, description thereof is omitted. Around the photosensitive drum 1a, a charging device 2a, a developing device 4a, and a cleaning device 5a are arranged along the drum rotation direction (counterclockwise in FIG. 2), and the primary transfer roller 6a with the intermediate transfer belt 8 interposed therebetween. Is arranged. Further, a belt cleaning device 19 including a belt cleaning roller 19a facing the driven roller 10 with the intermediate transfer belt 8 interposed therebetween is disposed on the upstream side in the rotation direction of the intermediate transfer belt 8 with respect to the photosensitive drum 1a.

  The charging device 2 a includes a charging roller 21 that contacts the photosensitive drum 1 a and applies a charging bias to the drum surface, and a charging cleaning roller 23 for cleaning the charging roller 21. The developing device 4a is a touch-down developing type having two agitating and conveying screws 25, a magnetic roller 27, and a developing roller 29. A developing bias having the same polarity (positive) as that of toner is applied to the developing roller 29. Toner flies over the drum surface.

  The cleaning device 5 a includes a rubbing roller 30, a cleaning blade 31, and a collection screw 33. The rubbing roller 30 is pressed against the photosensitive drum 1a with a predetermined pressure, and is driven to rotate in the same direction on the contact surface with the photosensitive drum 1a by a driving means (not shown). The speed is controlled faster than the peripheral speed of 1a (1.2 times here). Examples of the rubbing roller 30 include a structure in which a foam layer made of EPDM rubber and having an Asuka C hardness of 55 ° is formed as a roller body around a metal shaft. The material of the roller body is not limited to EPDM rubber, but may be rubber or foam rubber body of other materials, and those having an Asuka C hardness in the range of 10 to 90 ° are preferably used.

  A cleaning blade 31 is fixed in contact with the photosensitive drum 1a on the surface of the photosensitive drum 1a downstream of the contact surface with the rubbing roller 30 in the rotation direction. As the cleaning blade 31, for example, a polyurethane rubber blade having a JIS hardness of 78 ° is used, and is attached at a predetermined angle with respect to the tangential direction of the photosensitive member at the contact point. The material, hardness, dimensions, the amount of biting into the photosensitive drum 1a, the pressure contact force, and the like of the cleaning blade 31 are appropriately set according to the specifications of the photosensitive drum 1a.

  Residual toner removed from the surface of the photosensitive drum 1a by the rubbing roller 30 and the cleaning blade 31 is discharged to the outside of the cleaning device 5a (see FIG. 2) as the recovery screw 33 rotates. As the toner used in the present invention, silica, titanium oxide, strontium titanate, alumina or the like is embedded in the toner particle surface as an abrasive and held so as to partially protrude on the surface. Those that are electrostatically attached to the surface are used.

  The surface of the photosensitive drum 1a is polished by the residual toner containing the abrasive by rotating the rubbing roller 30 with a speed difference with respect to the photosensitive drum 1a, and the drum is formed by the rubbing roller 30 and the cleaning blade 31. Remove surface moisture and contaminants along with residual toner.

  Further, depending on the printing rate and environmental conditions in each of the image forming units Pa to Pd, the polishing of the photosensitive drums 1a to 1d may not be sufficient only by polishing with residual toner after image formation. Therefore, the step of forcibly discharging the toner in the developing devices 4a to 4d to the photosensitive drums 1a to 1d and the toner discharged to the photosensitive drums 1a to 1d are supplied to the rubbing rollers 30 of the cleaning devices 5a to 5d. Then, the photosensitive member refresh mode including the step of polishing the surfaces of the photosensitive drums 1a to 1d at least once is executed.

  The photosensitive drum refresh mode is executed when, for example, the image forming apparatus is started from the power-off state or the sleep (power saving) mode to the copy start state, or after every predetermined number of printings. The kimono can be removed in a short time, and an effective polishing system capable of effectively preventing image flow over a long period of time and making the polishing amount of the photosensitive layer on the drum surface constant. The execution time of the photoreceptor refresh mode is appropriately set according to the configuration of the photosensitive layer to be used, the use environment of the apparatus, the refresh mode execution interval, and the like.

  Next, the control path of the image forming apparatus of the present invention will be described. FIG. 3 is a block diagram showing an example of a control path used in the image forming apparatus of the present invention. It should be noted that since various control of each part of the apparatus is performed when the image forming apparatus 100 is used, the control path of the entire image forming apparatus 100 becomes complicated. Therefore, here, a portion of the control path that is necessary for the implementation of the present invention will be mainly described.

  The control unit 90 includes a central processing unit (CPU) 91 as a central processing unit, a read only memory (ROM) 92 that is a read-only storage unit, a random access memory (RAM) 93 that is a read / write storage unit, A plurality of (two in this case) that transmit a control signal to each device in the temporary storage unit 94, the counter 95, and the image forming apparatus 100 for storing image data and the like, and receive an input signal from the operation unit 50. The I / F (interface) 96 is provided. Further, the control unit 90 can be arranged at an arbitrary location inside the apparatus main body.

  The ROM 92 stores a control program for the image forming apparatus 100, data necessary for control, and the like that are not changed during use of the image forming apparatus 100. The RAM 93 stores necessary data generated during the control of the image forming apparatus 100, data temporarily required for controlling the image forming apparatus 100, and the like. Further, the ROM 92 (or RAM 93) supplies the photosensitive drums 1a to 1d with the number of printed sheets for determining whether or not the photosensitive member refresh mode needs to be executed and the residual toner on the intermediate transfer belt 8 after printing is finished as will be described later. In this case, the reference print rate used for determining the supply photosensitive drum is also stored. The counter 95 adds up the number of printed sheets and counts it. Note that the number of times may be stored in the RAM 93, for example, without providing the counter 95 separately.

  In addition, the control unit 90 transmits a control signal from the CPU 91 to the respective units and apparatuses in the image forming apparatus 100 through the I / F 96. In addition, a signal indicating the state and an input signal are transmitted from each part or device to the CPU 91 through the I / F 96. For example, the image forming units Pa to Pd, the exposure unit 4, the primary transfer rollers 6 a to 6 d, the fixing unit 7, the secondary transfer roller 9, the image input unit 40, and bias control are included in each part and device controlled by the control unit 90. Examples include the circuit 41, the operation unit 50, and the like.

  When the image forming apparatus 100 is a printer as shown in FIG. 1, the image input unit 40 is a receiving unit that receives image data transmitted from a personal computer or the like. When the image forming apparatus 100 is a copying machine, Scanning optical system equipped with a scanner lamp that illuminates the document during copying and a mirror that changes the optical path of the reflected light from the document, a condensing lens that focuses the reflected light from the document and forms an image It is an image reading unit composed of a CCD or the like that converts image light into an electrical signal. The image signal input from the image input unit 40 is converted into a digital signal and then sent to the temporary storage unit 94.

  The bias control circuit 41 is connected to the charging bias power source 42, the developing bias power source 43, the transfer bias power source 44, and the belt cleaning bias power source 45, and operates each of these power sources by an output signal from the control unit 90. Each of these power supplies applies a predetermined bias to the charging devices 2a to 2d, the developing roller 29, the primary transfer rollers 6a to 6d, the secondary transfer roller 9, and the belt cleaning roller 19a according to a control signal from the bias control circuit 41. .

  The operation unit 50 is provided with a liquid crystal display unit 51, LEDs 52 indicating various states, and a numeric keypad 53, and the user operates the operation unit 50 to input instructions, thereby making various settings of the image forming apparatus 100. And execute various functions such as image formation. The liquid crystal display unit 51 displays the state of the image forming apparatus 100, displays the image forming status and the number of copies, and can be used as a touch panel to perform various settings such as functions such as double-sided printing and black-and-white reversal, magnification setting, and density setting. It has become. The numeric keypad 53 is used for setting the number of copies to be printed and for inputting the other party's FAX number when the image forming apparatus 100 also has a FAX function.

  In addition, the operation unit 50 includes a start button for instructing the user to start image formation, a stop / clear button used when the image formation is stopped, and various settings of the image forming apparatus 100 in a default state. A reset button or the like is provided for use.

  In the tandem type color image forming apparatus as shown in FIG. 1, the toner transfer amount to the intermediate transfer belt 8 increases toward the downstream side of the intermediate transfer belt 8. Then, part of the toner once transferred to the intermediate transfer belt 8 at the time of transfer on the downstream photosensitive drum is reattached to the surface of the photosensitive drum. Therefore, even when each image forming unit Pa to Pd outputs an image with the same printing rate (toner consumption), the toner supply amount to the photosensitive drum 1a located on the most upstream side is the smallest, and the photosensitive member As the drums 1b, 1c, and 1d go downstream, the amount of toner supplied to the photosensitive drum sequentially increases. That is, the polishing on the upstream photosensitive drum is not sufficiently performed.

  If the photosensitive member refresh mode is executed, the upstream photosensitive drum can be similarly polished. However, if the photosensitive member refresh mode is frequently executed, useless toner consumption other than printing increases. Further, since it is necessary to stop normal printing during execution of the photoreceptor refresh mode, the image forming efficiency is also reduced.

  Therefore, in the image forming apparatus of the present invention, after the single color printing or after the final color continuous printing, the cleaning of the intermediate transfer belt 8 by the belt cleaning device 19 is forgotten, and the intermediate transfer belt 8 is rotated once. A transfer reverse bias having the same polarity as the toner is applied to the primary transfer rollers 6a to 6b to supply the residual toner on the intermediate transfer belt 8 to one or more photosensitive drums 1a to 1d. I am going to do the cleaning. For example, when residual toner is supplied to the uppermost photosensitive drum 1a, a reverse transfer bias is applied to the primary transfer roller 6a disposed to face the photosensitive drum 1a.

  At this time, the charging bias applied to the charging device 2a is turned off, and the surface potential of the photosensitive drum 1a is set to near zero by a static eliminating device (not shown) arranged on the downstream side of the cleaning device 5a. The residual toner on the intermediate transfer belt 8 can be smoothly moved onto the photosensitive drum 1a without excessively increasing the reverse transfer bias applied to the primary transfer roller 6a.

  As a result, the residual toner on the intermediate transfer belt 8 can be supplied to the upstream photosensitive drum 1a with a small amount of remaining toner after the end of the normal printing operation, so that the surface of the photosensitive drum 1a is polished by the rubbing roller 30. And the increase in the friction coefficient of the drum surface can be suppressed. In addition, since the frequency of execution of the photoreceptor refresh mode can be reduced, wasteful toner consumption is suppressed and image formation efficiency is improved. By selecting the primary transfer rollers 6a to 6d to which the reverse transfer bias is applied and the photosensitive drums 1a to 1d having a surface potential near 0, residual toner can be supplied to any photosensitive drum.

  The intermediate transfer belt 8 may be rotated two or more times without cleaning, but most of the residual toner on the intermediate transfer belt 8 moves to the photosensitive drums 1a to 1d in the first rotation. Further, as the number of rotations is increased, the time required for toner feeding becomes longer. Accordingly, it is sufficient to rotate the intermediate transfer belt 8 once.

  Incidentally, the increase in the surface friction coefficient (friction resistance) of the photosensitive drums 1a to 1d is closely related to the printing rate in each of the image forming units Pa to Pd. If the printing rate is 4% or more, a sufficient polishing effect can be obtained with only untransferred toner remaining on the photosensitive drum, and therefore, an increase in the friction coefficient on the drum surface can be suppressed. On the other hand, when the printing rate is less than 2%, particularly 1% or less, the toner supply amount to the rubbing roller 30 is insufficient with only untransferred toner, and the friction coefficient of the drum surface increases.

  Therefore, the average printing rate in each of the image forming units Pa to Pd is calculated, and the residual toner on the intermediate transfer belt 8 is applied to the photosensitive drum disposed in the image forming unit whose average printing rate is a predetermined value (for example, 2%) or less. It is preferable to supply with priority. As a result, the polishing effect of the photosensitive drums 1a to 1d can be substantially equalized, and the surface friction coefficient can be maintained substantially constant. The average print rate is calculated as a print rate (dot ratio with respect to the total number of pixels) bn for each image based on a digital signal in the temporary storage unit 94, and an integrated print rate Σbn obtained by integrating the print rate bn. Then, the average printing rate Σbn / A (%) per predetermined number of sheets is calculated by dividing the integrated printing rate Σbn by the number of printed sheets A counted by the counter 95. The number of printed sheets A can be set as appropriate according to the usage status of the user, the characteristics of each color toner, and the like.

  Note that in the case of monochrome printing, only the black image forming unit Pd is driven and the other image forming units Pa to Pc are stopped, so that the photosensitive drums 1a to 1c are not charged and ions are applied to the drum surface. Product adhesion does not occur. Accordingly, the toner feeding operation to the upstream side photosensitive drum by the intermediate transfer belt 8 as described above may be performed only after the end of color printing.

  FIG. 4 is a flowchart showing an example of toner feeding operation control after completion of printing executed in the image forming apparatus of the present invention. With reference to FIG. 1 to FIG. 3, a toner feeding execution procedure will be described along the steps of FIG. 4.

  First, when the color printing process is started by the operation of the operation panel 50 or a personal computer by the user, the counter 95 counts the number of printed sheets n (step S1), and determines whether or not printing of the designated number of sheets is finished. (Step S2). The control unit 90 calculates a printing rate bn for each image in each of the image forming units Pa to Pd based on the digital signal in the temporary storage unit 94, and further calculates an integrated printing rate Σbn obtained by integrating the printing rate bn. Then, the average printing rate is calculated from Σbn / n (step S3).

  Next, the control unit 90 determines whether or not there is an image forming unit having an average printing rate of a predetermined value (here 2%) or less (step S4). If there is an image forming unit of 2% or less, belt cleaning is performed. The intermediate transfer cleaning bias applied to the roller 19a is kept OFF (step S5). Then, the elapsed time t1 from the end of printing (secondary transfer) is compared with the moving time Tn of the intermediate transfer belt 8 from the secondary transfer roller 9 to the image forming unit Pn selected in step S4 (step S6). When t1 = Tn, a reverse transfer bias is applied to the primary transfer roller corresponding to the image forming unit (step S7). Movement times Ta, Tb, Tc, and Td of the intermediate transfer belt 8 from the secondary transfer roller 9 to the image forming units Pa to Pd are stored in advance in the ROM 92 (or RAM 93).

  For example, when the image forming unit Pa has a printing rate of 2% or less, the elapsed time t1 after the secondary transfer becomes the moving time Ta of the intermediate transfer belt 8 from the secondary transfer roller 9 to the image forming unit Pa. Then, a reverse transfer bias is applied to the primary transfer roller 6a. As a result, the residual toner after the secondary transfer that has reached the image forming unit Pa due to the rotation of the intermediate transfer belt 8 is supplied onto the photosensitive drum 1a. Similarly, when there are a plurality of image forming units having a printing rate of 2% or less, a transfer reverse bias is applied to the corresponding primary transfer roller at the timing when the residual toner reaches each image forming unit.

  Thereafter, it is determined whether or not the intermediate transfer belt 8 has made one rotation (step S8). If the intermediate transfer belt 8 has made one rotation, the reverse transfer bias applied to the primary transfer rollers 6a to 6d is turned off (step S9). ) The intermediate transfer cleaning bias is turned ON to remove residual toner on the intermediate transfer belt 8 (step S10). If there is no image forming unit having a printing rate of 2% or less in step S4, the intermediate transfer cleaning bias is turned on immediately after the printing is finished, and the residual toner on the intermediate transfer belt 8 is removed (step S10).

  According to the above procedure, it is possible to suppress an increase in the frictional resistance of the drum surface by a polishing operation using residual toner that is performed in association with a normal printing operation, and it is possible to continuously output a good image. it can. Therefore, the execution frequency of the photoreceptor refresh mode can also be reduced, and wasteful toner consumption other than the printing operation can be suppressed, and a decrease in image forming efficiency can be suppressed.

  Further, by calculating the average printing rate in each of the image forming units Pa to Pd and applying a transfer reverse bias to the primary transfer roller corresponding to the image forming unit having a low average printing rate, residual toner on the intermediate transfer belt 8 is removed. It is possible to preferentially feed the photosensitive drum with a small amount of toner supply. As a result, the amount of toner supplied to each of the photosensitive drums 1a to 1d is averaged, and all the photosensitive drums 1a to 1d can be polished without excess or deficiency.

  By the way, in the case of a user who mainly outputs an image with a low printing rate such as a character image, the total amount of toner transferred onto the intermediate transfer belt 8 in each of the image forming units Pa to Pd becomes very small. Therefore, the residual toner on the intermediate transfer belt 8 is also reduced, and the toner supply amount to each of the photosensitive drums 1a to 1d may not be sufficiently increased even if the above-described toner feeding operation is executed after printing is completed.

  Accordingly, immediately after the end of color printing, a polishing image is formed in the black image forming unit Pd and transferred onto the intermediate transfer belt 8. Then, a reverse transfer bias having the same polarity as the toner is applied to the secondary transfer roller 9, the intermediate transfer belt 8 is rotated once without applying a bias to the belt cleaning roller 19 a, and a predetermined timing is reached when the polishing image arrives. A reverse transfer bias is applied to the primary transfer roller. By controlling in this way, a sufficient amount of toner can be sent to the upstream photosensitive drum even when only an image with a low printing rate is output, and a reduction in polishing efficiency can be suppressed.

  Also, since the black toner needs to take into account the change in color due to the addition of abrasives, compared to color toners, the amount of abrasive added to the black toner should be increased compared to other color toners. Thus, the polishing effect of the photosensitive drums 1a to 1d can be further enhanced. In addition, the amount of black toner consumed is larger than that of other color toners due to the creation of an image for polishing, but since the initial filling amount of black toner is usually larger than that of other color toners, the toner container is frequently used. There will be no inconvenience such as replacement.

  The amount of toner used for forming the polishing image is preferably determined based on the average printing rate in each of the image forming units Pa to Pd. Specifically, the toner consumption (printing rate) of the developing device in the image forming unit to which the toner needs to be fed is converted into the toner consumption (average printing rate) per sheet and added to the average printing rate. A polishing image is formed with an amount of toner of 4% to 6%. This makes it possible to minimize the amount of black toner used other than normal printing.

  In order to polish the drum surface uniformly and efficiently, it is necessary to feed a certain amount of toner over the entire longitudinal direction of the photosensitive drum. Therefore, it is preferable that the polishing image is formed in the entire developable region (for example, A4 size), and the usage amount of the black toner is adjusted by the density (halftone) of the polishing image.

  In addition, when there are a plurality of image forming units that need to feed toner, a plurality of polishing images may be formed using a necessary amount of toner in each image forming unit. Alternatively, a necessary amount of toner may be added together in each image forming unit to form a single polishing image, which may be divided and sent.

  FIG. 5 is a flowchart showing another control example of the toner feeding operation after the printing is completed. In the control example of FIG. 5, if there is an image forming unit having a printing rate of 2% or less in step S4, a polishing image is formed in the black image forming unit Pd immediately after the final printing is finished (step S5), and intermediate transfer is performed. Transfer onto the belt 8. The polishing image is printed at a predetermined interval (when moved to the downstream side by 20 to 50 mm) after the final print area passes through the image forming unit Pd. Further, the polishing image is formed with black toner in an amount of 4% to 6% in addition to the average printing rate.

  Then, a transfer reverse bias having the same polarity as the toner is applied to the secondary transfer roller 9 (step S6). Further, the intermediate transfer cleaning bias applied to the belt cleaning roller 19a is kept off (step S7). Accordingly, the polishing image does not adhere to the secondary transfer roller 9 and the belt cleaning roller 19a. Alternatively, a mechanism for separating the secondary transfer roller 9 and the belt cleaning roller 19a from the intermediate transfer belt 8 may be provided.

  Then, the elapsed time t2 from the start of the transfer of the polishing image to the intermediate transfer belt 8 is compared with the moving time Tn ′ of the intermediate transfer belt 8 from the image forming unit Pd to the image forming unit Pn selected in step S4. (Step S8), and when t2 = Tn ', a reverse transfer bias is applied to the primary transfer roller corresponding to the image forming unit (Step S9). The movement times Ta ′, Tb ′, Tc ′, and Td ′ of the intermediate transfer belt 8 from the image forming unit Pd to each of the image forming units Pa to Pd are stored in advance in the ROM 92 (or RAM 93).

  Thereafter, it is determined whether or not the intermediate transfer belt 8 has made one rotation (step S10). If the intermediate transfer belt 8 has made one rotation, the reverse transfer bias applied to the primary transfer rollers 6a to 6d is turned off and the polishing is performed. After the image area passes the secondary transfer roller 9 again, the transfer reverse bias applied to the secondary transfer roller 9 is turned off (step S11). Finally, the intermediate transfer cleaning bias is turned on to remove residual toner on the intermediate transfer belt 8 (step S12). If there is no image forming unit having a printing rate of 2% or less in step S4, the intermediate transfer cleaning bias is turned on immediately after the printing is finished to remove the residual toner on the intermediate transfer belt 8 (step S12).

  According to the above procedure, it is possible to suppress an increase in the frictional resistance of the drum surface by a polishing operation that accompanies a normal printing operation even in a case where only character images are continuously printed. Therefore, similar to the control shown in FIG. 4, a good image can be output continuously, and the frequency of execution of the photoreceptor refresh mode can be reduced.

  Further, the average printing rate in each of the image forming units Pa to Pd is calculated, and the toner of the polishing image is preferentially sent to the photosensitive drum of the image forming unit having a low average printing rate, whereby each of the photosensitive drums 1a to 1d. The toner supply amount is averaged, and all the photoconductive drums 1a to 1d can be polished without excess or deficiency. Since the polishing image is formed with an amount of toner that is 4% to 6% in addition to the average printing rate, the amount of black toner used can be suppressed to the minimum amount that can ensure a sufficient polishing effect. .

  The control shown in FIGS. 4 and 5 is merely an example, and can be changed as appropriate according to the specifications of the apparatus. For example, the intermediate transfer cleaning bias applied to the belt cleaning roller 19a is OFF until the intermediate transfer belt 8 rotates once after the printing is completed. However, if a bias having the same polarity as that of the toner is applied, the residual toner remains on the belt cleaning roller 19a. The adhesion of the toner or the polishing image can be more effectively suppressed.

  Further, the application timing of the reverse transfer bias to the primary transfer roller may be determined by other methods. For example, when a sensor for detecting the density and color shift of the reference image on the intermediate transfer belt 8 is provided, the detection timing of the print image or the polishing image and the position of the intermediate transfer belt 8 from the sensor position to a predetermined image forming unit. It can also be determined based on the travel time.

  In addition, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the spirit of the present invention. For example, in the above-described embodiment, the belt cleaning device 19 including the belt cleaning roller 19a is disposed as a cleaning unit for the intermediate transfer belt 8. However, for example, the belt cleaning device 19 including a belt cleaning blade in place of the belt cleaning roller 19a. Can also be arranged. In that case, a mechanism for bringing the belt cleaning blade into contact with and separating from the intermediate transfer belt 8 is provided, and when the residual toner is supplied to the upstream photosensitive drum, the belt cleaning blade should be separated from the intermediate transfer belt 8. good.

  The present invention can be used in a tandem color image forming apparatus using an intermediate transfer method, and is 1 during the rotation of the intermediate transfer member one or more times after completion of single color printing or after final printing in continuous color printing. A transfer reverse bias having the same polarity as the toner is applied to one or more primary transfer means to supply residual toner on the intermediate transfer member onto one or more image carriers.

  As a result, it is possible to provide an image forming apparatus capable of supplying the residual toner on the intermediate transfer member to the upstream image carrier having a small amount of toner after the end of the normal printing operation. The polishing effect can be promoted to suppress an increase in the friction coefficient on the surface of the image carrier. Further, since the frequency of execution of the refresh mode can be reduced, wasteful toner consumption is suppressed and the image forming apparatus has high image forming efficiency.

  Also, after the normal printing operation is completed, a black polishing image is formed and transferred onto the intermediate transfer member, and the bias applied to the secondary transfer means is turned OFF, or the transfer reverse bias having the same polarity as the toner is applied. By rotating the intermediate transfer member once or more in the applied state, a sufficient amount of toner can be supplied to the upstream image carrier using the polishing image even when only an image with a low printing rate is output. An image forming apparatus is obtained. At this time, if a larger amount of abrasive is added to the black toner than the other toners, the polishing efficiency can be further improved and the color of the toner does not change.

  Further, by applying a transfer reverse bias only to the primary transfer means corresponding to the low printing unit whose average printing rate is a predetermined value or less, the residual toner on the intermediate transfer member is preferentially given to the image carrier having a small toner supply amount. Since the toner can be fed in, the amount of toner supplied to each image carrier is averaged, and the image forming apparatus can polish all the image carriers without excess or deficiency.

FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus of the present invention. FIG. 2 is a partially enlarged view around the image forming unit Pa in FIG. 1. FIG. 3 is a block diagram showing a control path of the image forming apparatus of the present invention. These are flowcharts showing an example of toner feeding operation control after the completion of printing, which is executed in the image forming apparatus of the present invention. FIG. 10 is a flowchart showing another example of control of toner feeding operation after printing is completed.

Explanation of symbols

1a to 1d Photosensitive drum (image carrier)
2a to 2d Charging device 3a to 3d Developing device 4 Exposure unit 5a to 5d Cleaning device 6a to 6d Primary transfer roller (primary transfer means)
8 Intermediate transfer belt (intermediate transfer member)
9 Secondary transfer roller (secondary transfer means)
19 Belt cleaning device (intermediate transfer member cleaning means)
19a Belt cleaning roller 30 Rub roller (abrasive member)
31 Cleaning blade (abrasive member)
90 control unit 100 image forming apparatus Pa to Pd image forming unit

Claims (7)

An image carrier, a developing device that forms a toner image corresponding to the electrostatic latent image by attaching a toner containing an abrasive to the surface of the image carrier, and is pressed against the surface of the image carrier at a predetermined pressure. A plurality of image forming units including a polishing member for polishing the surface of the image carrier;
An endless intermediate transfer member on which the developed toner images are sequentially laminated on the image carrier;
Primary transfer means for laminating the toner image developed on the image carrier on the intermediate transfer member;
In a color image forming apparatus comprising: a secondary transfer unit that transfers a toner image laminated on the intermediate transfer member onto a recording medium at once.
The intermediate transfer member is applied with a reverse transfer bias having the same polarity as that of the toner to one or more primary transfer means while the intermediate transfer member is rotated at least once after the end of single color printing or after the final printing in color continuous printing. An image forming apparatus, wherein residual toner on a transfer member is supplied onto one or more of the image carriers.   2. The image forming apparatus according to claim 1, wherein a surface potential of the image carrier to which residual toner on the intermediate transfer member is supplied is set to near zero.   An intermediate transfer member cleaning unit for removing residual toner on the intermediate transfer member is provided, and the bias applied to the intermediate transfer member cleaning unit is turned OFF while the intermediate transfer member is rotated one or more times. The image forming apparatus according to claim 1, wherein a bias having the same polarity as that of the toner is applied.   After completion of single color printing or after completion of final printing in continuous color printing, a black polishing image is formed by one of the image forming units, transferred onto the intermediate transfer member, and applied to the secondary transfer unit. The image according to any one of claims 1 to 3, wherein the intermediate transfer member is rotated at least once in a state where the bias is turned off or a transfer reverse bias having the same polarity as the toner is applied. Forming equipment.   5. The image forming apparatus according to claim 4, wherein among the toners of a plurality of colors used in each of the image forming units, the black toner contains more abrasive than the other color toners.   6. The reverse transfer bias is applied only to the primary transfer unit corresponding to a low printing unit having an average printing rate per predetermined number of sheets or less among the respective image forming units. The image forming apparatus according to any one of the above.   A transfer reverse bias is applied only to the primary transfer unit corresponding to a low print unit having an average printing rate per predetermined number of sheets or less among the image forming units, and the polishing image is stored in the low print unit. The image forming apparatus according to claim 4, wherein the image forming apparatus is formed with a predetermined amount of toner in combination with the amount of toner used per printed sheet.

Images