JP2013148756A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device that can suppress a filming phenomenon of an image carrier surface without changing a voltage applied to a first electrification auxiliary member when the first electrification auxiliary member is replaced for the replacement of a replacement unit and the like.SOLUTION: An electrification auxiliary high voltage power supply 80 commonly applies a voltage according to a cumulative amount of image formation of image forming parts PM, PC, PBk, which are not replaced, to a first electrification auxiliary member 8Y of image forming parts PY, PM, PC, PBk. An electrification auxiliary high voltage power supply 70Y makes the absolute value of a voltage applied to a second electrification auxiliary member 7Y of the replaced image forming part PY lower than the absolute value of the voltage applied to the second electrification member 7Y of the image forming parts PM, PC, PBk, which are not replaced. The absolute value of the voltage applied to the second electrification auxiliary member 7Y gets lowerer as a difference amount of a predetermined cumulative amount and a cumulative amount of image formation of the individual image forming part PY becomes bigger.

Description

  The present invention relates to an image forming apparatus in which a first charging auxiliary member and a second charging auxiliary member are disposed between a transfer unit and a charging unit to perform development simultaneous cleaning, and more specifically, a new first charging auxiliary member is replaced. The present invention relates to the control of the absolute value of the voltage applied to the second auxiliary charging member.

  The electrostatic image formed on the image carrier is developed into a toner image, the toner image on the image carrier is transferred to a recording material directly or via an intermediate transfer member, and the recording material onto which the toner image has been transferred is heated. 2. Description of the Related Art Image forming apparatuses that are pressed and fixed on a recording material are widely used. An image forming apparatus that forms a full-color image by superimposing toner images respectively formed on a plurality of image carriers on a recording material or an intermediate transfer member is also widely used. In an image forming apparatus including a plurality of image carriers, an image carrier, a charging member, a cleaning device, and the like are assembled together to form an exchange unit, and parts can be exchanged in units of exchange units.

  In Patent Document 1, the transfer residual toner on the image carrier that has passed through the transfer portion of the toner image is recharged to a normal charging polarity on the image carrier, and is reused for developing the electrostatic image again by the developing device. An image forming apparatus of the developing simultaneous cleaning system is shown. The simultaneous development cleaning system has an advantage that the image forming apparatus can be reduced in size because the amount of toner discarded is reduced and the image carrier is not provided with a cleaning device or a recovered toner pipe.

  Patent Document 2 discloses an image forming apparatus of the simultaneous development cleaning system in which a first charging auxiliary member is disposed on the downstream side of a transfer portion of an image carrier, and a second charging auxiliary member is disposed on the downstream side of the first charging auxiliary member. Is shown. The first charging auxiliary member is applied with a voltage having a polarity opposite to the charging polarity of the toner, traps the toner that has passed through the transfer portion, and gradually discharges the toner to disperse the toner on the surface of the image carrier. The second charging auxiliary member is charged with a voltage having the same polarity as the charging polarity of the toner, and charges the toner dispersed on the surface of the image carrier to a normal polarity usable in the developing device.

  In recent years, downsizing of image forming apparatuses and reduction of the number of parts have progressed, and a common power source is provided for a plurality of first charging auxiliary members, and the same voltage is applied to the plurality of first charging auxiliary members. It has been broken. Patent Document 3 shows an example in which power sources related to charging are shared.

JP 2008-145522 A JP 2009-192877 A JP 2002-148894 A

  In an image forming apparatus of the simultaneous development cleaning method, toner accumulates in the first charging auxiliary member gradually as image formation accumulates, and the charging performance deteriorates. Therefore, the absolute value of the voltage applied to the first charging auxiliary member is Control is performed to gradually increase as the formation is accumulated.

  In such an image forming apparatus, when an exchange unit in which the image carrier, the first charging auxiliary member, and the second charging auxiliary member are assembled together is exchanged, an image called a vertical white streak defective image is obtained by the exchanged exchange unit. It has been found that defects are likely to occur. A vertical white streak defective image is an image defect due to a filming phenomenon in which fragments of discharged toner particles and external additives are solidified and deposited on the surface of the image carrier.

  When the cause was investigated, in the exchange unit replaced, the absolute value of the surface potential of the image carrier rubbed by the first charging auxiliary member was relatively large compared to the exchange unit not exchanged. As a result, in a replacement unit that has been replaced, the potential difference between the second charging auxiliary member and the surface potential of the image carrier becomes too large, and the discharge between the second charging auxiliary member and the image carrier increases, so that replacement is not performed. The growth rate of the filming layer was higher than that of the unit (see FIG. 6).

  In other words, compared to the first charging auxiliary member on which toner before replacement is deposited, the first charging auxiliary member on which toner after replacement is not deposited has a higher charging performance on the surface of the image carrier. For this reason, when the same voltage as before replacement is applied to the first charging auxiliary member, the absolute value of the voltage on the surface of the image carrier slid by the first charging auxiliary member becomes too large. As a result, the potential difference between the second charging auxiliary member to which the same voltage as that before the replacement is applied and the surface potential of the image carrier becomes too large, and the discharge increases.

  In view of this, it has been studied to reduce the voltage applied to the first auxiliary charging member in the replaced exchange unit. However, in the device configuration in which the same voltage is applied from the common power source to the first charging auxiliary member of the plurality of replacement units, only the voltage applied to the first charging auxiliary member of the replaced replacement unit is independent. Cannot be reduced. When the overall voltage is lowered, the simultaneous development cleaning does not function properly in other replacement units that have not been replaced.

  The present invention provides an image forming method capable of suppressing the filming phenomenon on the surface of the image bearing member without changing the voltage applied to the first charging auxiliary member when the first charging auxiliary member is replaced by replacing the replacement unit. The object is to provide a device.

  The image forming apparatus of the present invention includes an image carrier on which a toner image is formed by simultaneous development cleaning, a first charging auxiliary member capable of rubbing the surface of the image carrier after the toner image is transferred, A second charging auxiliary member capable of rubbing the surface of the image bearing member rubbed by the first charging auxiliary member, and a charging polarity of the toner controlled so that an absolute value thereof increases as image formation accumulates. Based on a first charging auxiliary power source that applies a voltage of reverse polarity to the first charging auxiliary member, a detection unit that detects that the first charging auxiliary member is replaced with a new one, and a detection result of the detection unit, After the first charging auxiliary member is replaced with a new one, a voltage having the same polarity as the charging polarity of the toner controlled so that the absolute value is lower than that before the new charging is applied to the second charging auxiliary member. With a secondary charging auxiliary power supply .

  In the image forming apparatus of the present invention, even when the first charging auxiliary member is replaced with a new one and the absolute value of the potential of the surface slid on the first charging auxiliary member increases, the voltage applied to the second charging auxiliary member is increased. The potential difference between the second charging auxiliary member and the surface potential of the image carrier is reduced by the lowered amount. For this reason, the discharge between the second charging auxiliary member and the surface of the image carrier is suppressed, and the growth rate of the filming layer does not become excessive.

  Therefore, when the first charging auxiliary member is replaced by replacing the replacement unit or the like, the simultaneous development performance can be functioned normally without changing the voltage applied to the first charging auxiliary member, and the image carrier. Surface filming phenomenon can be suppressed.

1 is an explanatory diagram of a configuration of an image forming apparatus. It is explanatory drawing of a structure of an image formation part. It is a block diagram of the control system of a charging auxiliary high voltage power supply. 6 is a flowchart of charging auxiliary voltage control of Example 1 and a comparative example. It is explanatory drawing of the charge contrast of the 2nd auxiliary charging member in Example 1 and a comparative example. FIG. 6 is an explanatory diagram of a difference in filming growth of a photosensitive drum between Example 1 and a comparative example.

  Embodiments of the present invention will be described below with reference to the drawings. In the present invention, as long as the voltage applied to the second charging auxiliary member before and after the replacement of the first charging auxiliary member is different, a part or all of the configuration of the embodiment is replaced with the alternative configuration. It can also be implemented in the embodiment.

  Therefore, as long as simultaneous development cleaning is performed, the image forming apparatus using a two-component developer or the image forming apparatus using a one-component developer can be used. The image forming apparatus is implemented without distinction between charging method, electrostatic image writing method, developing method, two-component developer / one-component developer, transfer method, intermediate transfer method / recording material transport method / sheet-fed method, and fixing method. it can. In the present embodiment, only main parts related to toner image formation / transfer will be described. However, the present invention includes a printer, various printing machines, a copier, a fax machine, a composite machine, in addition to necessary equipment, equipment, and a housing structure. It can be implemented in various applications such as a machine.

  The dimensions, materials, shapes, and relative arrangements of the components described in the embodiments should be appropriately changed according to the configuration of the apparatus to which the invention is applied and various conditions. The scope of the present invention is as follows. It is not intended to limit the form according to the description.

<Image forming apparatus>
FIG. 1 is an explanatory diagram of the configuration of the image forming apparatus. FIG. 2 is an explanatory diagram of the configuration of the image forming unit.

  As shown in FIG. 1, the image forming apparatus 100 is a tandem intermediate transfer type full-color printer in which image forming portions PY, PM, PC, and PBk of yellow, magenta, cyan, and black are arranged along an intermediate transfer belt 90. is there.

  In the image forming unit PY, a yellow toner image is formed on the photosensitive drum 1Y and transferred to the intermediate transfer belt 90. In the image forming unit PM, a magenta toner image is formed on the photosensitive drum 1M and transferred to the intermediate transfer belt 90. In the image forming units PC and PBk, a cyan toner image and a black toner image are formed on the photosensitive drums 1C and 1Bk, respectively, and transferred to the intermediate transfer belt 90.

  The full-color toner image formed by superimposing the four-color toner images is conveyed to the secondary transfer portion T2 as the intermediate transfer belt 90 rotates, and is secondarily transferred to the recording material P. The recording material P taken out from the recording material cassette 15 is separated one by one by the separation roller 16 and conveyed to the registration roller 12. The registration roller 12 sends the recording material P to the secondary transfer portion T2 in synchronization with the toner image on the intermediate transfer belt 90. The recording material P on which the full-color toner image has been secondarily transferred at the secondary transfer portion T2 is heated and pressurized by the fixing device 17 to fix the image on the surface, and then is discharged outside the machine.

  The intermediate transfer belt 90 is supported around a driving roller 93, a tension roller 92, and a secondary transfer counter roller 91, and is driven by the driving roller 93 to rotate in the direction of arrow R2 at a predetermined process speed. The secondary transfer roller 11 is pressed against the intermediate transfer belt 90 whose inner surface is supported by the secondary transfer counter roller 91 to form a secondary transfer portion T2. The belt cleaning device 10 abuts a cleaning blade on the intermediate transfer belt 90 and collects transfer residual toner from the intermediate transfer belt 90 that has escaped transfer to the recording material P and passed through the secondary transfer portion T2.

  The image forming units PY, PM, PC, and PBk are substantially the same except that the toner colors used in the developing devices 4Y, 4M, 4C, and 4Bk are different. Hereinafter, the image forming unit PY will be described with reference to FIG. 2, and the image forming units PM, PC, and PBk will be described by replacing Y at the end of the code with M, C, and Bk.

  As shown in FIG. 2, the image forming unit PY has a charging roller 2Y, an exposure device 3Y, a developing device 4Y, a primary transfer roller 9Y, a first auxiliary charging brush 7Y, and a first auxiliary charging member 8Y around the photosensitive drum 1Y. Is arranged. The photosensitive drum 1Y has a photosensitive layer formed on the outer peripheral surface of an aluminum cylinder, and rotates at a predetermined process speed.

  The charging roller 2Y contacts and rotates with the photosensitive drum 1Y. The charging high-voltage power supply 20Y applies an oscillating voltage obtained by superimposing an AC voltage on a DC voltage to the charging roller 2Y, and charges the surface of the photosensitive drum 1Y to a uniform negative potential VD. Charging DC voltage = −550V.

  The exposure device 3Y writes an electrostatic image of the image on the charged photosensitive drum 1Y by scanning with a rotating mirror a laser beam obtained by ON / OFF modulating an image signal obtained by developing a yellow image. In the exposed portion, the dark portion potential VD discharges and decreases to the bright portion potential VL, so that the toner that is negatively charged can be attached more negatively than the unexposed portion. It becomes.

  The developing device 4Y forms a toner image by reversing and developing the electrostatic image on the photosensitive drum 1Y using a developer containing toner and a carrier. The developing device 4Y uses a two-component developer in which a toner and a carrier are mixed as a developer. A new amount of toner corresponding to the amount of toner consumed by the developing device 4Y by image formation is supplied from the developer supply device 5Y to the developing device 4Y. The toner replenishment amount is estimated by calculating an integrated video count or an integrated image ratio for each output image.

  The primary transfer roller 9Y presses the inner surface of the intermediate transfer belt 90 to form a primary transfer portion TY between the photosensitive drum 1Y and the intermediate transfer belt 90. The transfer high-voltage power supply 90Y applies a transfer voltage having a polarity opposite to the charging polarity of the toner to the primary transfer roller 9Y, and primarily transfers the negative toner image carried on the photosensitive drum 1Y to the intermediate transfer belt 90. The transfer voltage is controlled at a constant current so that the transfer current is 18 μA.

  In the image forming unit PY, when the toner image is transferred to the intermediate transfer belt 90 in the primary transfer unit TY, about 1 to 10% of residual toner is generated, passes through the primary transfer unit TY, and accompanies the photosensitive drum 1Y. spin. As a method of collecting the transfer residual toner from the photosensitive drum, it is common to use a cleaning device using a cleaning blade. However, in the image forming apparatus 100, a two-stage auxiliary charging member is provided, and development with the development simultaneous cleaning method is performed by the developing devices 4Y, 4M, 4C, and 4Bk, and the transfer residual toner is reused.

<Simultaneous development cleaning>
In the developing device 4Y, the developing screw 43 and the agitating screw 44 convey the developer in the reverse direction in the developing container 45 partitioned by the partition wall 46 and circulate the developer while agitating. With the stirring, the toner of the developer and the carrier are rubbed to charge the toner positively and the carrier negatively. The charged developer is coated on the developing sleeve 41 and conveyed to a portion facing the photosensitive drum 1Y to develop the electrostatic image on the photosensitive drum 1Y.

  The developing sleeve 41 is supported by the developing container 45 and rotates in the arrow R4 direction. A non-rotating magnet roller 42 is disposed at the center of the developing sleeve 41, and a carrier in the developer is carried on the surface of the developing sleeve 41 by the magnetic force. The developer supplied to the developing sleeve 41 by the developing screw 43 is regulated to have a constant layer thickness by the layer thickness regulating blade 47, and is struck by the magnetic force of the magnet roller 42 and rubs against the photosensitive drum 1Y.

  The developing high-voltage power supply 40Y applies an oscillating voltage obtained by superimposing the AC voltage Vac on the DC voltage Vdc to the developing sleeve 41, and transfers the toner in the sprouting developer to the electrostatic image on the photosensitive drum 1Y. Development DC voltage = −350V. At this time, the negatively charged toner adhering to the photosensitive drum 1Y is also driven to an oscillating voltage to reciprocate between the photosensitive drum 1Y and the developing sleeve 41, and a part is collected by the developing sleeve 41. The portion is transferred to the electrostatic image on the photosensitive drum 1Y. Such a development process is called simultaneous development cleaning. A system in which the transfer residual toner is not collected by a cleaning blade or the like and reused by the developing device 4Y is called a cleanerless system.

  In order to realize the simultaneous development cleaning, the transfer residual toner on the photosensitive drum 1Y that has passed through the primary transfer portion TY by escaping from the transfer to the intermediate transfer belt 90 is normal until it reaches the portion facing the developing sleeve 41. It is necessary to charge the negatively charged state. The transfer residual toner is charged to a potential having uniform negative polarity, so that it reaches the developing device 4Y without adhering to the charging roller 2Y, and the transfer residual toner in the developing device 4Y can be collected and reused.

  The first auxiliary charging member 8Y uses a deck brush-like brush so that the toner can be easily collected. The first auxiliary charging member 8Y is applied with a positive voltage having a polarity opposite to the charging polarity of the toner from the auxiliary charging high-voltage power supply 80. As will be described later, the auxiliary charging high-voltage power supply 80 outputs a common voltage to the first auxiliary charging member 8Y of the image forming units PY, PM, PC, and PBk. The first charging auxiliary member 8Y temporarily traps the transfer residual toner that has escaped from the transfer to the intermediate transfer belt 90 and passed through the primary transfer portion TY, and a large amount of transfer residual toner is charged at a time during image formation. It is prevented from being conveyed to the roller 2. The first auxiliary charging member 8Y has a function of discharging the transfer residual toner that has passed through the primary transfer portion TY. The first auxiliary charging member 8Y also has a function of discharging toner onto the photosensitive drum 1Y when the image forming job is completed.

  The second charging auxiliary member 7Y uses a roller brush-like brush that rotates in order to reliably contact the toner and perform charge transfer. The second auxiliary charging member 7Y is applied with a negative voltage from the auxiliary charging high-voltage power supply 70Y. The second auxiliary charging member 7Y charges the toner that has passed through the first auxiliary charging member 8Y and diffused on the photosensitive drum 1Y to a potential with uniform negative polarity that can be used in the developing device 4Y.

  The first auxiliary charging member 8Y immediately after the transfer position has a function of temporarily trapping or neutralizing the transfer residual toner. However, when the image formation is completed, the toner is discharged to recover the trap performance. Since the first charging auxiliary member 8Y has a lot of toner adhering to the accumulation of image formation, the first charging auxiliary member 8Y increases the charging auxiliary voltage Y to be applied according to the usage situation (the number of sheets used and the cumulative image ratio). The static neutralization and temporary trap functions are maintained.

  The second charging auxiliary member 7Y disposed between the first charging auxiliary member 8Y and the charging roller 2Y applies a negative DC voltage to the toner discharged from the first charging auxiliary member 8Y to re-transfer residual toner. It is electrified to facilitate simultaneous cleaning of development.

  As a characteristic, the same positive voltage is applied from the common auxiliary charging high voltage power supply 80 to the first auxiliary charging members 8Y, 8M, 8C, and 8Bk of the image forming units PY, PM, PC, and PBk. The auxiliary charging high-voltage power supply 80 applies a common voltage under constant voltage control to the first auxiliary charging members 8Y, 8M, 8C, and 8Bk. As a result, space saving and cost reduction of the power supply arrangement are realized as compared with the case where the auxiliary charging high-voltage power supply is individually provided. However, the common use of the auxiliary charging high-voltage power supply 80 includes the case where parts such as a transformer are used in common.

<Control system for auxiliary charging high-voltage power supply>
FIG. 3 is a block diagram of a control system of the charging auxiliary high voltage power source. As shown in FIG. 1, the image forming apparatus 100 includes at least two image forming units PY, PM, PC, and PBk that are examples of replacement units. As shown in FIG. 2, in the image forming portion PY, the photosensitive drum 1Y, the first charging auxiliary member 8Y, and the second charging auxiliary member 7Y are assembled so as to be interchangeable together. On the photosensitive drum 1Y as an example of the image carrier, a toner image is formed by simultaneous development cleaning.

  As shown in FIG. 3, memory units 15Y, 15M, 15C, and 15Bk, which are examples of memory elements, record and hold accumulated information of individual image formation. The cumulative information of image formation is at least one of output sheet number information for image formation and toner usage information for image formation.

  The control unit 1010, which is an example of a detection unit, reads recorded information in the memory units 15Y, 15M, 15C, and 15Bk, and detects that the first charging auxiliary member 8Y has been replaced with a new one. The control unit 1010, which is an example of a control unit, calculates the absolute voltage applied to the first charging auxiliary member 8Y and the second charging auxiliary member 7Y based on the accumulated image formation information read from the memory units 15Y, 15M, 15C, and 15Bk. Set the value.

  The image forming units PY, PM, PC, and PBK are provided with memory boards as memory units 15Y, 15M, 15C, and 15Bk, respectively. The counter calculation unit 150 measures the output number of image formation for each of the image forming units PY, PM, PC, and PBK, and cumulatively records the output number on the integrated value Pn of the used number counter of the memory units 15Y, 15M, 15C, and 15Bk. Yes. The image forming units PY, PM, PC, and PBK have memory units 15Y, 15M, 15C, and 15Bk independently. The accumulated value Pn of the used sheet counter is obtained by adding the image forming number of the image forming job to the original accumulated value Pn-1 by the counter calculation unit 150 at the end of each image forming job. Write to 15C and 15Bk.

  In this embodiment, the memory unit 15 is provided in the image forming unit. However, a similar memory unit is provided on the main body side of the image forming apparatus 100 to individually accumulate accumulated information of image formation. It is also possible.

  In this embodiment, as the cumulative information of image formation, the number of recording materials used in the image forming job is cumulatively counted. However, the toner contamination amount of the first charging auxiliary members 8Y, 8M, 8C, and 8Bk is estimated. Other parameters to obtain may be used. It is desirable to be able to estimate the amount of toner used in an image forming job. In that sense, an integrated video count, an integrated image ratio, or the like can be used.

<Comparative example>
FIG. 4 is a flowchart of charging auxiliary voltage control according to the first embodiment and the comparative example. FIG. 5 is an explanatory diagram of the charge contrast of the second charging auxiliary member in Example 1 and the comparative example. The comparative example is an example in which the voltage applied to the second auxiliary charging member 7Y is not changed even when the first auxiliary charging member 8Y is replaced with a new one.

  As shown in FIG. 4 with reference to FIG. 3, when an instruction signal for an image forming job is received from the operation unit 1000, the control unit 1010 receives the current usage number counter from each of the memory units 15 Y, 15 M, 15 C, and 15 Bk. The integrated value Pn is read. The control unit 1010 selects the maximum value Pmax from the four integrated values Pn of the image forming units PY, PM, PC, and PBk (S101).

  The controller 1010 determines the auxiliary charging voltage Y to be applied to the first auxiliary charging members 8Y, 8M, 8C and 8Bk by referring to the auxiliary charging table in Table 1 with the maximum value Pmax (S102).

  As shown in Table 1, when the maximum value Pmax is 100 sheets, the auxiliary charging voltage Y is + 100V. However, when the maximum value Pmax is 20000 or more, the auxiliary charging voltage Y is constant at 400V. This is because when the number of printed sheets reaches 20000, the toner contamination amount of the first charging auxiliary member 8 becomes saturated, and the current I flowing from the first charging auxiliary member 8 to the photosensitive drum 1 becomes substantially constant. .

  The control unit 1010 determines the auxiliary charging voltage Z to be applied to the second auxiliary charging members 7Y, 7M, 7C, and 7Bk with reference to the auxiliary charging table in Table 2 according to the “conventional image forming flow” indicated by the broken line. (S201).

  The auxiliary charging voltage Z is −700 V regardless of the maximum value Pmax.

  As shown in FIG. 5A, in the initial state (Pn = 0) of the comparative example, since the auxiliary charging voltage Y is low, the photosensitive drum potential X after passing through the first auxiliary charging member is also low. For this reason, the charging contrast of the second auxiliary charging member 7Y to which the auxiliary charging voltage Z is applied does not become excessive, and an appropriate range of charging current flows from the photosensitive drum 1Y to the second auxiliary charging member 7Y. Specifically, since the dark portion potential VD of the photosensitive drum 1Y is −550V and the primary transfer current is 18 μA, the photosensitive drum potential after passing through the primary transfer portion TY is −300V. When a charging auxiliary voltage Y of 100 V was applied to the first charging auxiliary member 8Y, the photosensitive drum potential X after passing through the first charging auxiliary member became −200V. At this time, when the auxiliary charging voltage Z = −700 V was applied to the second auxiliary charging member 7Y, the current I = 15 μA flowed from the photosensitive drum 1 to the second auxiliary charging member 7.

  With this current I = 15 μA, the transfer residual toner on the photosensitive drum 1Y can be sufficiently recharged to minus, and filming does not occur on the photosensitive drum 1. Filming is a phenomenon in which electric discharge occurs between the second auxiliary charging member 7Y and the photosensitive drum 1Y, and toner or the like adheres to the photosensitive drum 1 to form an insulating film. When filming occurs, charging using the charging roller 2Y and development using the developing sleeve 41 are affected, and image quality is degraded.

  As shown in FIG. 5B, in the cumulative image formation state (Pn = 40000) of the comparative example, the charging auxiliary voltage Y is set high based on Table 1, but the charging performance of the first charging auxiliary member 8Y. Therefore, the photosensitive drum potential X is low after passing through the first auxiliary charging member. For this reason, the charging contrast of the second auxiliary charging member 7Y to which the auxiliary charging voltage Z is applied does not become excessive, and an appropriate range of charging current flows from the photosensitive drum 1Y to the second auxiliary charging member 7Y. As shown in Table 1, since Pmax = 20000 sheets, the applied voltage Y to the first auxiliary charging member 8Y is + 400V. This is because a large amount of toner accumulates in the first charging auxiliary member 8 and the contact with the photosensitive drum 1Y is hindered, so that the voltage is increased in order to pass a current in an appropriate range. Specifically, the photosensitive drum potential after passing through the primary transfer portion TY is −300V. When a voltage Y of 400V is applied to the first auxiliary charging member 8Y, the photosensitive drum potential X becomes −200V after passing through the first auxiliary charging member. At this time, when a charging auxiliary voltage Z = −700 V was applied to the second charging auxiliary member 7Y, a current I = 15 μA flowed from the photosensitive drum 1Y to the second charging auxiliary member 7Y.

  With this current I = 15 μA, the transfer residual toner on the photosensitive drum 1Y can be sufficiently recharged to minus, and filming does not occur on the photosensitive drum 1Y.

  As shown in FIG. 5C, it is assumed that the image forming unit PY shown in FIG. 1 is replaced with a new one in the image forming cumulative state (Pn = 40000) of the comparative example. At this time, the image forming portions PM, PC, and PBk are in the image forming cumulative state (Pn = 40000), but the image forming portion PY is in the initial state (Pn = 0), and therefore applied to the first charging auxiliary member 8Y. The charged auxiliary voltage Y is excessive. Although the auxiliary charging voltage Y is set high based on Table 1, since the charging performance of the first auxiliary charging member 8Y has greatly recovered, the photosensitive drum potential X becomes higher after passing through the first auxiliary charging member. For this reason, the charging contrast of the second charging auxiliary member 7Y to which the auxiliary charging voltage Z is applied becomes excessive, and an excessive charging current outside the appropriate range flows from the photosensitive drum 1Y to the second charging auxiliary member 7Y. .

  When the integrated value Pn of the image forming portions PM, PC, and PBk = 40000 sheets, while the integrated value Pn = 0 of the image forming portion PY = 0, the maximum value Pmax is 20000 sheets. The voltage Y applied to 8Y is + 400V. At this time, a good image is output in the image forming units PM, PC, and PBk. However, in the image forming unit PY, excessive current flows from the first auxiliary charging member 8Y to the photosensitive drum 1Y, and the surface potential of the photosensitive drum 1Y contacting the second auxiliary charging member 7Y becomes (a), (b). Higher than the case. Specifically, the photosensitive drum potential after passing through the primary transfer portion TY is −300V. When a voltage Y of 400 V was applied to the first auxiliary charging member 8Y, the photosensitive drum potential X increased to −100 V after passing through the first auxiliary charging member. At this time, when a charging auxiliary voltage Z = −700 V was applied to the second charging auxiliary member 7Y, a current I = 30 μA flowed from the photosensitive drum 1Y to the second charging auxiliary member 7Y.

  When this current I = 30 μA, filming occurred on the photosensitive drum 1. It is considered that the filming grows because the speed at which the toner or the like adheres to the photosensitive drum 1 by the discharge at the second charging auxiliary member 7 is larger than the speed at which the second charging auxiliary member 7 scrapes the filming.

  In many cases of use in the market, one color image forming unit reaches the end of its life and is often replaced. In that case, in order to maintain the static elimination and the one-time trap function, the DC voltage of the second charging auxiliary member is adjusted to the color image forming unit having a large number of used sheets.

  Therefore, the DC voltage of the second charging auxiliary member of the replaced one-color image forming unit is set too high, and the potential difference between the first charging auxiliary member and the photosensitive member is widened. As a result, an excessive current flows from the first charging auxiliary member to the photosensitive member, and filming is likely to occur, where toner or shaving powder of the photosensitive member adheres on the photosensitive member.

  Therefore, in the following embodiment, as shown in FIG. 5D, when the image forming unit PY is replaced with a new one, the absolute value of the auxiliary charging voltage Z applied to the second auxiliary charging member 7 is reduced. I am letting. As a result, the current I flowing from the photosensitive drum 1Y to the second auxiliary charging member 7Y is reduced to 15 μA, which is an appropriate range. The main point is to adjust the auxiliary charging voltage Z applied to the second auxiliary charging member 7Y so that the potential difference between the second auxiliary charging member 7Y and the photosensitive drum 1Y is made uniform with the other image forming parts PM, PC, PBk.

<Example 1>
As shown in FIG. 2, the first charging auxiliary member 8Y, which is an example of the first charging auxiliary member, can rub the surface of the image carrier after the toner image is transferred. The second charging auxiliary member 7Y, which is an example of the second charging auxiliary member, can rub the surface of the photosensitive drum 1Y rubbed by the first charging auxiliary member 8Y.

  The auxiliary charging high-voltage power supply 80, which is an example of the first auxiliary charging power source, applies a voltage having a polarity opposite to the charging polarity of the toner whose absolute value is set higher as the image formation is accumulated to the first auxiliary charging member 8Y. . Based on the detection result of the control unit 1010, the charging auxiliary high-voltage power supply 70Y, which is an example of the second charging auxiliary power supply, has a lower absolute value after the first charging auxiliary member 8Y is replaced with a new one than after the new replacement. A voltage having the same polarity as the set toner charging polarity is applied to the second auxiliary charging member 7Y.

  The charging auxiliary high-voltage power supply 80 applies a voltage corresponding to the cumulative amount of image formation of the image forming portions PM, PC, and PBk that are not replaced to the first charging auxiliary member 8Y of all the image forming portions PY, PM, PC, and PBk. Apply in common. The auxiliary charging high-voltage power supply 80 applies a constant voltage of 400 V to the first auxiliary charging member 8Y when the cumulative amount of image formation exceeds a predetermined cumulative amount.

  The auxiliary charging high-voltage power supplies 70Y, 70M, 70C, and 70Bk use the absolute value of the voltage applied to the second charging auxiliary member 8Y of the replaced image forming unit PY as the second charging of the image forming units PM, PC, and PBk that are not replaced. Lower than the absolute value of the voltage applied to the auxiliary member. The charging auxiliary high-voltage power supplies 70Y, 70M, 70C, and 70Bk have an absolute value of the voltage applied to the second charging auxiliary member 7Y as the difference between the predetermined accumulation amount and the image formation accumulation amount of the individual image forming unit PY increases. Lower.

  The charging auxiliary high voltage power sources 70Y, 70M, 70C, and 70Bk set the absolute value of the voltage applied to the second charging auxiliary member 7Y as follows. The current flowing from the photosensitive drum 1Y to the second charging member 7Y in the replaced image forming unit PY is made equal to the current flowing from the photosensitive drum 1Y to the second charging member 7Y in the image forming units PM, PC, and PBk that are not replaced. It is set to.

  As shown in FIG. 4 with reference to FIG. 3, prior to the start of the image forming job, the control unit 1010 reads the accumulated value Pn of each current used number counter from the memory units 15Y, 15M, 15C, and 15Bk. . The control unit 1010 selects the maximum value Pmax from the four integrated values Pn of the image forming units PY, PM, PC, and PBk (S101). The controller 1010 determines the auxiliary charging voltage Y to be applied to the first auxiliary charging members 8Y, 8M, 8C and 8Bk by referring to the auxiliary charging table in Table 1 with the maximum value Pmax (S102).

  The control unit 1010 confirms whether or not the maximum value Pmax is greater than 20000 (S103), and if it is smaller than 20000 (No in S103), uses the value as the maximum value Pmax. However, when the maximum value Pmax is 20000 or more (Yes in S103), Pmax = 20000 is set (S103).

The control unit 1010 calculates the correction parameter Pcal for each of the four integrated values Pn of the image forming units PY, PM, PC, and PBk (S105).
Pcal = Pmax−Pn

  When the correction parameter Pcal is negative, the control unit 1010 sets Pcal = 0 (S105).

  The control unit 1010 refers to the offset DC voltage table in Table 3 using the correction parameter Pcal, and determines the offset voltage ΔV that decreases the absolute value of the auxiliary charging voltage Z applied to the second auxiliary charging member 7 (S106).

Based on the offset voltage ΔV in Table 3, the corrected auxiliary charging voltage Z ′ actually applied to the second auxiliary charging member 7 is as follows.
Z ′ = Z + ΔV

  That is, when the maximum value Pmax is 20000 or more, the image forming units PY, PM, PC, and PBk are actually applied to the second auxiliary charging member 7 as the difference between the integrated value Pn and the maximum value Pmax is larger. The absolute value of the auxiliary charging voltage Z ′ is set low. If the integrated value Pn exceeds 20000 sheets, no correction is made, and the auxiliary charging voltage Z = −700 V is set as it is, as in the comparative example.

  As shown in FIG. 5D, since the number of prints of the image forming sections PM, PC, and PBk is 30000, Pmax = 20000 (Yes in S103). As shown in Table 1, since Pmax = 30000 sheets, the auxiliary charging voltage Y applied to the first auxiliary charging member 8Y is + 400V. The correction parameter Pcal is 20000-0 = 20000. The offset DC voltage corresponding to Pcal: 20000 is 100V. Therefore, the auxiliary charging voltage Z ′ actually applied to the second auxiliary charging member 7 is −700 V + 100 V = −600 V. As a result, the potential difference between the photosensitive drum potential before contacting the second charging auxiliary member 7 and the charging auxiliary voltage Z ′ actually applied to the second charging auxiliary member 7 is expressed by the image forming portions PY, PM, PC, and PBk. The point is to make them substantially the same.

  As shown in FIG. 5D, the surface potential of the photosensitive drum 1Y that has passed through the first auxiliary charging member 8Y is −100 V as in FIG. However, since the charging auxiliary voltage Z applied to the second charging auxiliary member 7Y is corrected to −600 V, the current I flowing from the photosensitive drum 1Y to the second charging auxiliary member 7 is reduced to 15 μA within the appropriate range, Filming does not occur on the photosensitive drum 1Y.

<Example 2>
FIG. 6 is an explanatory diagram of the difference in filming growth of the photosensitive drum between Example 1 and the comparative example.

  The control program of Example 1 and the control program of the comparative example were mounted on the control unit 1010, and a continuous image formation experiment was performed. In the experiment, a continuous image was formed by forming a halftone image having an image density (exposure duty) of 5% on the entire surface of A4 size plain paper. The evaluation of the experiment was performed by comparing the cumulative number of images formed until a vertical white streak defective image caused by filming occurred in an image output in continuous image formation. Further, in order to more directly evaluate the filming growth of the photosensitive drum 1Y, the 10-point surface roughness Rz of the photosensitive drum 1Y was obtained for every 10000 sheets of image formation. The vertical white streak defective image is caused by filming formed on the photosensitive drum 1Y, and the degree of filming growth can be determined by the surface roughness Rz.

  As shown in FIG. 6, in Example 2, the gradient of transition of the surface roughness Rz of the photosensitive drum 1 with respect to the number of used sheets is smaller than that in the comparative example.

  In the comparative example, when the number of prints of the image forming units PY, PM, and PC is 30000, the image forming unit PBk is replaced with a new print of 0, and further 20000 continuous images are formed. Vertical white streak defective image occurred on the halftone image. At that time, the surface roughness Rz of the photosensitive drum 1Bk was measured and found to be Rz = 3.0 μm.

  In the second embodiment, when the number of prints of the image forming units PY, PM, and PC is 30,000, the image forming unit PBk is replaced with a new one with zero prints, and another 50,000 continuous images are formed. Finally, a vertical white streak image occurred on the halftone image. At that time, the surface roughness Rz of the photosensitive drum 1Bk was measured and found to be Rz = 3.2 μm. When the surface roughness Rz of the photosensitive drum 1Bk was measured at the time when the image was replaced with a new one and 20,000 continuous images were formed, Rz = 1.5 μm.

  Therefore, as shown in FIG. 6, the surface roughness Rz decreases almost linearly with the accumulation of image formation. When the surface roughness Rz exceeds 3.0 μm, the surface roughness Rz is actually on the halftone image. It was confirmed that a vertical white streak defective image was generated. In either case, the halftone image was uniform and clean until a vertical white streak defective image was generated.

  Further, in Example 2, the cumulative number of vertical white streak defective images generated on the halftone image and the measurement result of the surface roughness Rz are all 50,000 with the same usage frequency in the image forming portions PY, PM, PC, and PBk. It was the same level as when the image formation of sheets was accumulated. As a result, even when the image forming portions PY, PM, PC, and PBk are replaced one by one in the market, it has been confirmed that no problem due to filming occurs until the original life of the image forming portion without any problem.

  Since the image forming apparatus 100 shares the auxiliary charging power source 80 of the first charging auxiliary members 8Y, 8M, 8C, and 8Bk of the image forming units PY, PM, PC, and PBk, rather than separately providing a high-voltage power source. Space saving and cost reduction of the arrangement of high-voltage power supply has been realized. At the same time, it is possible to suppress filming in the image forming unit when one image forming unit is replaced with a new one.

1Y, 1M, 1C, 1Bk Photosensitive drums 2Y, 2M, 2C, 2Bk Charging rollers 3Y, 3M, 3C, 3Bk Exposure devices 4Y, 4M, 4C, 4Bk Developing devices 5Y, 5M, 5C, 5Bk Developer supply devices 7Y, 7M , 7C, 7Bk First charging auxiliary member 8Y, 8M, 8C, 8Bk Second charging auxiliary member 9Y, 9M, 9C, 9Bk Primary transfer roller 15Y, 15M, 15C, 15Bk Memory unit 41 Developing sleeve,
70Y, 70M, 70C, 70Bk Charge auxiliary high voltage power supply 80 Charge auxiliary high voltage power supply, 90 Transfer high voltage power supply 1010 Control unit PY, PM, PC, PBk Image forming unit (process cartridge)

Claims (6)

An image carrier on which a toner image is formed by simultaneous development cleaning;
A first charging auxiliary member capable of rubbing the surface of the image carrier after the toner image is transferred;
A second charging auxiliary member capable of rubbing the surface of the image carrier rubbed by the first charging auxiliary member;
A first auxiliary charging power source for applying to the first auxiliary charging member a voltage having a polarity opposite to the charging polarity of the toner controlled so as to increase in absolute value as the image formation is accumulated;
Detecting means for detecting that the first charging auxiliary member has been replaced with a new one;
Based on the detection result of the detection means, after the first charging auxiliary member is replaced with a new one, a voltage having the same polarity as the charging polarity of the toner is controlled so that the absolute value becomes lower than before the new replacement. An image forming apparatus comprising: a second auxiliary charging power source applied to the second auxiliary charging member. Comprising at least two exchange units assembled so that the image carrier, the first auxiliary charge member, and the second auxiliary charge member can be integrally replaced;
The first charging auxiliary power supply applies a voltage corresponding to the cumulative amount of image formation of the replacement unit that is not replaced to the first charging auxiliary member of all the replacement units in common.
The second charging auxiliary power source has an absolute value of a voltage applied to the second charging auxiliary member of the exchange unit replaced, based on an absolute value of a voltage applied to the second charging auxiliary member of the exchange unit not exchanged. The image forming apparatus according to claim 1, wherein the image forming apparatus is also lowered. The first charging auxiliary power supply applies a constant voltage to the first charging auxiliary member when the cumulative amount of image formation exceeds a predetermined cumulative amount,
The second charging auxiliary power source lowers the absolute value of the voltage applied to the second charging auxiliary member as the difference amount between the predetermined accumulation amount and the image formation accumulation amount of the individual replacement unit is larger. The image forming apparatus according to claim 2.   The second charging auxiliary power supply includes a current that flows from the image carrier to the second charging member in the exchange unit that has been replaced, and a current that flows from the image carrier to the second charging member in the exchange unit that is not replaced. 4. The image forming apparatus according to claim 3, wherein an absolute value of a voltage applied to the second auxiliary charging member is set so that the two are equal to each other. The exchange unit has a memory element that records and holds accumulated information of individual image formation,
Control means for setting absolute values of voltages to be applied to the first charging auxiliary member and the second charging auxiliary member based on cumulative information of image formation read from memory elements of all the replacement units. The image forming apparatus according to any one of claims 2 to 4.   6. The image forming apparatus according to claim 5, wherein the accumulated information of image formation is at least one of output sheet number information of image formation and toner usage amount information of image formation.

Images