JP2016075737A - Image forming apparatus, voltage application method, and voltage application program - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus that can suppress a reduction in surface potential of a photoreceptor drum while extending the life of the photoreceptor drum.SOLUTION: A control part 100 executes second control of applying, to a transfer roller 74, a second transfer bias that has the same polarity as that of a first transfer bias for transferring a toner image on a photoreceptor drum 51 to a sheet P and has the absolute value larger than that of the first transfer bias, during a period from when the rear end of a toner image to be preferentially transferred passes a nip part formed with the photoreceptor drum 51 and a transfer roller 74 arranged opposite to the photoreceptor drum 51 until when the front end of a toner image to be transferred subsequent to the toner image to be preferentially transferred passes the nip part.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus, a voltage application method, and a voltage application program.

  As a photosensitive drum of an image forming apparatus using an electrophotographic method, one using an organic photosensitive member is known. In the organic photoreceptor, in the exposure process in which the surface of the organic photoreceptor is exposed by an exposure device, charges tend to be injected into the photosensitive layer and the charges are likely to be accumulated in the photosensitive layer.

  As described above, it is known that when electric charges are accumulated in the photosensitive layer, the surface potential of the photosensitive drum is lowered. When the surface potential of the photosensitive drum is lowered, the chargeability of the photosensitive drum cannot be ensured, and problems such as fogging easily occur.

  As an image forming apparatus in which countermeasures against the reduction in the surface potential are taken, the image forming apparatus rotates in contact with the photosensitive drum, and transfers the toner image on the photosensitive drum to the sheet more than when the toner image is transferred. One that applies a large transfer voltage is known.

  For example, the image forming apparatus described in Patent Document 1 applies a transfer voltage having a value larger than that at the time of transferring a toner image to the transfer roller while the rotation of the photosensitive drum is continued after the end of the image forming operation. In this way, charges accumulated in the photosensitive layer are removed, and a decrease in the surface potential of the photosensitive drum is eliminated.

JP 2010-39168 A

  By the way, in recent years, there is a demand for an image forming apparatus having a long life that can reduce the number of maintenance operations by a user. One factor that greatly contributes to the life of the image forming apparatus is the life of the photosensitive drum.

  However, in the image forming apparatus described in Patent Document 1, the transfer voltage is applied to the transfer roller while the rotation of the photosensitive drum is continued every time the image forming operation is completed. That is, in order to eliminate the decrease in the surface potential of the photoconductive drum, the photoconductive drum is rotated even during an image forming operation, which may shorten the life of the photoconductive drum.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of suppressing a decrease in surface potential of a photosensitive drum while extending the life of the photosensitive drum. And

  In order to achieve the above object, an image forming apparatus of the present invention is disposed opposite to an image carrier that carries a developer image and conveys a transfer medium between the image carrier and the image carrier. A transfer member that forms a nip portion and transfers the developer image from the image carrier to the transfer medium, and a control unit, and the control unit transfers the developer image from the image carrier to the transfer medium. The first control for applying the first transfer bias to the transfer member is executed, and when the plurality of developer images are continuously transferred to the transfer medium by the transfer member, the transfer is first performed by the transfer member. The first developer image passes through the nip portion until the front end portion of the next developer image transferred next to the previous developer image passes through the nip portion. A second transfer via having the same polarity as the transfer bias and greater than the absolute value of the first transfer bias The and executes a second control to be applied to the transfer member.

  The image forming apparatus includes a feeding mechanism that feeds the transfer medium toward the nip portion, and the control unit is a time when a plurality of transfer media are continuously fed by the feeding mechanism, and is sent by the feeding mechanism first. The second control is executed after the rear end portion of the previous transfer medium passes through the nip portion and before the front end portion of the next transfer medium fed next to the previous transfer medium passes through the nip portion. It may be a configuration.

  In the image forming apparatus, the control unit controls the operation of the delivery mechanism to control the timing of sending out the transfer medium, and the interval between the rear end of the previous transfer medium and the front end of the next transfer medium is the image carrier. A configuration may be employed in which the next transfer medium is sent out at a timing that is equal to or more than one rotation of the body.

  In the image forming apparatus, the control unit counts the number of transfer media transferred by the transfer member, and executes the second control when the counted number of transfer media exceeds a predetermined number. May be.

  In the image forming apparatus, the control unit may count the number of rotations of the image carrier and reduce the predetermined number when the counted number of rotations of the image carrier exceeds a predetermined number of rotations. Good.

  In the image forming apparatus, the control unit is configured to reset the counted number of transfer media when a non-operating state in which the transfer member does not perform a transfer operation with respect to the transfer medium continues for a predetermined time or more. May be.

  In the image forming apparatus, the control unit counts the number of dots formed on the image carrier, and executes the second control when the counted number of dots becomes a predetermined number or more. Also good.

  In the image forming apparatus, the control unit may count the number of rotations of the image carrier and reduce the predetermined number when the counted number of rotations of the image carrier exceeds a predetermined number of rotations. Good.

  In the image forming apparatus, the control unit is configured to reset the number of counted dots when a non-operating state in which the transfer member does not perform a transfer operation on the transfer medium continues for a predetermined time or longer. Also good.

  The image forming apparatus includes a humidity detection unit that detects humidity in the apparatus, and the control unit performs the second control when it is determined that the apparatus has low humidity from the humidity detected by the humidity detection unit. It may be configured to execute.

  The invention disclosed in this specification includes various devices such as a control device, a control method, a printing device, a printing method, a computer program for realizing the functions of these methods or devices, and a recording medium on which the computer program is recorded. It is realizable with the aspect of.

  According to the present invention, the control unit performs the next development to be transferred next to the previous developer image after the rear end of the previous developer image transferred by the transfer member passes through the nip portion. The same polarity as the first transfer bias applied to the transfer member during transfer of the developer until the front end portion of the agent image passes through the nip portion, and more than the absolute value of the first transfer bias By applying a large second transfer bias to the transfer member, charges accumulated in the photosensitive layer of the image carrier are removed.

  As described above, since the rotation operation of the dedicated image carrier for removing the charge is not required, the charge accumulated in the photosensitive layer of the image carrier is reduced while reducing the number of rotations of the image carrier. Can be removed. Thereby, it is possible to suppress the decrease in the surface potential of the image carrier while extending the life of the image carrier.

1 is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to an embodiment. It is a block diagram which shows the electric constitution of the control part which concerns on this embodiment. It is a flowchart which shows the process sequence of a judgment process. This is a subroutine for suppression processing. It is a flowchart which shows the process sequence of a reset process. It is a schematic sectional drawing which shows the structure of the image forming apparatus which concerns on other embodiment.

  Embodiments will be described in detail with reference to the drawings. After describing the overall configuration of the image forming apparatus 1, features of the present invention will be described. The directions in the following description will be described with reference to the user when using the image forming apparatus 1. Specifically, in FIG. 1, the left side “front”, the right side “rear”, the upper side “up”, and the lower side “lower” as viewed in the drawing. Also, the front side of the page is “right” and the back side is “left”.

  As shown in FIG. 1, the image forming apparatus 1 includes, in the apparatus main body 2, a paper feeding unit 20 that feeds a sheet P as an example of a transfer medium, and an image forming that forms an image on the fed paper P. The unit 30 includes a paper discharge unit 90 that discharges the paper P on which an image is formed, and a control unit 100.

  An opening 2A is formed in the upper part of the apparatus main body 2, and the opening 2A is opened and closed by an upper cover 3 that is rotatably supported by the apparatus main body 2. The upper surface of the upper cover 3 is a paper discharge tray 4 that accumulates the paper P discharged from the apparatus main body 2, and a plurality of LED attachment members 5 that hold the LED units 40 are provided on the lower surface.

  The paper feed unit 20 is provided in the lower part of the apparatus main body 2 and is detachably attached to the apparatus main body 2. A paper feed roller 22, a separation roller 23, and a separation pad 24 as an example of a delivery mechanism. It has. In the paper supply unit 20, the paper P in the paper supply tray 21 is separated one by one by the separation roller 23 and the separation pad 24, and is sent out toward the image forming unit 30 by the paper supply roller 22.

  Above the paper feed unit 20, a humidity sensor 130 is provided as an example of a humidity detection unit that detects humidity in the apparatus main body 2. The humidity sensor 130 detects the humidity in the apparatus main body 2 and outputs the detection result to the control unit 100.

  The image forming unit 30 mainly includes four LED units 40, four cartridges 50, a transfer unit 70, and a fixing unit 80.

  The LED unit 40 is connected to the LED mounting member 5 and is positioned and supported by a positioning member (not shown) provided in the apparatus main body 2.

  The cartridge 50 is arranged side by side between the upper cover 3 and the paper feeding unit 20 in the front-rear direction, and a photosensitive drum 51 as an example of an image carrier on which an electrostatic latent image is formed, a charger 52, and a developing unit. A roller 53 and a toner storage chamber 54 that stores toner as an example of a developer are provided.

  The cartridge 50 includes cartridges 50K, 50Y, 50M, and 50C that store toner of each color of black, yellow, magenta, and cyan, and are arranged in this order from the upstream side in the conveyance direction of the paper P. In the present specification and drawings, when the photosensitive drum 51, the charger 52, the developing roller 53, and the transfer roller 74 corresponding to the color of the toner are specified, they are associated with black, yellow, magenta, and cyan, respectively. The symbols K, Y, M, and C are attached.

  The photoconductive drum 51 includes photoconductive drums 51K, 51Y, 51M, and 51C, and is arranged in this order from the upstream side in the transport direction of the paper P. In the present embodiment, the configurations of the respective photosensitive drums 51 are the same as each other.

  The transfer unit 70 is provided between the paper feeding unit 20 and each cartridge 50, and mainly includes a driving roller 71, a driven roller 72, a conveyance belt 73, and a transfer roller 74.

  The driving roller 71 and the driven roller 72 are arranged in parallel with a space in the front-rear direction, and a conveyance belt 73 formed of an endless belt is stretched between them. The outer surface of the conveyance belt 73 is in contact with each photosensitive drum 51.

  Inside the conveyance belt 73, a transfer roller 74 that sandwiches the conveyance belt 73 with each photoconductive drum 51 is disposed so as to face each photoconductive drum 51. The transfer roller 74 includes transfer rollers 74K, 74Y, 74M, and 74C.

  The fixing unit 80 is disposed downstream of each cartridge 50 and the transfer unit 70 in the conveyance direction of the paper P, and includes a heating roller 81 and a pressure roller 82 that is disposed to face the heating roller 81 and presses the heating roller 81. Yes.

  In the image forming unit 30 configured as described above, when performing color printing, first, the surface of each photosensitive drum 51 is uniformly positively charged by the charging bias applied to the charger 52, and then each LED is turned on. The unit 40 is exposed. As a result, the potential of the exposed portion is lowered, and an electrostatic latent image based on the image data is formed on each photosensitive drum 51.

  Thereafter, positively charged toner by the developing roller 53 is supplied to the electrostatic latent image, so that a toner image as an example of a developer image is carried on the photosensitive drum 51.

  When the paper P sent out onto the transport belt 73 by the paper feed roller 22 passes through the nip portion 8 for nipping and transporting the paper P between each photosensitive drum 51 and each transfer roller 74, each transfer roller 74 described later. The toner image on each photoconductive drum 51 is transferred onto the paper P in the order of K, Y, M, and C by the transfer bias applied to.

  When performing monochrome printing, an electrostatic latent image based on image data is formed on the photosensitive drum 51K by the LED unit 40 corresponding to the photosensitive drum 51K, and then a toner image is carried on the photosensitive drum 51K. Is done. Next, when the paper P sent out by the paper supply roller 22 passes through the nip portion 8, the toner image on the photosensitive drum 51K is transferred onto the paper P by the transfer bias applied to the transfer roller 74K.

  Then, as the paper P passes between the heating roller 81 and the pressure roller 82, the toner image transferred onto the paper P is thermally fixed.

  The paper discharge unit 90 mainly includes a transport path 91 that extends upward from the exit of the fixing unit 80 and a plurality of pairs of transport rollers 92 that transport the paper P. The sheet P on which the toner image has been transferred and heat-fixed is transported along a transport path 91 by a transport roller 92, discharged to the outside of the apparatus main body 2, and discharged to the paper discharge tray 4.

  In this embodiment, the humidity in the apparatus main body 2 is detected by the humidity sensor 130. However, the present invention is not limited to this, and each transfer roller 74 and each photosensitive member in a state where the paper P is sandwiched between the nip portions 8 are used. The structure etc. which measure the electrical resistance value between the body drums 51, and detect the humidity in the apparatus main body 2 based on the said electrical resistance value may be sufficient.

  In this embodiment, the surface of each photosensitive drum 51 is exposed by each LED unit 40. However, the present invention is not limited thereto, and includes a laser light emitting unit, a polygon mirror, a lens, a reflecting mirror, and the like. The structure exposed by a well-known exposure apparatus may be sufficient. A configuration in which the surface of each photosensitive drum 51 is exposed by light emitted from an EL element, a phosphor, or the like may be used.

  Next, the electrical configuration of the control unit 100 will be described.

  As shown in FIG. 2, the control unit 100 includes a CPU 111, a ROM 112, a RAM 113, and an NVRAM 114, and is electrically connected to each transfer roller 74 and the humidity sensor 130.

  The ROM 112 stores various control programs and image processing programs for controlling the image forming apparatus 1, various settings, initial values, and the like.

  The RAM 113 is used as a work area from which various control programs are read, or as a storage area for temporarily storing print data such as image data sent via various interfaces.

  The NVRAM 114 is a non-volatile storage unit, and is used as a storage area for storing various settings and print data.

  The CPU 111 executes arithmetic operations for realizing various functions such as a printing function in the image forming apparatus 1 and is the center of control. The CPU 111 controls each component of the image forming apparatus 1 while storing the processing result in the RAM 113 or the NVRAM 114 according to the control program read from the ROM 112.

  Next, the control performed by the control unit 100 will be described in detail.

  The controller 100 counts and accumulates the number of rotations of the photosensitive drum 51 corresponding to each cartridge 50 with a drum counter provided in the NVRAM 114 after each cartridge 50 is replaced. Hereinafter, the number of rotations of the photosensitive drum 51 accumulated by the drum counter is referred to as a total number of rotations. Further, in this specification, the time when the cartridge 50 is replaced means that an unused cartridge 50 is mounted on the apparatus main body 2.

  The control unit 100 reads predetermined information from an IC tag, an RFID tag, or the like fixed to the cartridge 50, and determines whether or not the mounted cartridge 50 has been replaced with an unused cartridge 50 based on the information. Thus, the replacement of the cartridge 50 is detected.

  Specifically, the control unit 100 sets the cumulative number of rotations of each photosensitive drum 51 to zero when each cartridge 50 is replaced. Then, based on the print data input to the control unit 100, the cumulative number of revolutions of each photosensitive drum 51 is counted by incrementing the cumulative number of revolutions. The drum counter is provided corresponding to each cartridge 50.

  In the present embodiment, the control unit 100 detects the replacement of each cartridge 50 and sets the cumulative number of rotations of each photosensitive drum 51 to zero when each cartridge 50 is replaced. However, the present invention is not limited to this, and the configuration may be such that the replacement of each photosensitive drum 51 is detected, and the cumulative number of rotations of each photosensitive drum 51 is set to zero when each photosensitive drum 51 is replaced.

  The control unit 100 counts and accumulates the number of sheets of paper P transferred since each cartridge 50 was replaced. Specifically, when each cartridge 50 is replaced, the control unit 100 sets the count value of the number counter provided in the NVRAM 114 and corresponding to each cartridge 50 to zero.

  Then, based on the print data input to the control unit 100, the count value of the number counter is incremented to count the cumulative number of sheets P transferred to each transfer roller 74.

  When performing monochrome printing and color printing, the control unit 100 applies a first charging bias having a voltage having the same polarity as the charge of the toner image to the charger 52 so that the surface of each photoconductive drum 51 is made uniform. It is positively charged. Further, when the sheet P passes through each nip portion 8, a first control is performed to apply a first transfer bias to each transfer roller 74K, 74Y, 74M, 74C.

  Here, the first transfer bias is a voltage applied between each transfer roller 74 and each photoconductor drum 51 in order to transfer the toner image on each photoconductor drum 51 onto the paper P.

  The first transfer bias is a transfer bias having a reverse polarity to the charge of the toner image on each photoconductive drum 51, and the toner image on the photoconductive drum 51 is transferred onto the paper P by the reverse polarity transfer bias. . In this embodiment, since positively charged toner is used, the first transfer bias applied to each transfer roller 74 is a negative transfer bias.

  In this embodiment, positively charged toner is used and a negative transfer bias is applied to each transfer roller 74. However, the present invention is not limited to this, and negatively charged toner is used. The configuration may be such that a positive transfer bias is applied to 74.

  The control unit 100 executes a determination process for determining whether or not to execute a suppression process to be described later for removing charges accumulated in the photosensitive layer of each photosensitive drum 51. The determination process will be described with reference to FIG.

  As shown in FIG. 3, when starting the determination process, the control unit 100 determines whether there is print data in the control unit 100 (S100), and determines that there is print data (S100: YES), Based on the cumulative number of rotations accumulated by the drum counter, it is determined whether or not the total number of rotations of each photosensitive drum 51 is equal to or greater than a predetermined number (S105).

  When the control unit 100 determines that the cumulative number of rotations of each photosensitive drum 51 is equal to or greater than the predetermined number (S105: YES), the control unit 100 sets the count value threshold of each number counter to the first threshold (S110). If it is determined that the cumulative number of rotations of the body drum 51 is not equal to or greater than the predetermined number (S105: NO), the threshold value of the count value of each number counter is set to the second threshold value (S115). Note that in S105, S110, and S115, the count values of the respective photosensitive drums 51 and the respective number counters are individually processed individually by the control unit 100.

  In the present embodiment, a value larger than the first threshold is set as the second threshold. Note that the second threshold value may be set to an arbitrary value larger than the first threshold value.

  Here, the control unit 100 sets the threshold value as a reference for determining whether or not to execute a suppression process described later.

  In the present embodiment, the threshold value is changed according to the total number of rotations of each photosensitive drum 51. This is because a phenomenon in which charges are easily accumulated in the photosensitive layer of each photosensitive drum 51 as the cumulative number of rotations of each photosensitive drum 51 increases is taken into consideration. That is, when the cumulative number of rotations of each photoconductor drum 51 is equal to or greater than a predetermined number, the frequency of execution of the suppression process is higher than when the cumulative number of rotations is less than the predetermined number.

  In this embodiment, the threshold value of the count value of each sheet counter is uniformly set to the same value, but the present invention is not limited to this, and the threshold value may be different for each sheet counter. Hereinafter, the threshold value of the count value of each sheet counter is simply referred to as a threshold value.

  Next, the control unit 100 drives the paper feed roller 22 based on the print data input to the control unit 100, and starts printing on the paper P (S120). In S <b> 120, a first transfer bias is applied between each transfer roller 74 and each photoconductor drum 51 in order to transfer the toner image on each photoconductor drum 51 to the paper P.

  And the control part 100 judges whether the inside of the apparatus main body 2 is low humidity based on the detection result output from the humidity sensor 130 (S125). In the present embodiment, when the humidity detected by the humidity sensor 130 is 20% or less, the control unit 100 determines that the inside of the apparatus main body 2 is low humidity. In this embodiment, the reference humidity for determining whether the humidity is low is 20%. However, the present invention is not limited to this, and an arbitrary value may be set.

  When the control unit 100 determines that the inside of the apparatus main body 2 is low humidity (S125: YES), the control unit 100 causes each number counter to count the cumulative number of sheets P transferred to each transfer roller 74 (S130). Here, the control unit 100 increments the count value of each number counter when performing color printing, and increments the count value of the number counter corresponding to the cartridge 50K when performing monochrome printing.

  If the control unit 100 determines that the inside of the apparatus main body 2 is not low humidity (NO in S125), the control unit 100 proceeds to S135 without causing the number counter to count the cumulative number of sheets P transferred to each transfer roller 74.

  In the present embodiment, as described above, the control unit 100 counts the cumulative number only when it is determined that the inside of the apparatus main body 2 is low in humidity. This is because a phenomenon in which charges are more likely to be accumulated in the photosensitive layer of each photosensitive drum 51 when the inside of the apparatus main body 2 is low humidity is considered as compared with the case where the inside of the apparatus main body 2 is not low humidity.

  Here, in this embodiment, the control unit 100 counts the cumulative number of sheets P transferred to each transfer roller 74, but when a predetermined condition is satisfied, the control unit 100 calculates the cumulative number. Configured to reset. In other words, the control unit 100 sets the count value of each sheet counter to zero when a predetermined condition is satisfied.

  The reset process of the count value executed by the control unit 100 will be described with reference to FIG. As shown in FIG. 5, the control unit 100 determines whether or not the non-operating state in which each transfer roller 74 does not perform the transfer operation on the paper P has continued for a predetermined time or more (S150).

  When the control unit 100 determines that the non-operating state has continued for a predetermined time or longer (S150: YES), the count value of each sheet counter is reset (S155), and the cumulative number of sheets P transferred to each transfer roller 74 is zero. To. If the control unit 100 determines that the non-operating state has not continued for a predetermined time or longer (S150: NO), the control unit 100 maintains the count value of each sheet counter. The reset process is called at predetermined time intervals.

  Next, as shown in FIG. 3, the control unit 100 determines whether or not there is print data in the control unit 100 (S135), and determines that there is print data in the control unit 100 (S135: YES), S110. Alternatively, based on the threshold set in S115, it is determined whether or not the count value of each sheet counter is equal to or greater than the threshold (S140). If it is determined that there is no print data in the control unit 100 (S135: NO), the determination process ends. The determination process is called at predetermined time intervals.

  When the control unit 100 determines that at least one of the count values of the respective number counters is equal to or greater than the threshold value (S140: YES), the control unit 100 executes a potential decrease suppressing process described later (S145).

  Here, the potential decrease suppressing process executed by the control unit 100 is a process for removing charges accumulated in the photosensitive layer of each photosensitive drum 51 and suppressing a decrease in the surface potential of each photosensitive drum 51. is there. The potential drop suppression process will be described in detail with reference to FIG.

  As shown in FIG. 4, the control unit 100 controls the driving of the paper feed roller 22, so that the rear end of the previous paper P that has already been sent out by the paper feed roller 22 and the paper feed roller 22. The next paper P is continuously sent to the previous paper P at a timing at which the distance from the front end of the next paper P that has not been sent is at least one rotation of the photosensitive drum 51 (S160). . Hereinafter, the interval between the rear end portion of the previous paper P and the front end portion of the next paper P is referred to as a paper interval.

  In other words, the control unit 100 controls the driving of the paper feed roller 22 so that the next paper P is continuously sent to the previous paper P at a timing at which the interval between the sheets becomes equal to or greater than the circumferential length of each photosensitive drum 51. .

  Next, the control unit 100 includes a photosensitive drum 51 provided in the cartridge 50 corresponding to the number counter having a count value equal to or greater than a threshold value, and a nip portion formed by the transfer roller 74 facing the photosensitive drum 51. 8, the second transfer bias is applied to the transfer roller 74 between the time when the rear end portion of the previous paper P passes and the time when the front end portion of the next paper P passes through the nip portion 8. The second control is executed (S165). Here, the second transfer bias is a transfer bias having the same polarity as the first transfer bias and larger than the absolute value of the first transfer bias.

  As described above, during printing, the charge accumulated in the photosensitive layer of each photosensitive drum 51 is removed by utilizing the space between the sheets. In order for the image forming unit 30 to appropriately perform the image forming operation on the paper P, a gap between sheets is necessarily required. For this reason, in the conventional image forming apparatus, each photosensitive member is in a period from when the rear end portion of the previous paper P passes through the nip portion 8 until the front end portion of the next paper P passes through the nip portion 8. The body drum 51 was rotating.

  Therefore, in this embodiment, the second control is executed for each transfer roller 74 until the nip portion 8 is passed. That is, the rotation of each photosensitive drum 51 until it passes through the nip portion 8 is effectively used to remove the charge accumulated in the photosensitive layer of each photosensitive drum 51.

  That is, since the rotation operation of the dedicated photoconductive drum 51 for removing the electric charge is not required, the electric charge can be removed while reducing the rotation speed of each photoconductive drum 51. Thereby, it is possible to suppress a decrease in the surface potential of each photoconductive drum 51 while extending the life of each photoconductive drum 51.

  When executing the second control, the control unit 100 applies the second transfer bias for a time during which the photosensitive drum 51 that is the target of the second control is rotated once or more. In this embodiment, since the next sheet P is sent out by the control unit 100 at a timing when the interval between the sheets becomes equal to or longer than the circumferential length of each photosensitive drum 51, the photosensitive drum 51 to be subjected to the second control rotates once. The second transfer bias can be applied for the above time.

  As a result, the charge in the photosensitive layer can be more uniformly removed as compared with the case where the second transfer bias is applied for a time shorter than the time for which the photosensitive drum 51 makes one or more rotations. It is possible to suppress a decrease in the surface potential of each photosensitive drum 51 while suppressing variations in potential.

  In the present embodiment, as described above, the second control is performed after the rear end portion of the previous paper P passes through the nip portion 8 and before the front end portion of the next paper P passes through the nip portion 8. However, the present invention is not limited to this, and after the rear end of the previous toner image transferred to the previous paper P passes through the nip portion 8, the next toner image transferred to the next paper P The configuration may be such that the second control is executed until the front end portion passes through the nip portion 8.

  Further, in this embodiment, the control unit 100 causes each number counter to count the cumulative number of sheets P transferred to each transfer roller 74 only when the inside of the apparatus main body 2 is determined to be low humidity. As described above, the control unit 100 causes each number counter to count only when the inside of the apparatus main body 2 is low in humidity and charges are likely to be accumulated in the photosensitive layer of each photosensitive drum 51.

  As a result, the suppression process is executed at an optimal timing at which the charge in the photosensitive layer needs to be removed. Therefore, in the case where it is not necessary to remove the charge in the photosensitive layer, the unnecessary photosensitive drums 51 are provided. Rotational motion can be reduced.

  Furthermore, in this embodiment, it is determined whether or not the cumulative number of rotations of each photosensitive drum 51 is greater than or equal to a predetermined value, and the threshold value is set according to the determination result. As a result, the suppression process is executed at an optimal timing at which the charge in the photosensitive layer needs to be removed. Therefore, in the case where it is not necessary to remove the charge in the photosensitive layer, the unnecessary photosensitive drums 51 are provided. Rotational motion can be reduced.

  Next, the control unit 100 resets the count value of each number counter (S170), and sets the cumulative number of sheets P transferred to each transfer roller 74 to zero.

  In the present embodiment, the control unit 100 is configured to determine whether to execute the suppression process based on the count value of each sheet counter. However, the present invention is not limited to this, and from when each cartridge 50 is replaced. The number of dots formed on each of the photoconductive drums 51 may be counted, and based on the count value, it may be determined whether to execute the suppression process. An example of this configuration will be described in detail below.

  The controller 100 counts and accumulates the number of dots formed on each photosensitive drum 51 from when each cartridge 50 is replaced. Specifically, when each cartridge 50 is replaced, the control unit 100 sets the count value of the dot counter provided in the NVRAM 114 to zero, and each LED included in the print data input to the control unit 100 Based on the information for exposing the unit 40, the number of dots formed on each photosensitive drum 51 is counted. A dot counter is provided for each cartridge 50.

  The control unit 100 refers to the count value of the dot counter for the count value in the determination process illustrated in FIG. 3, and determines whether to execute the suppression process based on the count value of the dot counter.

  In the embodiment described above, the paper P is used as an example of the transfer medium. However, the present invention is not limited to this. For example, as illustrated in FIG. 6, an intermediate transfer belt 273 to which toner images are transferred from a plurality of photosensitive drums 251 is used. The configuration may be employed as an example of a transfer medium. The image forming apparatus 201 provided with the intermediate transfer belt 273 will be described with reference to FIG.

  The image forming apparatus 201 mainly includes a plurality of photosensitive drums 251 that carry toner images, and a transfer unit 270. The transfer unit 270 performs intermediate transfer between the intermediate transfer belt 273 that contacts the plurality of photosensitive drums 251, the driving roller 271 and the driven roller 272 for stretching the intermediate transfer belt 273, and each photosensitive drum 251. A secondary transfer roller 275 that forms a nip portion 7 that sandwiches and conveys the belt 273 and sandwiches the intermediate transfer belt 273 between a plurality of primary transfer rollers 274 that transfer the toner image to the intermediate transfer belt 273 and the drive roller 271. And.

  The image forming apparatus 201 includes a paper feed tray 221 that stores paper, a feeding mechanism 222 that feeds the paper in the paper feed tray 221 toward the intermediate transfer belt 273 and the secondary transfer roller 275, and the intermediate transfer belt 273. And a secondary transfer roller 275, and a fixing device 280 for thermally fixing the toner image transferred onto the paper.

  In the image forming apparatus 201, the control unit 100 controls the feeding mechanism 222, so that the rear end portion of the previous paper that has already been sent out by the sending mechanism 222 and the next paper that has not yet been sent out by the sending mechanism 222. A configuration in which the next sheet is continuously fed to the preceding sheet at a timing at which the distance from the front end of the photosensitive drum 251 is equal to or more than one rotation of each photosensitive drum 251 may be employed.

  Further, in the image forming apparatus 201, the control unit 100 passes through the nip portion 7 after the rear end portion of the previous toner image transferred to the previous paper and then transfers the next toner image transferred to the next paper. The configuration may be such that the second control is performed on each photosensitive drum 251 until the front end portion passes through the nip portion 7.

  In the above-described embodiment, the control unit 100 is configured to apply the second transfer bias in the second control. However, the present invention is not limited to this, and instead of applying the second transfer bias, the first transfer bias is applied. A configuration may be adopted in which a second charging bias having the same polarity as the charging bias and larger than the absolute value of the first charging bias is applied to the charger 52.

  As an example of such a configuration, for example, a photosensitive drum carrying a toner image, a charging member to which a first charging bias is applied to charge the photosensitive drum, and a photosensitive drum are arranged so as to face the transfer medium. A transfer member that transfers a toner image from the photosensitive drum to a transfer medium, and a control unit. The control unit includes a plurality of transfer members that transfer a toner image to and from the transfer drum. When the toner image is continuously transferred to the transfer medium, after the rear end of the previous toner image transferred by the transfer member passes through the nip portion, the transfer is performed next to the previous toner image. The second charging bias having the same polarity as the first charging bias and larger than the absolute value of the first charging bias is charged until the front end of the next toner image passes through the nip portion. Executes the control applied to the member And it may be formed.

  Thereby, since the electric charge accumulated in the photosensitive layer of each photoconductive drum 51 can be removed, a decrease in the surface potential of each photoconductive drum 51 can be suppressed.

  In the embodiment described above, an example in which the control unit 100 executes each step has been described. However, the present invention is not limited to this, and at least a part of the steps is executed by another CPU, or one or a plurality of ASICs execute. It may be a configuration.

  The cartridge 50 includes the cartridges 50K, 50Y, 50M, and 50C that store the toners of black, yellow, magenta, and cyan. However, the present invention is not limited to this, and only the cartridge 50K that stores the black toner is used. The structure provided with may be sufficient.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 2 Apparatus main body 8 Nip part 20 Paper feed part 22 Paper feed roller 30 Image formation part 51 Photosensitive drum 70 Transfer unit 74 Transfer roller 90 Paper discharge part 100 Control part 130 Humidity sensor

Claims (12)

An image carrier for carrying a developer image;
A transfer member disposed facing the image carrier, forming a nip portion for nipping and conveying the transfer medium to and from the image carrier, and transferring a developer image from the image carrier to the transfer medium;
A control unit,
The controller is
Executing a first control to apply a first transfer bias to the transfer member when transferring a developer image from the image carrier to a transfer medium;
When a plurality of developer images are continuously transferred onto the transfer medium by the transfer member, the rear end portion of the developer image transferred by the transfer member first passes through the nip portion. Until the front end of the next developer image transferred next to the previous developer image passes through the nip portion, and has the same polarity as the first transfer bias and the first An image forming apparatus that executes a second control for applying a second transfer bias larger than the absolute value of the transfer bias to the transfer member. A feeding mechanism for feeding the transfer medium toward the nip portion;
The controller is
When a plurality of transfer media are continuously sent out by the delivery mechanism and the rear end of the previous transfer medium sent out by the delivery mechanism passes through the nip portion, the previous transfer 2. The image forming apparatus according to claim 1, wherein the second control is executed until a front end portion of a next transfer medium fed next to the medium passes through the nip portion.   The control unit controls the operation of the delivery mechanism to control the timing of sending out the transfer medium, and the interval between the rear end portion of the previous transfer medium and the front end portion of the next transfer medium is the image carrier. The image forming apparatus according to claim 2, wherein the next transfer medium is sent out at a timing of an interval of one rotation or more.   The control unit counts the number of transfer media transferred by the transfer member, and executes the second control when the counted number of transfer media exceeds a predetermined number. The image forming apparatus according to claim 2 or 3.   The control unit counts the number of rotations of the image carrier, and reduces the predetermined number when the counted number of rotations of the image carrier exceeds a predetermined number of rotations. Item 5. The image forming apparatus according to Item 4.   The control unit resets the counted number of transfer media when a non-operating state in which the transfer member does not perform a transfer operation on the transfer medium continues for a predetermined time or longer. The image forming apparatus according to claim 4 or 5.   The control unit counts the number of dots formed on the image carrier, and executes the second control when the counted number of dots is equal to or greater than a predetermined number. The image forming apparatus according to claim 2 or 3.   The control unit counts the number of rotations of the image carrier, and reduces the predetermined number when the counted number of rotations of the image carrier exceeds a predetermined number of rotations. Item 8. The image forming apparatus according to Item 7.   The control unit resets the counted number of dots when a non-operating state in which the transfer member does not perform a transfer operation on a transfer medium continues for a predetermined time or more. The image forming apparatus according to claim 7 or 8. It has a humidity detector that detects the humidity inside the device.
The said control part performs said 2nd control, when it is judged from the humidity detected by the said humidity detection part that the inside of an apparatus is low humidity, Any of Claim 1-9 The image forming apparatus according to claim 1. An image carrier that carries a developer image, and a nip portion that is disposed opposite to the image carrier and that sandwiches and conveys a transfer medium with the image carrier, and transfers the developer image from the image carrier. A first applying step of applying a first transfer bias to the transfer member when transferring the developer image carried on the image carrier of the image forming apparatus to the transfer medium. When,
When a plurality of developer images are continuously transferred onto the transfer medium by the transfer member, the rear end portion of the developer image transferred by the transfer member first passes through the nip portion. Until the front end of the next developer image transferred next to the previous developer image passes through the nip portion, and has the same polarity as the first transfer bias and the first Applying a second transfer bias larger than the absolute value of the transfer bias to the transfer member. An image carrier that carries a developer image, and a nip portion that is disposed opposite to the image carrier and that sandwiches and conveys a transfer medium with the image carrier, and transfers the developer image from the image carrier. An image forming apparatus comprising: a transfer member that transfers to a medium;
A first application process of applying a first transfer bias to the transfer member when transferring the developer image carried on the image carrier to a transfer medium;
When a plurality of developer images are continuously transferred onto the transfer medium by the transfer member, the rear end portion of the developer image transferred by the transfer member first passes through the nip portion. Until the front end of the next developer image transferred next to the previous developer image passes through the nip portion, and has the same polarity as the first transfer bias and the first And a second application process for applying a second transfer bias larger than the absolute value of the transfer bias to the transfer member.

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