JP6182502B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly to a technique for improving the image quality of a direct transfer type image forming apparatus that does not include a static elimination lamp.

  The image forming apparatus employs a direct transfer system in which a toner image on the surface of the photosensitive drum is directly transferred from the surface to a recording sheet at a transfer nip portion between the surface of the photosensitive drum and a transfer roller. Some direct transfer type image forming apparatuses do not have a static elimination lamp unit for the purpose of cost reduction. In this case, since the drum surface potential is not initialized by the charge eliminating light, in the direct transfer type image forming apparatus, the difference in the transfer discharge state at the transfer nip portion directly affects the drum surface potential. Therefore, it is necessary to adjust the drum surface potential for the next image forming process.

  A technique for adjusting the drum surface potential is an adjustment technique that takes into consideration the influence of charging by the contact charging method, but is disclosed in Patent Document 1 below. In Patent Document 1, in a contact charging type image forming apparatus, the charge amount on the surface of the photoreceptor is reduced by stepwise decreasing the charge removal amount of the charge removal lamp and the charge bias applied to the charge roller according to the cumulative number of printed sheets. An image forming apparatus that suppresses the generation of pinholes over a long period of time without degrading the sensitivity and charging performance of the photoreceptor even when the process is performed near the surface is shown.

JP 2010-204322 A

  As described above, in the direct transfer type image forming apparatus, when there is no static elimination lamp unit, the difference in the transfer discharge state at the transfer nip portion directly affects the drum surface potential. For this reason, in the transfer nip portion, the impedance changes depending on the presence or absence of recording paper and the discharge state at the nip differs, and the drum surface potential in the non-sheet passing area changes compared to the normal sheet passing area. As a result, the history of the non-sheet passing area remains at the next development, and an image defect called a ghost may occur. In order to eliminate this ghost, it is necessary to appropriately set the transfer bias (inter-paper transfer bias) when not passing paper. Alternatively, a separation charger provided downstream of the transfer nip portion in the recording paper conveyance direction is used to charge the drum surface by applying an appropriate separation bias to the drum surface located in a region corresponding to the separation charger. Thus, it is necessary to align the drum surface potential in the non-sheet passing area with the drum surface potential in the normal sheet passing area. However, when the photosensitive layer wears down and the film thickness decreases, the characteristics of the photosensitive drum change, and the transfer bias and separation bias described above may deviate from the proper settings. To solve this problem, it is difficult to solve the above-described technique disclosed in Patent Document 1 by applying the above-described problem caused by the contact charging method. In addition, the technique disclosed in Patent Document 1 is based on a direct transfer method. The solution to the above problem is not an issue.

  The present invention has been made to solve the above-described problem, and aims to suppress the occurrence of a ghost image due to a drum potential fluctuation over a long period in an image forming apparatus of a direct transfer type that does not have a charge eliminating lamp. To do.

  An image forming apparatus according to an aspect of the present invention includes: a photosensitive drum having a photosensitive layer formed on a surface thereof; a charging unit that applies a charging bias to the photosensitive drum to charge the surface of the photosensitive drum; An exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image, and supplies toner to the surface of the photosensitive drum, and the electrostatic unit formed by the exposure unit. The toner image generated on the surface of the photosensitive drum by the developing unit is transferred to a recording sheet nipped between the developing unit that generates a toner image from the latent image and the surface of the photosensitive drum by a transfer bias. A transfer unit; and a voltage control unit that applies the transfer bias to the transfer unit. The voltage control unit includes a transfer unit before the recording paper is nipped between the photosensitive drum and the transfer unit. A transfer bias, is intended to lower the predetermined value in accordance with the wear amount of the photosensitive layer.

  The invention according to another aspect of the present invention includes a photosensitive drum having a photosensitive layer formed on a surface thereof, a charging unit that applies a charging bias to the photosensitive drum to charge the surface of the photosensitive drum, and the charging An exposure unit that exposes the surface of the photosensitive drum charged by the unit to form an electrostatic latent image; and supplies toner to the surface of the photosensitive drum to form the electrostatic latent image formed by the exposure unit. Transfer in which a toner image generated on the surface of the photosensitive drum by the developing unit is transferred by a transfer bias to a recording sheet nipped between a developing unit that generates a toner image from the image and the surface of the photosensitive drum A separation unit that separates the recording paper on which the toner image has been transferred in the transfer unit by a separation bias, and a voltage control that applies the transfer bias to the transfer unit and applies the separation bias to the separation unit. The voltage control unit includes a region in which a surface of the photosensitive drum to which the transfer bias is applied corresponds to the separation unit in a period when the recording paper is not nipped between the photosensitive drum and the transfer unit. The separation bias to be applied when passing through is reduced to a predetermined value according to the amount of wear of the photosensitive layer.

  According to the present invention, since the transfer bias applied to the transfer roller or the separation bias applied to the separation charger can be changed in accordance with the change in the film thickness of the photosensitive layer of the photosensitive drum, the discharge lamp is not provided. Also in the direct transfer type image forming apparatus, it is possible to suppress the generation of a ghost image due to drum potential fluctuation over a long period of time.

1 is a front sectional view showing a structure of an image forming apparatus according to an embodiment of the present invention. FIG. 2 is a detailed front cross-sectional view around the photosensitive drum in FIG. 1. It is a functional block diagram which shows roughly the main internal structures of an image forming apparatus. It is a graph showing the relationship between the cumulative number of printed sheets and the film thickness of the photosensitive layer on the surface of the photosensitive drum. 6 is a graph showing the relationship between the film thickness of the photosensitive layer on the surface of the photosensitive drum or the cumulative number of printed sheets and a desired inter-paper transfer bias. 6 is a graph showing the relationship between the film thickness of the photosensitive layer on the surface of the photosensitive drum or the cumulative number of printed sheets and a desired paper separation bias.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. FIG. 1 is a front sectional view showing the structure of an image forming apparatus 1 according to an embodiment of the present invention.

  The image forming apparatus 1 is a multifunction machine having a plurality of functions such as a copy function, a printer function, a scanner function, and a facsimile function. The image forming apparatus 1 includes a main body unit 2, a stack tray 3 disposed on the left side of the main body unit 2, an image reading unit 5 disposed on the upper part of the main body unit 2, and an upper side of the image reading unit 5. And a document feeding unit 6 disposed. Further, an operation unit 47 and a display unit 473 are provided on the front part of the image forming apparatus 1.

  The image reading unit 5 includes a scanner unit 51 including a CCD (Charge Coupled Device) sensor and an exposure lamp, and a document table 52 and a document reading slit 53 made of a transparent member such as glass. The scanner unit 51 scans the document image and outputs the acquired image data to the control unit 10 (FIG. 3) described later. The document feeding unit 6 feeds the documents placed on the document placing table 61 one by one and conveys them to a position facing the document reading slit 53.

  The main body 2 includes a plurality of paper feeding cassettes 461, a paper feeding roller 462 that feeds recording paper from the paper feeding cassette 461 one by one and transports it to the image forming unit 40, and a recording paper transported from the paper feeding cassette 461. And an image forming unit 40 for forming an image.

  The image forming unit 40 includes a photosensitive drum 43, a cleaning unit 49 that removes toner remaining on the photosensitive drum 43, a charging roller (charging unit) 422 that charges the surface of the photosensitive drum 43, and a scanner unit 51. An exposure unit 423 that outputs a laser beam based on the image data acquired in step 1 to expose the surface of the photosensitive drum 43 to form an electrostatic latent image on the surface of the photosensitive drum 43; The recording paper is nipped at a transfer nip portion between the photosensitive drum 43 and the developing portion 44 that forms a toner image on the photosensitive drum 43, and the toner image carried on the photosensitive drum 43 is recorded on the recording paper. A transfer roller (transfer section) 41 to be transferred to the recording paper, and provided downstream of the transfer nip portion in the recording paper conveyance direction, and neutralizes the charge of the recording paper charged by the transfer of the toner image by the transfer roller 41. A separation charger (separation unit) 411, a fixing unit 45 for fixing the toner image to the recording sheet by heating the recording sheet to which the toner image has been transferred, and a sheet conveying path in the image forming unit 40 are provided. Are provided with conveying rollers 463, 464 and the like.

  FIG. 2 is a detailed front sectional view of the periphery of the photosensitive drum 43 in FIG. The image forming apparatus 1 employs an image forming mechanism based on a direct transfer method in which a toner image is directly transferred from a surface of the photosensitive drum 43 onto a recording sheet. Around the photosensitive drum 43, a charging roller 422, an exposure unit 423, a developing unit 44, and a cleaning unit 49 are disposed along the drum rotation direction (clockwise in FIG. 2).

  The photosensitive drum 43 is an image carrier having a single-layer or stacked organic photosensitive layer (OPC) formed on the surface. The initial thickness of the photosensitive layer is, for example, 35 μm.

  The charging roller 422 is a roller that contacts the photosensitive drum 43 and applies a charging bias to the drum surface. In the present embodiment, an example in which a conductive rubber layer in which an ion conductive material is mixed with epichlorohydrin is formed on the surface of the charging roller 422 is used. The charging roller 422 applies, for example, a positive DC constant voltage of 1250 V to the surface of the photosensitive drum 43 as a charging bias.

  The developing unit 44 includes two stirring and conveying screws 441 and a developing roller 442. A thin toner layer is formed on the surface of the developing roller 442. For example, when a developing bias having the same polarity (positive) as the toner is applied to the developing roller 442, the toner flies to the surface of the photosensitive drum 43.

  The cleaning unit 49 includes a cleaning blade 491 and a recovery screw 492. The cleaning blade 491 is fixed in contact with the photosensitive drum 43. The residual toner removed from the surface of the photosensitive drum 43 by the cleaning blade 491 is discharged to the outside of the cleaning unit 49 as the recovery screw 492 rotates.

  The transfer roller 41 is also an ion conductive type roller. On the surface of the transfer roller 41, for example, a transfer bias having a polarity (negative) opposite to the charging bias applied to the surface of the photosensitive drum 43 is applied by the voltage control unit 60 (FIG. 3).

  The separation charger 411 is a member in which a plurality of needle portions are formed on a thin metal plate. The separation charger 411 is electrically connected to the voltage control unit 60, and a separation bias having a reverse polarity (positive) to the transfer bias is applied by the voltage control unit 60. As a result, a charge having a polarity opposite to that of the transfer bias is supplied to the surface of the photosensitive drum 43 passing through a region corresponding to the separation charger 411, and the charge of the recording paper charged by the transfer of the toner image by the transfer roller 41 is neutralized. The Note that the region corresponding to the separation charger 411 is a region to which charges discharged from the separation charger 411 are supplied. The area is determined by the arrangement relationship of the separation charger 411 and the photosensitive drum 43, and in this embodiment, the surface area of the photosensitive drum 43 facing the separation charger 411 corresponds to the area.

  In the above description, the case where the separation charger 411 formed of a member in which a plurality of needle portions are formed on a thin metal plate is used as the separation portion for separating the recording paper onto which the toner image is transferred has been described. However, the present invention is not necessarily limited to this case. Not. Depending on the characteristics and settings such as the toner, the developing unit 44, the transfer roller 41, and the printing speed, a configuration in which the separation needle and the static elimination brush are arranged in a grounded state can be used as the separation unit.

  Next, the electrical configuration of the image forming apparatus 1 will be described. FIG. 3 is a functional block diagram showing the main internal configuration of the image forming apparatus 1.

  The image forming apparatus 1 includes a control unit 10. The control unit 10 includes a CPU (Central Processing Unit), a RAM, a ROM, a dedicated hardware circuit, and the like, and controls overall operation of the image forming apparatus 1.

  The image reading unit 5 includes the scanner unit 51 having a CCD sensor, an exposure lamp, and the like under the control of the control unit 10. The image reading unit 5 reads an image from the document by irradiating the document with an exposure lamp and receiving the reflected light with a CCD sensor.

  The image processing unit 31 performs image processing on the image data of the image read by the image reading unit 5 as necessary. For example, the image processing unit 31 performs predetermined image processing such as shading correction in order to improve the quality after the image read by the image reading unit 5 is formed by the image forming unit 40.

  The image memory 32 is an area for temporarily storing document image data obtained by reading by the image reading unit 5 and temporarily storing data to be printed by the image forming unit 40.

  The image forming unit 40 performs image formation such as print data read by the image reading unit 5 and print data received from the computer 200 connected to the network.

  The operation unit 47 receives instructions from the operator regarding various operations and processes that can be executed by the image forming apparatus 1. The operation unit 47 includes a display unit 473.

  The facsimile communication unit 71 includes an unillustrated encoding / decoding unit, modulation / demodulation unit, and NCU (Network Control Unit), and performs facsimile transmission using a public telephone network.

  The network interface unit 91 includes a communication module such as a LAN board, and transmits / receives various data to / from the computer 200 in the local area via a LAN connected to the network interface unit 91.

  The HDD 92 is a large-capacity storage device that stores document images and the like read by the image reading unit 5.

  The driving motor 70 is a driving source that applies a rotational driving force to each rotating member of the image forming unit 40, for example, each rotating member of the fixing unit 45.

  The voltage control unit 60 applies a transfer bias to the transfer roller 41 and applies a separation bias to the separation charger 411. For example, the voltage control unit 60 applies a transfer bias having a polarity opposite to the charging bias of the photosensitive drum 43 to the transfer roller 41. In this case, the transfer current is, for example, a DC constant current of −14 μA. Further, the voltage control unit 60 applies a separation bias having a polarity opposite to the transfer bias to the separation charger 411. The separation current is, for example, a 10 μA DC constant current.

  The control unit 10 includes a control unit 100. The control unit 100 includes an image reading unit 5, a document feeding unit 6, an image processing unit 31, an image memory 32, an image forming unit 40, an operation unit 47, a facsimile communication unit 71, a network interface unit 91, and an HDD (hard disk drive) 92. These are connected to the voltage control unit 60 and the like, and drive control of these parts is performed.

  In the direct transfer type image forming apparatus 1 as in the present embodiment, when there is a recording sheet at the nip portion between the transfer roller 41 and the photosensitive drum 43 (at the time of sheet feeding) and when there is no recording sheet (non-sheet passing). ), The surface discharge potential of the photosensitive drum 43 varies between the sheet passing area and the non-sheet passing area. Therefore, the voltage control unit 60 sets the transfer bias when not passing, that is, the transfer bias between the recording sheets conveyed to the nip portion, to a value different from the transfer bias when passing. The potential of the surface of the photoconductive drum 43 is prevented from changing due to each position in the circumferential direction.

  In the initial state where the photosensitive layer on the surface of the photosensitive drum 43 is not worn, the voltage control unit 60 applies, for example, a DC constant voltage of −1200 V to the transfer roller 41 as an inter-sheet transfer bias.

  Here, in the image forming apparatus 1 of the direct transfer method, the thickness of the photosensitive layer on the surface of the photosensitive drum 43 is likely to be reduced due to wear, but when the thickness of the photosensitive layer is reduced, the electrical characteristics on the surface of the photosensitive drum 43 are reduced. Change. In the case where the photosensitive layer is a single layer type, the change in the electrical characteristics of the surface of the photosensitive drum 43 due to the decrease in the film thickness is more noticeable than in the laminated type. Accordingly, it is not appropriate to continuously apply a constant DC voltage of −1200 V to the transfer roller 41 as the inter-sheet transfer bias, and the inter-sheet transfer bias is reduced according to the decrease in the film thickness of the photosensitive layer. It is necessary to adjust in the direction. The voltage control unit 60 appropriately changes the inter-sheet transfer bias according to the film thickness of the photosensitive layer on the surface of the photosensitive drum 43.

  FIG. 4 is a graph showing the relationship between the cumulative number of printed sheets and the film thickness of the photosensitive layer on the surface of the photosensitive drum 43. As shown in FIG. 4, as the cumulative number of printed sheets increases, the thickness of the photosensitive layer decreases at a substantially constant rate. Specifically, the photosensitive layer, which was 35 μm at the start of use, was worn to 31 μm when the cumulative number of printed sheets was 50,000 (50k), 25 μm when 125,000, and 20 μm when 200,000. Yes.

  On the other hand, FIG. 5 is a graph showing the relationship between the film thickness of the photosensitive layer on the surface of the photosensitive drum 43 or the cumulative number of printed sheets and a suitable inter-sheet transfer bias used in this embodiment. As shown in FIG. 5, in the image forming apparatus 1, the absolute value of the inter-sheet transfer bias is decreased as the cumulative number of printed sheets increases, that is, as the cumulative number of printed sheets increases. For example, as the cumulative number of printed sheets increases, the absolute value of the inter-sheet transfer bias is decreased at a substantially constant rate.

  For example, in the initial state in which the film thickness of the photosensitive layer is 35 μm, that is, the cumulative number of printed sheets is 0, the inter-sheet transfer bias is set to −1200 V, and the film thickness is 31 μm, that is, the cumulative number of printed sheets is 50,000 sheets (50k sheets). Then, the inter-paper transfer bias is set to −1100 V and the film thickness is 25 μm, that is, when the cumulative number of printed sheets is 125,000, the inter-paper transfer bias is set to −1000 V, and the film thickness is 20 μm, that is, the cumulative number of printed sheets is 200,000. When it is a sheet, the transfer bias between sheets is set to -900V.

  For this reason, the accumulated number of printed sheets and a suitable inter-sheet transfer bias value corresponding to each accumulated number of printed sheets are set in advance, and the relationship between the two is set as a table, and a memory such as a RAM or ROM built in the control unit 10; Alternatively, it is stored in advance in the HDD 92. The control unit 100 counts the cumulative number of printed sheets by the image forming apparatus 1 using, for example, a built-in counter, and records the count value as the cumulative number of printed sheets in the RAM, ROM, or HDD 92.

  The voltage control unit 60 refers to the above table, acquires the value of the inter-sheet transfer bias corresponding to the cumulative number of printed sheets at the time of adjusting the inter-sheet transfer bias, and sets the inter-sheet transfer bias of the transfer roller 41 to the acquired value. Set. In this way, the voltage control unit 60 changes the transfer bias of the transfer roller 41 in accordance with the wear amount of the photosensitive layer on the surface of the photosensitive drum 43.

  This makes it possible to change the sheet-to-paper transfer bias in accordance with the change in the film thickness of the photosensitive layer of the photosensitive drum 43. Therefore, even in a direct transfer type image forming apparatus that does not have a charge eliminating lamp, It is possible to suppress the generation of a ghost image due to the fluctuation of the drum surface potential.

  Further, there is a linear relationship between the cumulative number of printed sheets and the cumulative driving time of the photosensitive drum 43. Therefore, the relationship between the cumulative driving time of the photosensitive drum 43 instead of the cumulative number of printed sheets and a suitable paper-to-paper transfer bias corresponding thereto is used as a table, and is stored in the RAM, ROM, or HDD 92 built in the control unit 10. It may be stored in advance. In this case, the control unit 100 measures the cumulative driving time of the photosensitive drum 43 using a built-in timer or the like, and records the measured time in the RAM, ROM, or HDD 92.

  Alternatively, in place of the table, a function that returns a desired sheet-to-sheet transfer bias using the accumulated number of printed sheets or the accumulated driving time of the photosensitive drum 43 as an argument is defined, and the function is stored in the RAM, ROM, or HDD 92 in the control unit 10. It may be stored in advance. In this case, the voltage control unit 60 obtains the cumulative number of printed sheets or the cumulative driving time of the photosensitive drum 43 at the time when adjusting the inter-sheet transfer bias, and applies the obtained value to the above function to obtain a suitable paper. Inter-transfer bias is calculated.

  Next, another embodiment of the present invention will be described. In this embodiment, the recording roller 41 is applied to the transfer roller 41 when there is recording paper at the nip portion between the transfer roller 41 and the photosensitive drum 43 (when paper is passed) and when there is no recording paper (when paper is not passed). The polarity of the transfer bias is switched.

  When the recording paper is passed, a transfer bias having a polarity (negative) opposite to the charging bias on the surface of the photosensitive drum 43 is applied to the transfer roller 41, and the toner image on the surface of the photosensitive drum 43 is transferred to the surface of the recording paper. Is done. Further, when the recording paper is not passing, such as between paper sheets, a reverse bias of the transfer bias, that is, a reverse transfer bias having the same polarity (positive) as the photosensitive drum 43 and the charging bias is applied as the paper transfer bias. Due to the reverse transfer bias, a toner image formed in a non-sheet passing area that does not face the recording paper on the surface of the photosensitive drum 43, or a so-called fog toner other than the image is not attracted to the transfer roller 41. Further, the toner adhering to the surface of the transfer roller 41 is reversely transferred to the surface of the photosensitive drum 43 by the reverse transfer bias and collected by the cleaning unit 49.

  Also in this embodiment, the transfer discharge state is different between when there is recording paper at the nip portion between the transfer roller 41 and the photosensitive drum 43 (when paper is passed) and when there is no recording paper (when paper is not passed). Therefore, the surface potential of the photosensitive drum 43 varies between the paper passing area and the non-paper passing area. Therefore, in this embodiment, the voltage control unit 60 further applies a paper separation bias having a polarity (negative) opposite to the separation bias at the time of paper feeding to the separation charger 411 at the time of non-paper feeding. The fluctuation of the surface potential of the photosensitive drum 43 due to the reverse transfer bias when the paper is not fed is adjusted to the potential of the surface of the photosensitive drum 43 when the paper is passed by the paper separation bias. It does not occur.

  The separation bias at the time of paper passing is an area where the surface of the photosensitive drum 43 to which the transfer bias is applied corresponds to the separation charger 411 during a period in which the recording paper is nipped between the photosensitive drum 43 and the transfer roller 41. Is a separation bias to be applied when passing through. Further, the non-sheet-passing separation bias (inter-paper separation bias) is the separation of the surface of the photosensitive drum 43 to which the transfer bias is applied in a period in which the recording paper is not nipped between the photosensitive drum 43 and the transfer roller 41. This is a separation bias applied when passing through a region corresponding to the charger 411.

  As in the previous embodiment, the voltage control unit 60 applies a DC constant current of −14 μA, for example, to the transfer roller 41 as a transfer bias when passing paper. In addition, a DC constant voltage of 300 V is applied to the transfer roller 41 as a reverse transfer bias when not passing paper. A DC constant current of 10 μA is applied to the separation charger 411 as a separation bias at the time of paper feeding. In this embodiment, a DC constant current of −8 μA is applied as a paper separation bias to the separation charger 411 when no paper is passed.

  Here, as in the previous embodiment, as shown in FIG. 5, the film thickness of the photosensitive layer on the surface of the photosensitive drum 43 decreases in accordance with the cumulative number of printed sheets. When the film thickness of the photosensitive layer decreases, the electrical characteristics on the surface of the photosensitive drum 43 change in the same manner. Therefore, it is necessary to reduce the separation bias according to the decrease in the thickness of the photosensitive layer, that is, to adjust in the positive direction. As shown in FIG. 6, the voltage control unit 60 reduces the absolute value of the paper separation bias at the time of non-sheet passing at a substantially constant rate according to the film thickness of the photosensitive layer on the surface of the photosensitive drum 43.

  For example, in the initial state where the photosensitive layer thickness is 35 μm, that is, the cumulative number of printed sheets is 0, the paper separation bias is set to a DC constant current of −8 μA, and the film thickness is 31 μm, that is, the cumulative number of printed sheets is 50,000 sheets ( 50k sheets), the paper separation bias is set to a DC constant current of -7 μA, and when the film thickness is 25 μm, that is, the accumulated number of printed sheets is 125,000, the paper separation bias is set to a DC constant current of -6 μA. When the thickness is 20 μm, that is, when the cumulative number of printed sheets reaches 200,000, the paper separation bias is set to a DC constant current of −5 μA.

  As a specific method of changing the paper separation bias according to the cumulative number of printed sheets, the same method as the control of the paper transfer bias in the previous embodiment can be used.

  The present invention is not limited to the configuration of the above embodiment, and various modifications can be made. For example, in the above-described embodiment, the case where the photosensitive layer of the photosensitive drum 43 is a single layer type is described as an example. The effect of the present invention is remarkably exhibited in the case of a single layer type. Even with the photosensitive drum 43, it is possible to enjoy the effects of the present invention.

  The image forming apparatus according to an embodiment of the present invention has been described using a multifunction peripheral. However, this is only an example, and other electronic devices such as a printer, a copier, a facsimile machine, and the like are used. An image forming apparatus may be used.

  Moreover, you may combine said embodiment and each modification partially. For example, in the above embodiment, (1) when the transfer bias is lowered to a predetermined value according to the wear amount of the photosensitive layer, and (2) the separation bias is predetermined according to the wear amount of the photosensitive layer. However, a combination of these may be used to reduce both the transfer bias and the separation bias to a predetermined value according to the wear amount of the photosensitive layer.

  Moreover, in the said embodiment, the structure and process which were shown by the said embodiment using FIG. 1 thru | or FIG. 6 are only one Embodiment of this invention, and are not the meaning which limits this invention to the said structure and process.

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 43 Photosensitive drum 422 Charging roller 41 Transfer roller 411 Separation charger 60 Voltage control unit

Claims (7)

A photosensitive drum having a photosensitive layer formed on the surface;
A charging unit that applies a charging bias to the photosensitive drum to charge the surface of the photosensitive drum;
An exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image; and
A developing unit that supplies toner to the surface of the photosensitive drum and generates a toner image from the electrostatic latent image formed by the exposure unit;
A transfer unit for transferring a toner image generated on the surface of the photoconductive drum by the developing unit to a recording sheet niped with the surface of the photoconductive drum by a transfer bias;
A voltage control unit for applying the transfer bias to the transfer portion,
The voltage control unit reduces the transfer bias during a period when the recording paper is not nipped between the photosensitive drum and the transfer unit to a predetermined value according to the amount of wear of the photosensitive layer , and An image forming apparatus having a polarity opposite to a transfer bias during a period in which a recording sheet is nipped between the photosensitive drum and the transfer unit . A photosensitive drum having a photosensitive layer formed on the surface;
A charging unit that applies a charging bias to the photosensitive drum to charge the surface of the photosensitive drum;
An exposure unit that exposes the surface of the photosensitive drum charged by the charging unit to form an electrostatic latent image; and
A developing unit that supplies toner to the surface of the photosensitive drum and generates a toner image from the electrostatic latent image formed by the exposure unit;
A transfer unit for transferring a toner image generated on the surface of the photoconductive drum by the developing unit to a recording sheet niped with the surface of the photoconductive drum by a transfer bias;
A separation unit for separating the recording paper onto which the toner image has been transferred in the transfer unit by a separation bias;
A voltage control unit that applies the transfer bias to the transfer unit and applies the separation bias to the separation unit;
The voltage control unit is configured such that when the surface of the photosensitive drum to which the transfer bias is applied passes through a region corresponding to the separation portion in a period in which the recording paper is not nipped between the photosensitive drum and the transfer portion. An image forming apparatus in which a separation bias to be applied is reduced to a predetermined value in accordance with a wear amount of the photosensitive layer.   3. The image formation according to claim 1, wherein the voltage control unit uses a predetermined value according to the cumulative number of printed sheets by the image forming apparatus as a predetermined value according to the wear amount of the photosensitive layer. apparatus.   3. The voltage control unit according to claim 1, wherein the voltage control unit uses a value that is predetermined according to an accumulated driving time of the photosensitive drum as a value that is predetermined according to the wear amount of the photosensitive layer. Image forming apparatus. The voltage control unit includes a transfer bias in a period in which the recording paper is nipped between the photosensitive drum and the transfer unit in a period in which the recording paper is not nipped between the photosensitive drum and the transfer unit. the image forming apparatus according to 請 Motomeko 2 you apply a reverse polarity transfer bias is. A storage unit for storing a table indicating a correspondence relationship between the abrasion amount of the photosensitive layer and the cumulative number of printed sheets;
A counter for counting the cumulative number of printed sheets in the image forming apparatus,
The said power supply control unit calculates the amount of wear of the said photosensitive layer using the said cumulative printing number counted by the said counter, and the said table memorize | stored in the said memory | storage part. Image forming apparatus.   The image forming apparatus according to claim 1, wherein the photosensitive layer is a single-layer organic photosensitive layer.