JP2007271798A - Image forming apparatus - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus obtaining a big preventive effect for image deterioration due to transfer bias voltage control by minimizing the image quality deterioration due to a difference in density in a transfer image. <P>SOLUTION: Secondary transfer bias voltage obtaining a specified transfer current is outputted to a secondary transfer outside roller 42 from a power source 14 immediately before recording paper P is proceeded to a nip in a secondary transfer part N2 in the image forming apparatus. A control part 15 retains a transfer voltage table defining the length in a back edge region for which voltage is lowered according to a type of the recording material and distinction between both sides printing and one side printing and the amount of lowered voltage. The control part 15 detects the type of the recording medium and the distinction between both sides printing and the one side printing set by a user, sets a point for starting lowering of the secondary transfer voltage and then a final lowered voltage amount is obtained at the back edge by gradually lowering the secondary transfer bias voltage. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to an image forming apparatus that transfers a toner image to a recording material using a transfer bias voltage, and more particularly to an image forming apparatus that lowers the transfer bias voltage in the trailing end region of the recording material.

  As an electrophotographic multi-color or full-color image forming apparatus, an intermediate transfer type image forming apparatus has been put into practical use. In the intermediate transfer method, the toner images of the respective colors formed on the photosensitive drum are sequentially primary-transferred and superimposed on the intermediate transfer belt, and a plurality of toner images carried on the intermediate transfer belt are collectively stacked on the recording material. Secondary transfer.

  Patent Document 1 discloses an image forming apparatus that forms a full-color image on a recording material using one photosensitive drum and one intermediate transfer belt. Here, a rotation exchange type developing device is used in which a plurality of developing devices having different development colors are exchanged and brought into contact with the photosensitive drum.

  Patent Document 2 discloses an image forming apparatus that forms a full-color image on a recording material using one photosensitive drum and one intermediate transfer belt. Here, a back surface printing path is shown in which the recording material having the toner image fixed on the front surface through the fixing device is switched back and conveyed to the secondary transfer portion again in the reverse state.

  By the way, when the toner image is secondarily transferred to the back surface of the recording material having the toner image fixed on the front surface, the recording material curled during the image formation (particularly fixing) on the surface may pass through the secondary transfer portion. is there. Since the curl is corrected by the guide structure that guides the recording material to the secondary transfer portion, the secondary transfer is normally performed in the front end region to the intermediate region of the curled recording material. However, in the rear end region after the rear end of the recording material passes through the guide structure, curling appears and the distance between the recording material and the intermediate transfer belt increases, preventing normal secondary transfer to the recording material. There is a possibility that. When the toner is scattered on the surface of the recording material at an enlarged distance, there is a possibility of causing image degradation called scattering. If a discharge occurs at an enlarged distance, the charging polarity of the toner image on the intermediate transfer belt is reversed, and the toner image cannot be attracted to the recording material using the secondary transfer bias voltage, which may cause image deterioration called blanking. There is.

  Japanese Patent Application Laid-Open No. 2003-228561 discloses a technique for preventing scattering and white spots by lowering the transfer bias voltage in the rear end region of the recording material by one step compared to the front end region to the intermediate region.

JP 2006-064748 A JP 2000-351534 A JP-A-7-334018

  In the control of the transfer bias voltage disclosed in Patent Document 3, since the transfer bias voltage is decreased stepwise at the rear end of the recording material, the transfer efficiency of the toner image is decreased and the image density is decreased in the rear end region. The difference in image density between the front end region to the intermediate region and the rear end region is not noticeable if the image is monochrome and halftone, but a gray scale causes a sense of incongruity, and in a full color image, the change in color tone is often noticeable.

  In the control of the transfer bias voltage disclosed in Patent Document 3, the transfer bias voltage in the rear end region is uniformly reduced for a curled recording material, a recording material that is likely to curl, and a recording material that is not curled at all. Therefore, a recording material that does not curl may cause unnecessary image deterioration due to unnecessary control.

  In addition, since the curl amount varies depending on the type of recording material, fixing conditions, the front / back surface of the recording material, the leaving condition of the recording material, and the like, a sufficient effect cannot be achieved by uniformly controlling the transfer bias voltage. This is because when the amount of decrease in the transfer bias voltage is insufficient, white spots and scattering occur, and a sufficient image deterioration prevention effect cannot be achieved, and when the amount of decrease is excessive, unnecessary image deterioration occurs.

  Also, depending on the recording material, the curl amount is small or the curl is generated, but the recording material with weak paper stiffness may cause a gap between the intermediate transfer belt and the recording material in the rear end region of the recording paper. There is no need to lower the transfer bias. Therefore, reducing the transfer bias at the trailing edge of the recording material with respect to these recording materials results in excessive reduction of the transfer bias, and further reducing the density at the trailing edge of the recording material for all recording materials. Cause problems.

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of minimizing image quality deterioration due to density difference of a transferred image and realizing a large image deterioration preventing effect by transfer bias voltage control.

  An image forming apparatus of the present invention includes an image carrier that carries a toner image, a transfer electrode unit that transfers the toner image from the image carrier to a recording material using a transfer bias voltage, and a variable for the transfer electrode unit. Power supply means for outputting the transfer bias voltage. Detecting means for detecting information relating to curling of the trailing edge of the recording material passing through the transfer electrode means; and when the information on occurrence of the curling is detected, controlling the power supply means to Control means for reducing the transfer bias voltage in the end region.

  In the image forming apparatus of the present invention, when curling occurs in the rear end region of the recording material to which the toner image is transferred, the transfer bias voltage in the rear end region is applied from the front end of the recording material to the rear end region. Lower than the transfer bias voltage.

  Therefore, it is possible to cope with the situation more finely than the control of Patent Document 3 in which the transfer bias voltage is uniformly reduced in the rear end region. When no curl has occurred, control that does not reduce the transfer bias voltage can be automatically set. As a result, it is possible to avoid a situation in which the image density in the edge region is lowered after the meaningless transfer bias voltage drop control is executed when no curling occurs.

  The curl state in the rear end region of the recording material is identified from the information detected by the detection means. It is possible to identify information and select or adjust control contents. It is also possible to finely classify the curl occurrence status by combining various types of information, and to deal with a detailed response for each classification.

  Hereinafter, a color copying machine as an embodiment of an image forming apparatus of the present invention will be described in detail with reference to the drawings. The image forming apparatus of the present invention is not limited to the limited configuration of the embodiments described below. As long as the toner image on the image carrier is transferred to the recording material using the transfer bias voltage, another embodiment in which a part or all of the configuration of the embodiment is replaced with the alternative configuration can be realized.

  Note that some of the general structure, material, control, operation method, and the like of the image forming apparatus disclosed in Patent Documents 1 to 3 are not shown, and detailed descriptions are also omitted.

<First Embodiment>
1 is a cross-sectional view illustrating a schematic configuration of the image forming apparatus according to the first embodiment, FIG. 2 is an enlarged view illustrating a configuration around a photosensitive drum, and FIG. 3 is an enlarged view illustrating a configuration around a secondary transfer unit. It is. 4 is an explanatory diagram of a gap between the intermediate transfer belt and the recording material in the secondary transfer portion, FIG. 5 is a diagram of control of the secondary transfer bias voltage when passing through the recording material, and FIG. 6 is a setting of the secondary transfer bias voltage. It is explanatory drawing of a procedure. FIG. 7 is a flowchart of secondary transfer bias voltage control.

  As shown in FIG. 1, the image forming apparatus 100 forms a toner image on the photosensitive drum 1 by an electrophotographic process, and secondarily transfers the toner image primarily transferred to the intermediate transfer belt 81 to the recording material P. In the main body of the image forming apparatus 100, an endless intermediate transfer belt 81 that runs in the direction of the arrow X is disposed. The intermediate transfer belt 81 is made of a dielectric resin such as polycarbonate, polyethylene terephthalate resin film, polyvinylidene fluoride resin film, polyimide, ethylene tetrafluoroethylene copolymer, or the like. The intermediate transfer belt 81 can employ other materials, volume resistivity, and thickness, and may employ an intermediate transfer belt having an elastic layer as a surface layer. Since an intermediate transfer belt having an elastic layer on the surface layer has a large influence on the image due to curling, in the first embodiment, an intermediate transfer belt having an elastic layer on the surface layer is used.

  The control unit 15 is a normal computer control device that has a calculation function and is program-controlled, and comprehensively controls each unit of the image forming apparatus 100 to form a full-color image on the recording material P.

  As shown in FIG. 2, a photosensitive drum 1 is a cylindrical drum-shaped electrophotographic photosensitive member having a conductive base 1b made of aluminum or the like and a photoconductive layer 1a formed on the outer periphery thereof as a basic configuration. Is the body. The photosensitive drum 1 has a support shaft 1c at the center, and the photosensitive drum 1 is rotationally driven by a driving means (not shown) in the direction of arrow R1 around the support shaft 1c. Around the photosensitive drum 1a, process devices such as a charging roller 2 serving as a primary charger, a developing device 4 that performs development with toner of four colors, a cleaning device 7, and an exposure device 3 are disposed.

  The charging roller 2 is arranged in parallel to the photosensitive drum 1 and is configured to be a roller that rotates in contact with the surface of the photosensitive drum 1, and the surface of the photosensitive drum 1 is uniformly at a predetermined polarity and potential. Charge uniformly.

  The charging roller 2 includes a conductive roller (core metal) 2c disposed in the center and a conductive layer 2b formed on the outer periphery thereof, and both ends of the core metal 2c are rotatably supported by a bearing member (not shown). Is done. The bearing members at both ends are urged toward the photosensitive drum 1 by pressing means (not shown), whereby the charging roller 2 is pressed against the surface of the photosensitive drum 1 with a predetermined pressing force. Yes.

  The charging roller 2 is driven to rotate in the direction of arrow R2 as the photosensitive drum 1 rotates in the direction of arrow R1. The cored bar 2 c of the charging roller 2 is in contact with an electrical contact connected to the power source 10. The charging roller 2 is applied with a primary charging bias voltage by the power source 10, and thereby uniformly and uniformly charges the surface of the photosensitive drum 1.

  The developing device 4 arranged on the downstream side of the exposure device 3 rotates 45 degrees in the direction of the arrow R4 around the rotation shaft 4e, and the developing devices 4a, 4b, 4c, and 4d are sequentially arranged on the photosensitive drum 1. Move to the opposite position. By this operation, separated color toner images having different development colors are sequentially formed on the photosensitive drum 1. The difference between the developing devices 4a, 4b, 4c, and 4d is that each of them accommodates yellow, magenta, cyan, and black toners to form yellow, magenta, cyan, and black separated color toner images. .

  Since the developing devices 4a, 4b, 4c, and 4d are configured in the same manner except that the stored toner is different, the developing device 4a will be described as a representative. The developing device 4 a includes a developing container 116 that contains a two-component developer 119, and a developing sleeve 111 is rotatably installed in an opening of the developing container 116 facing the photosensitive drum 1. In the developing sleeve 111, a magnet roller 112 that carries the developer on the developing sleeve 111 is fixedly disposed so as not to rotate with respect to the rotation of the developing sleeve 111.

  A regulating blade 115 that regulates the developer carried on the developing sleeve 111 to form a thin developer layer is installed above the developing sleeve 111 of the developing container 116. In the developing container 116, a developing chamber C1 and a stirring chamber C2 defined by a partition wall 117 are provided in a substantially lower half portion thereof. The developer 119 is mainly composed of toner and a carrier that is a magnetic material, and the toner is negatively charged and the carrier is positively charged.

  First, as the developing sleeve 111 rotates, the developer in the developing chamber C <b> 1 is pumped up by the magnetic pole of the magnet roller 112 and is carried on the developing sleeve 111. The carried developer is conveyed by the rotation of the magnet roller 112, and the toner is negatively charged in the conveyance process up to that time. The developer is regulated by a regulating blade 115 arranged perpendicular to the developing sleeve 111 and formed into a thin developer layer.

  When the developer formed in the thin developer layer is transported to the development area facing the photosensitive drum 1, it is spiked by the magnetic force of the development main pole located in the development area of the magnet roller 112, A developer magnetic brush is formed. A developing bias voltage is output from the power source 121 to the developing sleeve 111 while rubbing the surface of the photosensitive drum 1 with this magnetic brush.

  As a result, the toner adhering to the carrier constituting the ears of the magnetic brush adheres to the visible portion of the electrostatic latent image (exposure pattern by laser light) and develops, and a toner image is formed on the photosensitive drum 1. The Below the photosensitive drum 1, a primary and primary transfer roller 6, which is a roller-shaped transfer device, is disposed.

The primary transfer roller 6 includes a conductive roller shaft 6a connected to a power source 11 and a conductive layer 6b formed in a cylindrical shape on the outer peripheral surface thereof. The conductive layer 6b of the primary transfer roller 6 has a resistance value of about 10 ⁵ to 10 ⁸ Ω · cm, and single bubble or continuous bubble EPDM, SBR, BR, and the like are preferable. Both ends of the primary transfer roller 6 are urged toward the photosensitive drum 1 by a pressing member such as a spring (not shown). Thereby, the conductive layer 6b of the primary transfer roller 6 is pressed against the photosensitive drum 1 side with a predetermined pressing force so as to sandwich the intermediate transfer belt 81, and a nip of the primary transfer portion N1 is formed.

  An image signal based on the yellow component color of the document is projected through a polygon mirror (not shown) of the exposure device 3 to form an electrostatic latent image on the photosensitive drum 1. As the photosensitive drum 1 rotates, yellow toner is supplied from the developing device 4a to the electrostatic latent image, and the electrostatic latent image is developed into a yellow toner image. The yellow toner image arrives at the transfer portion N where the photosensitive drum 1 and the intermediate transfer belt 81 come into contact with the rotation of the photosensitive drum 1. In the transfer portion N, the yellow toner image is primarily transferred to the intermediate transfer belt 81 by the primary transfer bias voltage applied to the primary transfer roller 6.

  A magenta toner image is prepared on the photosensitive drum 1 until the intermediate transfer belt 81 carrying the yellow toner image is circulated once and conveyed to the primary transfer portion N1 again. The developing device 4 is rotated by 45 ° in the direction of the arrow R4 about the rotation shaft 4e, and the developing device 4b is moved to the photosensitive drum facing position. The magenta toner image formed on the photosensitive drum 1 in the same procedure as the yellow toner image is primarily transferred to be superimposed on the yellow toner image on the intermediate transfer belt 81.

  Thereafter, similarly, a cyan toner image and a black toner image are primarily transferred to the toner image on the intermediate transfer belt 81, and the four-color toner images thus superimposed are transferred to the secondary transfer portion N2 shown in FIG. Be transported. By this time, the recording paper P taken out from the paper feed cassette 60 has reached the secondary transfer portion N2, and the toner images of four colors are recorded on the recording material P by the secondary transfer bias voltage applied to the secondary transfer device 40. Are collectively transferred.

As shown in FIG. 3, the secondary transfer device 40 includes a secondary transfer inner roller 41 positioned inside the intermediate transfer belt 81 and a secondary transfer outer roller 42 positioned outside. The outer secondary transfer roller 42 has a diameter of 24 mm, and is formed of a conductive shaft 42a and a conductive layer 42b covering the surface thereof. The conductive layer 42b of the secondary transfer outer roller 42 has a resistance value of about 10 ⁵ to 10 ⁷ Ω · cm, and is preferably solid or foaming EPDM, SBR, BR or the like. The secondary transfer inner roller 41 is a conductive roller, and its diameter is preferably 21 mm, and the material is preferably SUS, Al or the like.

  The control unit 15 controls the power supply 14 to adjust the secondary transfer bias voltage applied to the secondary transfer outer roller 42. The power supply 14 can output a variable high voltage and includes an output current detection circuit. The control unit 15 controls the power supply 14 before the recording material P reaches the secondary transfer unit N2, and obtains a constant voltage as a secondary transfer bias voltage so that the output current becomes a necessary transfer current. Then, the recording material P is guided to the nip of the secondary transfer portion N2 to which this constant voltage is applied.

  In the first embodiment, the secondary transfer inner roller 41 is set to the ground potential, and a positive secondary transfer bias voltage is applied to the secondary transfer outer roller 42. However, the toner image is secondarily transferred from the intermediate transfer belt 81 to the recording material P by applying a negative secondary transfer bias voltage to the secondary transfer inner roller 41 with the secondary transfer outer roller 42 as a ground potential. Also good.

  The toner image on the intermediate transfer belt 81 is transferred to the recording paper P that passes through the secondary transfer portion N2. The recording material P onto which the toner image has been secondarily transferred is conveyed to the fixing device 9 shown in FIG. 1, and the toner image is fixed on the surface. The fixing device 9 includes a fixing roller 9 a and a pressure roller 9 b that sandwich and convey the transfer material P and fix the developer on the transfer paper P.

  After the recording material P passes through the fixing device 9, if single-side printing is designated by the user, the recording material P is discharged as it is to a paper discharge tray (not shown) arranged downstream of the fixing device 9. However, if double-sided printing has been designated by the user, the recording material P on which the toner image has been fixed by the fixing device 9 is drawn into the reverse conveyance path 31 and switched back, and is again subjected to the secondary transfer in the reverse state. Enter device 40.

  In the reverse conveyance path 31, a reverse paper feed roller (not shown) rotates reversely, and the recording material P is fed reversely via a reverse paper discharge flapper (not shown). Thereafter, the recording material P waits at a predetermined position in the vicinity of the registration roller pair 28 serving as a refeeding unit, and the recording material P is conveyed to the secondary transfer portion N2 in a direction in which the back surface is in contact with the intermediate transfer belt 81.

  In the first embodiment, in the secondary transfer of the recording material P that has been curled due to the leaving condition or the recording material P (back surface) that has been curled through the fixing device 9, the secondary transfer bias is applied in the rear end region of the recording material P. Reduce voltage. The control unit 15 continuously changes the secondary transfer bias voltage from the predetermined position of the recording material P through the nip of the secondary transfer unit N2 to the rear end, that is, in the rear end region. This prevents secondary transfer failure caused by a discharge phenomenon that occurs in the gap formed between the recording material P and the intermediate transfer belt 81.

  The control unit 15 determines whether or not to change the secondary transfer bias voltage from information such as the type of the recording material P, the leaving condition of the recording material P, and the front / back surface. When the secondary transfer bias voltage is changed, a reduction amount for reducing the secondary transfer bias voltage and a timing for starting the reduction are determined, and a finer response than the control disclosed in Patent Document 3 is performed.

<Secondary transfer bias voltage control>
The characteristic part of 1st Embodiment is demonstrated in detail below. The curl amount of the recording material P and the transfer failure at the secondary transfer portion N2 caused by the curl differ depending on the state of the recording material P and the type of the recording material P.

  For example, the curl amount does not change between 80 g paper and 209 g paper by basis weight, but there is a difference in the white spot phenomenon occurring in the rear end region of the recording material P. There is no white spot phenomenon in the rear end region of the recording material P with 80g paper, but white spot phenomenon occurs with 209g paper.

  Even with the recording material P having the same basis weight, the curl amount varies depending on the type of the recording material P, the difference between the plain paper and the coated paper, the surface property of the recording material P, and the like. In the recording paper A having a basis weight of 209 g, the curl amount is 5 mm and the white spot phenomenon in the rear end region occurs up to a position of 8 mm from the rear end. However, in the recording paper B having the same basis weight of 209 g, the curl amount is 3 mm and the white spot phenomenon in the rear end region occurs up to a position of 6 mm from the rear end.

  Such a difference is determined by the amount of gap between the recording material P and the intermediate transfer belt 81 before entering the nip of the secondary transfer portion N2. FIG. 4 shows a gap between the recording material P and the intermediate transfer belt 81 in the process in which the recording material P passes through the secondary transfer portion N2.

  As shown in FIG. 4A, the recording material P passing through the secondary transfer portion N2 is regulated by the pre-transfer guide 45 while being sandwiched by the secondary transfer portion N2. After the trailing end of the recording material P passes through the pre-transfer guide 45, the recording material P enters the nip of the secondary transfer portion N2 along the intermediate transfer belt 81. However, when the recording material P is curled, as shown in FIG. 4B, a length L1, a height between the nip of the secondary transfer portion N2 and the rear end region of the recording material P is obtained. A gap of H1 is generated.

  Depending on the combination of the length L1 and the height H1 of the gap, the occurrence of the white spot phenomenon varies. In the recording paper A having a basis weight of 209 g, the gap length L1 was about 10 mm and the height H1 was about 1 mm. In the recording paper B having a basis weight of 209 g, the gap length L1 was about 7 mm and the height H1 was about 2 mm.

  Further, in the paper having a basis weight of 80 g, even if curl is generated, the paper is not strong and no gap is generated. The paper having a grammage of 80 g enters the nip of the secondary transfer portion N2 along the intermediate transfer belt 81 even after passing through the pre-transfer guide 45 while being sandwiched between the nip of the secondary transfer portion N2.

  Note that the curling of the recording material P in the reverse side printing varies depending on the type of the fixing device 9, the fixing temperature, and the fixing speed. The white spot phenomenon occurring in the rear end region of the recording material P is caused by the surface resistance of the intermediate transfer belt 81, the nip direction (sheet passing angle) of the secondary transfer portion N2, and the speed of the recording material P passing through the secondary transfer portion N2. It also differs depending on the voltage value of the secondary transfer bias voltage. Therefore, the conditions of these phenomena are not uniformly defined. Even when the curl of the recording material P is in the opposite direction as shown in FIG. 4B, that is, when curled upward, the gap between the intermediate transfer belt 81 and the recording material P is enlarged, so that the secondary transfer bias voltage is lowered. Is effective. However, since the behavior of the rear end portion is different between the case of convex upward and the case of convex downward, the voltage control pattern applied to each may be changed.

  Next, control of the secondary transfer bias voltage in the secondary transfer portion N2 will be described using the time chart of FIG. The horizontal axis of the time chart indicates time t, and the vertical axis indicates the secondary transfer bias voltage applied to the secondary transfer portion N2.

  As shown in FIG. 5, time t1 indicates the time when the leading edge of the recording material P reaches the secondary transfer portion N2. The control unit 15 starts to output the secondary transfer bias voltage from the power source 14 to the secondary transfer outer roller 42 at time t1 when a margin is given when the recording material P reaches the secondary transfer unit N2. As shown in Patent Document 3, the secondary transfer bias voltage at that time is controlled so that the power supply 14 is controlled at a constant current to detect the resistance of the secondary transfer portion N2, and the constant transfer voltage is obtained so as to obtain a predetermined transfer current. Determined by calculation.

  Resistance detection of the secondary transfer portion N2 is performed by detecting a voltage value V1 (V) when a transfer current I1 (μA) necessary for secondary transfer is applied, as shown in a diagram 6a of FIG. It can be used at the next transfer. However, the resistance detection of the secondary transfer portion N2 is performed before the recording material P1 is conveyed to the secondary transfer portion N2. Therefore, a voltage obtained by adding a voltage corresponding to the resistance value of the recording material P to the detected voltage value V1 (V) is applied as the secondary transfer bias voltage.

  When the resistance of the secondary transfer portion N2 is detected, a transfer current I3 (μA) as shown in the diagram 6a of FIG. 6 is applied to the current I1, and the transfer current of the secondary transfer portion N2 at the time of resistance detection. And the secondary transfer bias voltage are detected. When the secondary transfer bias voltage is lowered in the rear end region of the recording material P, the secondary transfer bias voltage V2 (V) with respect to the target transfer current I2 (μA) is detected.

  The final target current I2 when the secondary transfer bias voltage is lowered in the rear end region of the recording material P is determined by the transfer efficiency and image quality of the selected recording material P as shown in the diagram 6b. Usually, regardless of the selected recording material P, the target current I1 is set in correspondence with the transfer efficiency peak E1. However, as described above, when the secondary transfer bias voltage V1 with respect to the target transfer current I1 is applied, in the curled recording material P, a white spot phenomenon occurs in the rear end region of the recording material P. However, when the secondary transfer bias voltage is lowered in the rear end region of the recording material P, if the transfer efficiency becomes equal to or lower than E3 shown in the diagram 6b, the image quality after the transfer is extremely deteriorated, and unevenness is reduced. It becomes a lot of images.

  Accordingly, the transfer current I2 is selected so that the whitening phenomenon in the rear end region of the recording material P does not occur and the transfer efficiency E2 does not cause unevenness. As a result, although there is a slight decrease in transfer efficiency, unevenness of the image is not noticeable in the rear end region and the other regions, and an image without white spots in the rear end region is obtained.

  As shown in FIG. 5, the control unit 15 sets the secondary transfer bias voltage V2 according to the curl degree of the recording material P with respect to the secondary transfer bias voltage V1 applied from the leading end of the recording material P1. The controller 15 gradually decreases the secondary transfer bias voltage V when the leading end of the recording material P enters the secondary transfer portion N2 and time t2 has elapsed. Then, at time t3 when the recording material P passes through the secondary transfer portion N2, it is lowered to the secondary transfer bias voltage V2.

  The timing t2 at which the secondary transfer bias voltage starts to drop can be changed depending on the recording material P. The final secondary transfer bias voltage V2 can also be switched depending on the transferability of the recording material P. That is, when the length L1 of the gap of the recording material P is large as shown in FIG. 4B, the lowering control VP2 that accelerates the start of the reduction is selected as shown in FIG. However, when the gap length L1 is small and the height H is large, the descent control VP1 is selected.

  Hereinafter, the control flow of the first embodiment will be described with reference to the flowchart of FIG.

  A user inputs information through the operation unit of the image forming apparatus 100. Before starting copying, the user inputs paper type information and basis weight information to the image forming apparatus 100 (S11).

  Thereafter, the user presses the copy button (S12). Then, the resistance detection of the secondary transfer portion N2 and the information of the internal environment at that time are taken into the control portion 15 (S13). Then, the secondary transfer bias voltage applied on the first surface is determined based on the detection result detected by the resistance detection of the secondary transfer portion N2 (S14). Immediately, the actual secondary transfer of the first surface is executed (S15).

  Next, when the recording paper designated by the user is single-sided (YES in S16), the copying ends (S21). However, in the case of both sides (NO in S16), the secondary transfer bias voltage applied on the second side is determined (S17 to S20).

  At this time, if the designated recording material P corresponds to a paper type in which a whiteout phenomenon occurs in the trailing edge region due to curling (YES in S17), the secondary transfer bias voltage is selected with reference to the transfer voltage table. (S18). The transfer voltage table defines a transfer voltage corresponding to the amount of curl generated depending on the type of the recording material P, and is particularly applied to the second side (back side printing). The first side is not affected by curling due to fixing, so there is no need to select it.

  However, the recording material P left in the paper feed cassette 60 may be curled. Therefore, a mechanism capable of setting control for lowering the secondary transfer bias voltage on the first surface (front surface) depending on the standing time and a mechanism that can be arbitrarily set by the user are incorporated. As a result, if the control for reducing the secondary transfer bias voltage can be set on the first surface, the effect is higher.

  Next, the recording material P stands by at a predetermined position near the registration roller pair 28 until the four color toner images are superimposed on the intermediate transfer belt 81. The recording material P is conveyed to the nip of the secondary transfer portion N2 so as to coincide with the timing of the four color toner images on the intermediate transfer belt 81. A secondary transfer bias voltage having a lowered pattern is applied to the secondary transfer outer roller 42 (S19), and the secondary transfer of the second surface is completed (S20).

  The recording paper on which the transfer of the first side and the second side has been completed is conveyed to the fixing unit, and the recording sheet on which the toner image has been fixed is conveyed onto the paper discharge tray, and the process ends (S21).

  As described above, in the first embodiment, the secondary transfer bias voltage from the predetermined position of the recording material P1 passing through the nip of the secondary transfer portion N2 to the rear end is gradually reduced. It is determined whether or not the secondary transfer bias voltage is to be changed from information such as the type of the recording material P, the leaving condition of the recording material P, and the first and second surfaces. From these pieces of information, a change pattern of the decrease, that is, a change amount for changing the secondary transfer bias voltage and a change timing are determined.

  As a result, the problem at the time of transfer of the recording material P that has been curled according to the leaving condition or the second surface (back surface) of the recording material P that has been curled by passing through the fixing device 9 is solved. It is possible to prevent a phenomenon that occurs due to a discharge phenomenon that occurs in a gap formed between the recording material P and the intermediate transfer belt 81. Further, even if the secondary transfer bias voltage is lowered at the trailing edge of the curled cardboard recording paper, an image with less unevenness can be obtained.

  In addition, the type of the recording material P that changes the secondary transfer bias voltage in the rear end region of the recording material P is not limited to the execution of the recording material P that is recorded in advance in the transfer voltage table. The recording material P that is not recorded in the transfer voltage table can be arbitrarily registered by the user as long as the recording material P needs to change the secondary transfer bias voltage in the rear end region of the recording material P. By providing such a mechanism, a wider range of recording materials P can be handled.

Second Embodiment
In the second embodiment, with the configuration shown in FIGS. 1 to 3, as in the first embodiment, the secondary from the predetermined position of the recording material P after passing through the nip of the secondary transfer portion N <b> 2 to the rear end. Reduce the transfer bias voltage. As in the first embodiment, whether or not to change the transfer bias is determined based on information such as the type of the recording material P, the leaving condition of the recording material P, and the first and second surfaces. In the case of changing, the change amount and change timing for changing the transfer bias are determined based on these pieces of information. This prevents a white spot phenomenon and transfer unevenness that occur at the rear end portion of the thick paper that has been curled once passing through the fixing device.

  In the second embodiment, the developing device 4 is controlled in association with such control for lowering the secondary transfer bias voltage, and the toner loading amount in the secondary transfer is also adjusted. That is, when the recording material P in which the trailing edge white spot phenomenon occurs due to curling is selected, the control unit 15 controls the developing device 4 in the direction in which the toner loading amount decreases over the entire image. As a result, as shown in the diagram 6b of FIG. 6, the amount of toner that is secondarily transferred over the entire image is reduced, and the required transfer current can be reduced. Thereby, the transfer efficiency difference with respect to the current difference between the transfer current I2 and the transfer current I1 can be further reduced. That is, even if the transfer current is decreased from I1 to I2 in order to prevent the white spot phenomenon in the rear end region of the recording material P, the transfer efficiency does not decrease so much and the transfer image of the high duty image is not impaired. Thus, an image with less unevenness than that of the first embodiment can be obtained.

  As shown in FIGS. 1 to 3, the image forming apparatus according to the second embodiment includes a photosensitive drum 1, and a primary transfer unit N <b> 1 that transfers a toner image formed on the photosensitive drum 1 to an intermediate transfer belt 81. Is provided. A secondary transfer portion N2 for secondary transfer of the toner image on the intermediate transfer belt 81 by applying a secondary transfer bias voltage to the recording material P, and the toner image secondarily transferred to the recording material P on the recording material P And a fixing device 9 for permanent fixing. The voltage value of the secondary transfer bias voltage applied to the secondary transfer portion N2 can be changed depending on the position of the recording material P passing through the secondary transfer portion N2.

  The controller 15 determines whether or not to change the secondary transfer bias voltage depending on the type and state of the recording material P. When changing the secondary transfer bias voltage, the change amount and change timing are determined according to the type or state of the recording material P and the secondary transfer bias voltage applied to the recording material P until the change. Control. This prevents a phenomenon that occurs due to a discharge phenomenon that occurs in the gap generated between the recording material P and the intermediate transfer belt 81 during the transfer (back surface printing) of the second surface of the recording material P in which curling has occurred. Even if the transfer bias is lowered in the trailing edge region of the curled cardboard recording paper, an image with less unevenness can be obtained.

<Image Forming Apparatus of Comparative Example>
An electrophotographic multi-color or full-color image forming apparatus has been put into practical use. In recent years, an intermediate transfer type image forming apparatus that forms a color image by sequentially superimposing toner images of respective colors formed on a photosensitive drum on an intermediate transfer body (intermediate transfer belt), and collectively transferring the image onto a recording sheet. Has been proposed. In the intermediate transfer method, a toner image is formed on the photosensitive drum by a charging unit, an exposure device, and a developing unit arranged around the photosensitive drum, and electrostatically transferred to the intermediate transfer belt by the transfer unit in the primary transfer unit. .

  In the case of forming a color image, a full color image is formed on the intermediate transfer belt by sequentially transferring the toner images to the intermediate transfer belt. The toner image transferred to the intermediate transfer belt is conveyed to the secondary transfer portion by the rotation of the intermediate transfer belt, and is electrostatically transferred to the recording material. The toner image secondarily transferred onto the recording material is conveyed to a fixing unit and permanently fixed on the surface of the recording material P.

  Here, when transfer is performed on both surfaces of the recording material, the toner image is transferred to the first surface and then passed through a fixing device to fix the toner image on the recording material. Thereafter, the recording material is turned upside down, the second side is conveyed to the secondary transfer unit with the second side as the front, the toner image is transferred to the second side, and the second side is permanently fixed by the fixing device.

  At this time, in particular, the recording material once passed through the fixing device may cause a curl phenomenon depending on the type, humidity, fixing conditions, and image ratio of the first surface. When an image is to be formed on the second surface of such a recording material, as shown in FIG. 4B, at the secondary transfer portion and before the trailing edge of the recording paper enters the secondary transfer nip. In some cases, a gap may be formed between the intermediate transfer belt and the recording material.

  When the recording material is transported to the secondary transfer section with the gap between the recording material and the intermediate transfer belt as described above, the secondary transfer bias voltage is applied to discharge between the intermediate transfer belt and the recording material. A phenomenon occurs. As a result, an image defect occurs in which the toner whose charging polarity is negative-positive-inverted by the discharge is not transferred to the recording material and the image is blank. With respect to this phenomenon, Patent Document 3 describes that the transfer bias voltage is lowered in the rear end region of the recording material to prevent image defects.

  However, in the control of Patent Document 3, since the secondary transfer bias voltage is reduced in the rear end region of the recording material, the transfer efficiency is lowered, and the image density in the rear end region is lighter than that in the region up to the rear end region. Become. In response to this problem, Japanese Patent Application Laid-Open No. H10-228561 describes that “the voltage applied to the transfer means is set to a level within a range in which transferable image scattering phenomenon can be alleviated and transfer efficiency without practical problems can be obtained”. It is said. However, in recent years, with the spread of full-color copiers, the amount of toner transferred to the recording material has increased, so that when transferring a low-density toner image, the reduction in transfer efficiency is not noticeable. However, when transferring a high density toner image, depending on the amount of transfer bias to be changed, the image density changes extremely due to a decrease in transfer efficiency.

  For such a problem, Patent Document 3 describes a method of reducing the transfer bias voltage all at once to a predetermined voltage. However, the amount of curl varies depending on the type of recording material, fixing conditions, the first and second surfaces of the recording material, the recording material leaving condition, and the resistance value also varies depending on the type of recording material. For this reason, depending on the type of the recording material, the amount of decrease in the transfer bias voltage is insufficient, and white spots may occur due to a discharge phenomenon between the intermediate transfer belt and the recording material. In addition, the transfer bias may be excessively lowered to cause unevenness in the transferred image, or image defects such as transfer loss may occur along the formation of the recording material.

  Furthermore, depending on the recording material, the curl amount is small or curl occurs, but the recording material with weak paper stiffness does not cause a gap between the intermediate transfer belt and the recording material at the trailing edge of the recording paper. There is no need to lower the transfer bias. Therefore, reducing the transfer bias at the trailing edge of the recording material with respect to these recording materials results in excessively lowering the transfer bias, and further reducing the density at the trailing edge of the recording material for all recording materials. It will cause problems.

<Correspondence with Invention>
The image forming apparatus 100 of the first embodiment includes an intermediate transfer belt 81 that carries a toner image, a secondary transfer outer roller 42 that transfers the toner image from the intermediate transfer belt 81 to the recording material P using a transfer bias voltage, And a power supply 14 that outputs a variable transfer bias voltage to the secondary transfer outer roller 42. A control unit 15 that detects information related to curling of the trailing end region of the recording material P that passes through the secondary transfer outer roller 42, and controls the power source 14 when information on occurrence of curling is detected, And a controller 15 for reducing the transfer bias voltage in the end region.

  When the curl is generated in the rear end region of the recording material P to which the toner image is transferred, the image forming apparatus 100 applies the transfer bias voltage in the rear end region from the front end of the recording material P to the rear end region. Lower than the transfer bias voltage.

  Therefore, it is possible to cope with the situation more finely than the control of Patent Document 3 in which the transfer bias voltage is uniformly reduced in the rear end region. When no curl has occurred, control that does not reduce the transfer bias voltage can be automatically set. As a result, it is possible to execute a meaningless transfer bias voltage lowering control when no curling occurs and avoid a situation in which the image density in the rear end region is lowered.

  The curl state in the rear end region of the recording material P is identified from information detected by the control unit 15. It is possible to identify information and select or adjust control contents. It is also possible to finely classify the curl occurrence status by combining various types of information, and to deal with a detailed response for each classification.

  Note that the information detected by the control unit 15 may be replaced with output information of a sensor that directly detects the curl amount in the trailing edge region of the recording material P. Such a sensor may be arranged in the conveyance path of the recording material P reaching the secondary transfer portion N2, and directly detects the rear end region of the recording material P entering the nip of the secondary transfer portion N2 in real time. It may be reflected in the secondary transfer bias voltage. Instead of relying on input information through the operation unit, the ease of occurrence of curling may be identified by the output information of a sensor that directly detects the moisture content and paper quality of the recording material P and the paper quality difference between the front and back sides. In any case, it is possible to respond more precisely to the situation than the control of Patent Document 3 in which the recording material is handled uniformly regardless of the presence or absence of curling.

  The control unit 15 in the image forming apparatus 100 reduces the transfer bias voltage in the process in which the recording material P passes through the secondary transfer outer roller 42 when the transfer bias voltage in the rear end region of the recording material is decreased. Both the maximum reduction amount of the transfer bias voltage in the process in which the recording material P passes through the secondary transfer outer roller 42 are adjusted based on the detected information.

  The control unit 15 in the image forming apparatus 100 continuously changes the transfer bias voltage from the beginning to the rear end of the rear end region where the transfer bias voltage is decreased. As a result, compared to Patent Document 3 where the transfer bias voltage is changed in stages, it is difficult to make a drop in image density with the position where the transfer bias voltage is changed as a boundary. Even if the image density changes due to a decrease in transfer efficiency, the density change is continuous and unnoticeable.

  The image forming apparatus of the second embodiment includes a photosensitive drum 1 on which an electrostatic latent image is formed on the surface by optical writing, a developing device 4 that attaches toner to the electrostatic latent image and develops the toner image. Is provided. The control unit 15 controls the developing device 4 to reduce the amount of toner attached to the entire electrostatic latent image when the amount of decrease in the transfer bias voltage is large than when the amount of decrease is small. As a result, the necessary transfer current is reduced over the entire image, so that the transfer efficiency does not decrease much even in the rear end area where the transfer current has decreased, and the image density between the portion before the rear end area and the rear end area is reduced. The difference is not noticeable.

  The image forming apparatus 100 includes a fixing device 9 that fixes the toner image by heating and pressing the recording material P to which the toner image is transferred. The information is the type of the recording material P and the distinction between double-sided printing and not. The control unit 15 has a transfer voltage table in which the control content of the secondary transfer bias voltage is recorded according to the type and the distinction. The control unit 15 identifies the combination of the type input by the operation and the distinction, and executes the control content selected from the transfer voltage table.

  The image forming apparatus 100 heats and presses the recording material P to which the toner image has been transferred to fix the toner image, and conveys the recording material P fixed by the fixing device 9 to the secondary transfer outer roller 42 again. And a reverse conveyance path 31 to be provided. The information is the number of times the recording material P has passed through the fixing device 9.

  The information in the image forming apparatus 100 can be the leaving environment and the leaving time of the recording material P. If left in a high-temperature and high-humidity environment, the amount of water on the front and back surfaces may change and curl.

  The information in the image forming apparatus 100 can be the basis weight and stiffness of the recording material P. As described above, the basis weight and the rigidity are related to the curl depth and the difficulty of returning.

  The information in the image forming apparatus 100 can be the paper feeding direction of the recording material P. In the recording material P having a directional structure, there are a paper feeding direction in which curling is likely to occur and a paper feeding direction in which curling is difficult to occur. In the paper passing direction where curling is unlikely to occur, curling does not occur, so there is no need to lower the bias voltage.

  The control unit 15 in the image forming apparatus 100 reduces the transfer bias voltage depending on the conveyance distance or the conveyance time after the leading edge of the recording material P is detected by a sensor (not shown) arranged on the conveyance path of the recording material P. Is controlling.

  The control unit 15 in the image forming apparatus 100 controls the power supply 14 to detect the transfer bias voltage corresponding to the necessary transfer current and apply the set transfer bias voltage to the recording material P until the time.

  The image forming apparatus 100 includes a photosensitive drum 1 on which an electrostatic latent image is formed on the surface by optical writing. The image carrier is an endless intermediate transfer belt 81 in which a toner image is primarily transferred from the photosensitive drum 1 and circulated. The transfer electrode means is a secondary transfer outer roller 42 that contacts the intermediate transfer belt 81 and forms a secondary transfer nip to the recording material P.

  The image forming apparatus of the second embodiment includes a photosensitive drum 1 on which an electrostatic latent image is formed on the surface by optical writing, a developing device 4 that attaches toner to the electrostatic latent image and develops the toner image. An endless intermediate transfer belt 81 in which a toner image is primarily transferred from the photosensitive drum 1 and circulated, and a toner image primarily transferred to the intermediate transfer belt 81 by applying a secondary transfer bias voltage to the recording material P. And a secondary transfer outer roller 42 for next transfer. The control unit 15 reduces the secondary transfer bias voltage in the rear end region of the recording material P. The control unit 15 increases the density of the toner image in the region corresponding to the rear end region of the photosensitive drum 1 at a rate commensurate with the decrease in transfer efficiency in the rear end region of the recording material P. This reduces the difference in image density between the area where the transfer bias voltage is lowered and the area where the transfer bias voltage is not lowered.

1 is a cross-sectional view illustrating a schematic configuration of an image forming apparatus according to a first embodiment. FIG. 2 is an enlarged view for explaining a configuration around a photosensitive drum. It is an enlarged view for explaining the configuration around the secondary transfer portion. FIG. 6 is an explanatory diagram of a gap between an intermediate transfer belt and a recording material in a secondary transfer unit. FIG. 5 is a diagram of control of a secondary transfer bias voltage when passing through a recording material. It is explanatory drawing of the setting procedure of a secondary transfer bias voltage. It is a flowchart of secondary transfer bias voltage control.

Explanation of symbols

1 Image forming body (photosensitive drum)
2 Primary charging roller 3 Exposure device 4 Development device 6 Primary transfer roller 9 Fixing device 14 Power supply means (power supply)
15 Control means (control part)
31 Transport means (reverse transport path)
40 Secondary transfer device 41 Secondary transfer inner roller 42 Transfer electrode means (secondary transfer outer roller)
45
81 Image carrier (intermediate transfer belt)
N1 Primary transfer part N2 Secondary transfer part

Claims (13)

An image carrier for carrying a toner image;
Transfer electrode means for transferring the toner image from the image carrier to a recording material using a transfer bias voltage;
An image forming apparatus comprising: a power supply unit that outputs the transfer bias voltage variable to the transfer electrode unit;
Detecting means for detecting information relating to curling of the trailing edge region of the recording material passing through the transfer electrode means;
An image forming apparatus comprising: control means for controlling the power supply means to reduce the transfer bias voltage in the rear end area when the information on occurrence of the curl is detected. When the control means reduces the transfer bias voltage in the rear end region,
(1) A time for reducing the transfer bias voltage in the process of passing the recording material through the transfer electrode means,
(2) a maximum decrease amount of the transfer bias voltage in the process of passing a recording material through the transfer electrode means;
2. The image forming apparatus according to claim 1, wherein at least one of them is adjusted based on the information.   3. The image forming apparatus according to claim 1, wherein the control unit continuously changes the transfer bias voltage from the beginning to the rear end of the rear end region where the transfer bias voltage is decreased. . An image forming body in which an electrostatic latent image is formed on the surface by optical writing;
A developing device that attaches toner to the electrostatic latent image and develops the toner image,
2. The control unit according to claim 1, wherein the controller controls the developing device to reduce the amount of toner adhering to the entire electrostatic latent image when the amount of decrease in the transfer bias voltage is large. 4. The image forming apparatus according to any one of items 3. A fixing device for fixing the toner image by heating and pressing the recording material to which the toner image has been transferred;
The information is a distinction between the type of recording material and double-sided printing,
The control means has a transfer voltage table that records the control content of the secondary transfer bias voltage according to the type and the distinction,
5. The image according to claim 1, wherein the control unit identifies a combination of the type input by the operation and the distinction, and executes the control content selected from the transfer voltage table. 6. Forming equipment. A fixing device for fixing the toner image by heating and pressing the recording material onto which the toner image has been transferred;
Conveying means for conveying the recording material fixed by the fixing device to the transfer electrode means again,
The image forming apparatus according to claim 1, wherein the information is a number of times the recording material has passed through the fixing device.   The image forming apparatus according to claim 1, wherein the information includes a recording material storage environment and a storage time.   The image forming apparatus according to claim 1, wherein the information includes a basis weight and rigidity of the recording material.   The image forming apparatus according to claim 1, wherein the information is a sheet feeding direction of the recording material.   10. The control unit according to claim 1, wherein the control unit controls the time when the transfer bias voltage is lowered according to a conveyance distance or a conveyance time after detecting a leading edge of the recording material on a conveyance path of the recording material. The image forming apparatus according to claim 1.   11. The control means controls the power supply means to detect a transfer bias voltage corresponding to a necessary transfer current and apply the set transfer bias voltage to the recording material until the time. The image forming apparatus described. An image forming body in which an electrostatic latent image is formed on the surface by optical writing,
The image carrier is an endless intermediate transfer belt that primarily transfers and circulates the toner image from the image forming body,
The image forming apparatus according to claim 1, wherein the transfer electrode unit is a secondary transfer roller that forms a secondary transfer nip to a recording material in contact with the intermediate transfer belt. . An image forming body in which an electrostatic latent image is formed on the surface by optical writing;
A developing device that attaches toner to the electrostatic latent image and develops the toner image;
An endless intermediate transfer belt in which the toner image is primarily transferred and circulated from the image forming body;
Secondary transfer electrode means for applying a secondary transfer bias voltage and secondarily transferring the toner image primarily transferred to the intermediate transfer belt to a recording material,
In the operation method of the image forming apparatus for reducing the secondary transfer bias voltage in the rear end region of the recording material,
By increasing the density of the toner image in the region corresponding to the rear end region of the image forming body at a rate commensurate with the decrease in transfer efficiency in the rear end region, the transfer bias voltage is reduced to a lower region. An operation method characterized by reducing a difference in image density from a non-existing region.

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