JP2018060044A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent the occurrence of a trouble in an entering passage for means that fixes an image to a recording material.SOLUTION: An image forming apparatus 100 comprises: a transfer roller 25 that transfers a developer image formed on a photoconductor drum 21 to a sheet P at a transfer nip part N1 formed by pressing the photoconductor drum 21; a fixing part 30 that fixes the developer image transferred to the sheet P by the transfer roller 25 to the sheet P; an upper conveyance guide 54 and a lower conveyance guide 55 that guide conveyance of the sheet P with respect to the fixing part 30; and a CPU 60 that changes a bias applied to the transfer roller 25 and controls the way in which the sheet P separated from the photoconductor drum 21 and fed to the upper conveyance guide 54 and lower conveyance guide 55 enters the upper conveyance guide 54 and lower conveyance guide 55.SELECTED DRAWING: Figure 3

Description

  The present invention relates to an image forming apparatus such as an electrophotographic recording type copying machine or printer.

  Many image forming apparatuses such as copiers or printers that employ an electrophotographic method have a fixing process for fixing an unfixed developer image on a transfer material such as a sheet carrying an unfixed developer image. A fixing device adopting a thermal fixing method is provided. As such a fixing device, a fixing film that rotates along a film guide, a heating source such as a heater that is disposed in the fixing film and heats the fixing film, and a heat-resistant elastic layer is formed on an aluminum or iron core metal. What is provided with a pressure roller is known. Such a fixing film is pressed against the pressure roller by a spring or the like. A pressure contact width formed by pressure contact between the fixing film and the pressure roller is called a fixing nip.

  Conventionally, when fixing with a fixing film, a spur provided with gear-shaped external teeth is provided in a conveyance portion between a fixing nip and a transfer portion to prevent the transfer material from lifting and to guide the conveyance of the transfer material. An image forming apparatus is known.

  Japanese Patent Application Laid-Open No. 2004-228561 discloses an image forming apparatus having a spur that guides conveyance of a transfer material located upstream in the conveyance direction with respect to a fixing belt. In the image forming apparatus disclosed in Patent Document 1, when the rear end of the transfer material is separated from the photosensitive drum, the conveyance state is disturbed and the transfer material comes into contact with the fixing belt by installing the spur at an optimal position. In addition, the image transferred to the transfer material is prevented from rubbing or shifting.

JP 2003-337491 A

  However, in Patent Document 1, depending on the curl amount of the transfer material, or when the movement of the spur deteriorates due to paper dust or the like, the transfer material is rubbed or caught on the spur. There is a problem that the image transferred to the surface is rubbed against a spur.

  An object of the present invention is to provide an image forming apparatus capable of preventing inconveniences in an approach path to a means for fixing an image on a recording material.

  An image forming apparatus according to the present invention includes an image carrier, a transfer unit that transfers a developer image formed on the image carrier to a recording material at a transfer nip formed by pressing the image carrier, A fixing unit that fixes the developer image transferred to the recording material by the transfer unit to the recording material; a guide unit that guides conveyance of the recording material to the fixing unit; and a bias that is applied to the transfer unit, and changes the image. And a control means for controlling the manner in which the recording material separated from the carrier and sent to the guide means enters the guide means.

  According to the present invention, it is possible to provide an image forming apparatus that does not cause inconvenience in an approach path to a means for fixing an image on a recording material.

1 is a cross-sectional view of an image forming apparatus according to Embodiment 1 of the present invention. 1 is a schematic diagram of a main part of an image forming apparatus according to Embodiment 1 of the present invention. 1 is a block diagram illustrating a configuration of an image forming apparatus according to Embodiment 1 of the present invention. It is a flowchart of the transfer conveyance process which concerns on Embodiment 1 of this invention. It is a figure which shows an example of the table which calculates | requires the distance A which concerns on Embodiment 1 of this invention. It is a figure which shows an example of the table which calculates | requires the distance between the guide member which concerns on Embodiment 1 of this invention, and a sheet | seat. FIG. 3 is a diagram illustrating a state of a recording material conveyed from a separation position to a fixing nip according to Embodiment 1 of the present invention. It is a figure which shows an example of the table which calculates | requires the rigidity used in the modification of the transfer conveyance process which concerns on Embodiment 1 of this invention. FIG. 10 is a diagram illustrating a distance between a guide member and a sheet for each sheet passing number in a modified example of the transfer conveyance process according to the first embodiment of the present invention, in comparison with the conventional art. It is a schematic diagram of the principal part of the image forming apparatus which concerns on Embodiment 2 of this invention.

  Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings.

(Embodiment 1)
<Configuration of image forming apparatus>
The configuration of the image forming apparatus 100 according to the first embodiment of the present invention will be described in detail with reference to FIGS.

  The image forming apparatus 100 includes a sheet feeding unit 10, an image forming unit 20, a fixing unit 30, a sheet discharging unit 40, a sheet refeeding unit 50, an upper conveyance guide 54, and a lower conveyance guide 55. And a cleaning device 56. The image forming apparatus 100 includes a CPU 60, a document reading device 61 a, an image data communication device 61 b, a dot counter 61 c, a RAM 62, and a ROM 63. Further, the image forming apparatus 100 includes an operation unit 64, a cassette open / close detection unit 65, a temperature / humidity sensor 66, a transfer bias variable unit 67, and a transfer bias power source 68.

  The document reading device 61a, the image data communication device 61b, and the dot counter 61c shown in FIG. 2 may be connected to the image forming device 100 as external devices of the image forming device 100, or the image forming device. 100 may be included. The upper conveyance guide 54 and the lower conveyance guide 55 constitute guide means for guiding the conveyance of the sheet P to the fixing unit 30.

  The sheet feeding unit 10 feeds the sheet P to the image forming unit 20. Specifically, the sheet feeding unit 10 includes a feeding cassette 11, a pickup roller 12, a separation roller pair 13, a feeding roller pair 14, and a registration roller pair 15. Further, the sheet feeding unit 10 includes a manual feed tray 16, a supply roller 17, a separation pad 18, a supply roller pair 19, a sheet detection sensor S1, and a pre-registration sensor S2.

  The feeding cassette 11 stores sheets P.

  The pickup roller 12 feeds the sheet P stored in the feeding cassette 11 to the separation roller pair 13.

  The separation roller pair 13 is composed of a normal rotation roller and a reverse rotation roller, and separates the multi-feed sheet P into one sheet when the sheet P fed from the pickup roller 12 is multi-feed. Transport to feeding path PS1.

  The feeding roller pair 14 conveys the sheet P conveyed from the separation roller pair 13 via the feeding path PS <b> 1 toward the registration roller pair 15.

  When the rotation of the registration roller pair 15 is stopped, the leading edge of the sheet P conveyed from the feeding roller pair 14 or the re-feeding roller pair 53 follows the nip so as to correct the skew of the sheet P. . The registration roller pair 15 starts to rotate at a predetermined timing, and conveys the sheet P whose skew has been corrected toward the image forming unit 20.

  The manual feed tray 16 can stack sheets P.

  The supply roller 17 and the separation pad 18 separate the sheets P stacked on the manual feed tray 16 into a single sheet and convey the sheet P toward the supply roller pair 19.

  The supply roller pair 19 conveys the sheet P conveyed by the supply roller 17 and the separation pad 18 toward the supply roller pair 14.

  The sheet detection sensor S <b> 1 detects whether or not the sheet P is stored in the feeding cassette 11.

  The pre-registration sensor S2 detects the timing at which the leading edge of the sheet P reaches the nip of the registration roller pair 15.

  The image forming unit 20 forms an image on the sheet P fed from the sheet feeding unit 10 and conveys the image toward the fixing unit 30. Specifically, the image forming unit 20 includes a photosensitive drum 21, a charging roller 22, a laser unit 23, a developing roller 24, and a transfer roller 25.

  The surface of the photosensitive drum 21 as an image carrier is uniformly charged by a charging roller 22. The photosensitive drum 21 is irradiated with laser light corresponding to the image information from the laser unit 23, and the charged electric charge of the portion irradiated with the laser light is removed, whereby an electrostatic latent image corresponding to the image information is removed. Is formed. A developer image is formed on the photosensitive drum 21 by the developer being attached to the electrostatic latent image by the developing roller 24. The photosensitive drum 21 rotates and conveys the developer image to the transfer nip portion N1. The photosensitive drum 21 is configured to separate the curvature of the sheet P.

  The charging roller 22 uniformly charges the surface of the photosensitive drum 21 by applying a charging bias.

  The laser unit 23 irradiates the surface of the photosensitive drum 21 with laser light in accordance with image information such as a document transmitted from a personal computer or a reading scanner (not shown) to expose the electrostatic latent image on the photosensitive drum 21. Form.

  The developing roller 24 supplies a developer to the electrostatic latent image formed on the photosensitive drum 21, develops the electrostatic latent image, and visualizes it as a developer image.

  The transfer roller 25 as a transfer means is composed of a conductive elastic foam containing a conductive agent mainly composed of an elastic foam. The transfer roller 25 presses the photosensitive drum 21 to form a transfer nip portion N1. The sheet P is conveyed from the registration roller pair 15 to the transfer roller 25 in synchronization with the timing at which the developer image is conveyed to the transfer nip portion N1 by the photosensitive drum 21. The transfer roller 25 sandwiches and conveys the conveyed sheet P with the photosensitive drum 21 at the transfer nip portion N1, and transfers the developer image formed on the photosensitive drum 21 to the sheet P.

  The transfer roller 25 transfers the developer image formed on the photosensitive drum 21 onto the sheet P when a transfer bias is applied from the transfer bias power source 68. The transfer bias needs to be set to a value that does not cause transfer failure or transfer omission. Here, 15 μA is exemplified. The transfer roller 25 rubs against the upper conveyance guide 54 and the lower conveyance guide 55 after reaching the separation position where the sheet P is separated from the photosensitive drum 21 when a predetermined condition described later is satisfied. A bias that is controlled so as not to be caught is applied.

  The fixing unit 30 as a fixing unit fixes the developer image transferred to the sheet P conveyed from the image forming unit 20 to the sheet P, and directs the sheet P on which the developer image is fixed to the sheet discharge unit 40. Transport. Specifically, the fixing unit 30 includes a fixing roller 31, a pressure roller 32, and a fixing sensor S3.

  The fixing roller 31 is composed of an aluminum roller or the like, and is heated to a predetermined fixing temperature by a heat source such as a halogen lamp disposed therein.

  The pressure roller 32 is in contact with the fixing roller 31 and presses the fixing roller 31 with a predetermined pressure to form a fixing nip portion N2. In the fixing nip portion N2, the developer image conveyed from the image forming portion 20 is nipped and conveyed, and the developer image is fixed to the sheet P by heating and pressing.

  The fixing sensor S3 detects that the leading edge of the sheet P has passed through the fixing nip portion N2.

  Note that the fixing unit 30 does not use the heating roller method in which the fixing roller 31 heats, but the pressure roller 32 presses a heat source such as a ceramic heater through an endless film to form the fixing nip portion N2. A demand fixing method may be used.

  The sheet discharge unit 40 discharges the sheet P conveyed from the fixing unit 30. Specifically, the sheet discharge unit 40 includes a discharge roller pair 41 and a discharge tray 42.

  The discharge roller pair 41 discharges the sheet P conveyed from the fixing unit 30 to the discharge tray 42. When forming images on both sides of the sheet P, the discharge roller pair 41 is after the leading edge of the sheet P having an image formed on one surface has passed through the fixing sensor S3 and the trailing edge of the sheet P has been removed. Pause before. The discharge roller pair 41 temporarily stops and then reversely rotates, thereby inverting the sheet P and conveying the sheet P toward the sheet refeeding unit 50.

  The discharge tray 42 stacks the sheets P discharged by the discharge roller pair 41.

  The sheet refeeding unit 50 conveys the sheet P conveyed from the sheet discharge unit 40 toward the image forming unit 20 again. Specifically, the sheet refeeding unit 50 includes a refeeding roller pair 51, a refeeding roller pair 52, and a refeeding roller pair 53.

  The refeed roller pair 51 conveys the sheet P conveyed from the discharge roller pair 41 to the refeed path PS2.

  The refeed roller pair 52 conveys the sheet P conveyed from the refeed roller pair 51 via the refeed path PS2 to the refeed path PS2.

  The refeed roller pair 53 transports the sheet P conveyed from the refeed roller pair 52 via the refeed path PS2 toward the registration roller pair 15.

The upper conveyance guide 54 as an upper guide member is provided on the side to which the developer image of the sheet P is first transferred (left side of the sheet P in FIG. 2), and fixes the sheet P conveyed from the transfer nip portion N1. The conveyance to the unit 30 is guided. The lower conveyance guide 55 provided as the lower guide member on the side where the developer image of the recording material is first transferred is provided on the side where the developer image of the sheet P is not transferred first (the right side of the sheet P in FIG. 2). ) And guides the conveyance of the sheet P conveyed from the transfer nip N1 to the fixing unit 30.

  Here, as shown in FIG. 2, the distance from the transfer nip portion N1 to the separation position where the sheet P is separated from the photosensitive drum 21 is A, and the sheet P is transferred from the transfer nip portion N1 to the upper conveyance guide 54 and the lower conveyance. Let B be the distance to reach the guide 55.

  The cleaning device 56 collects untransferred toner remaining on the photosensitive drum 21 with a cleaning blade (not shown).

  The CPU 60 as the control means performs various operations such as an image forming operation by reading and executing a control program stored in the ROM 63. The CPU 60 controls the transfer bias variable unit 67 to change the bias applied to the transfer roller 25, and is separated from the photosensitive drum 21 and sent to the upper conveyance guide 54 and the lower conveyance guide 55. And the manner of entering the lower conveyance guide 55 is controlled.

  Specifically, the CPU 60 acquires the image information input from the dot counter 61c, calculates the image printing rate from the measured value of the number of dots of the acquired image information, and temporarily stores the calculated image printing rate in the RAM 62. Let Here, the image printing rate can be obtained by dividing the number of dots measured by the dot counter 61c by the maximum number of dots that can be printed.

  The CPU 60 temporarily stores the selection result input from the operation unit 64 in the RAM 62. Based on the detection result input from the cassette open / close detection unit 65, the CPU 60 obtains a time from opening the feeding cassette 11 to closing it (hereinafter referred to as “open / close time”). The CPU 60 is associated with the detection result of the temperature or humidity and the obtained opening / closing time of the feeding cassette 11 in the table for each opening / closing time in which the temperature or humidity stored in the ROM 63 is associated with the amount of paper moisture. Select and predict the amount of moisture in the paper. The CPU 60 temporarily stores the paper moisture amount prediction result in the RAM 62.

  The CPU 60 obtains the distance A with reference to the image printing rate, the selection result and the paper moisture content stored in the RAM 62 and the table stored in the ROM 63. The CPU 60 uses the upper conveyance guide 54 when the sheet P reaches the upper conveyance guide 54 and the lower conveyance guide 55 based on the image printing rate, the selection result, the paper moisture content, and the table stored in the ROM 63. Further, the distance between the lower conveyance guide 55 and the sheet P is obtained. The distance between the upper conveyance guide 54 and the lower conveyance guide 55 and the sheet P when the sheet P reaches the upper conveyance guide 54 and the lower conveyance guide 55 is hereinafter referred to as “relative between the sheet P and the guide member. “Distance”.

  The CPU 60 sets a bias for setting a bias that prevents the sheet P from being rubbed or caught on the upper conveyance guide 54 or the lower conveyance guide 55 based on the obtained distance A and the relative distance between the sheet P and the guide member. Is output to the transfer bias variable section 67.

  The document reading device 61a reads an image of the document and outputs image data of the read image to the image data communication device 61b.

  The image data communication device 61b transmits the image data input from the document reading device 61a to the dot counter 61c.

  The dot counter 61c as a measuring unit measures the number of dots of the image data received from the image data communication device 61b, and outputs the measured value to the CPU 60 as image information.

  The RAM 62 temporarily stores an image printing rate, a selection result, and a paper moisture amount prediction result.

  The ROM 63 stores in advance a control program for executing an image forming operation and various tables.

  One of the various tables is a table in which the moisture content of the recording material, the image printing rate, the recording material type, the recording material size, and the distance A are associated with each other. One of the various tables is a table in which the moisture content of the recording material, the image printing rate, the recording material type, the recording material size, and the relative distance between the sheet P and the guide member are associated with each other. is there. Furthermore, one of the various tables is a table for each opening / closing time in which the temperature or humidity is associated with the paper moisture content.

  Here, as shown in FIG. 2, the relative distance between the sheet P and the guide member starts from the sheet passing surface of the lower conveyance guide 55 and increases as the sheet position approaches the upper conveyance guide 54. Therefore, the relative distance between the sheet P and the guide member is the distance from the lower conveyance guide 55. The relative distance between the sheet P and the guide member may be a distance from the upper conveyance guide 54.

  The operation unit 64 acquires the selection result of the type, basis weight, and size of the sheet P selected by the user's operation, and outputs the acquired selection result to the CPU 60.

  The cassette open / close detection unit 65 detects opening / closing of the feeding cassette 11 and outputs a detection result of opening / closing of the feeding cassette 11 to the CPU 60.

  The temperature / humidity sensor 66 detects temperature and humidity, and outputs the temperature and humidity detection results to the CPU 60.

  When the switching signal is input from the CPU 60, the transfer bias varying unit 67 sets the bias instructed by the switching signal. The transfer bias varying unit 67 applies a set bias to the transfer roller 25 with respect to the transfer bias power source 68 after reaching the separation position where the sheet P is separated from the photosensitive drum 21.

  The transfer bias power source 68 applies a transfer bias to the transfer roller 25 during transfer. The transfer bias power supply 68 applies the transfer bias set by the transfer bias variable unit 67 to the transfer roller 25 after reaching the separation position where the sheet P is separated from the photosensitive drum 21.

<Transfer conveyance process>
The transfer conveyance process according to the embodiment of the present invention will be described in detail with reference to FIGS.

  The CPU 60 starts execution of the transfer conveyance process shown in FIG. 4 by reading out the control program stored in advance in the ROM 63 from the ROM 63 and executing it.

  First, the CPU 60 acquires the selection result of the type and size of the sheet P from the operation unit 64 (S1).

  Next, when the image forming operation is started, the CPU 60 predicts the paper moisture content of the sheet P based on the detection result input from the temperature / humidity sensor 66 and the detection result input from the cassette open / close detection unit 65. (S2).

  Next, based on the image information acquired from the dot counter 61c, the CPU 60 calculates an image printing rate of less than 20 mm on the front end of the sheet P and an image printing rate of 20 mm or more on the front end of the sheet P (S3).

  Next, the CPU 60 determines the type and size of the sheet P acquired in S1, the paper moisture amount predicted in S2, the image printing rate of the sheet P less than 20 mm calculated in S3, the table shown in FIG. The distance A is determined based on (S5). Specifically, the CPU 60 selects the distance A associated with the type and size of the sheet P, the paper moisture content, and the image printing rate in the table shown in FIG.

  In this embodiment, the distance A is selected based on the type of the sheet P, the size of the sheet P, the amount of paper moisture, and the image printing rate. However, at least one of these conditions is selected. You may make it select the distance A from one.

  Next, the CPU 60 determines whether or not the distance A is greater than or equal to a predetermined reference value (greater than a predetermined distance) (S5). The reference value is exemplified here as 10 mm.

  When the distance A is not less than the reference value (S5: Yes), the CPU 60 determines the relative distance between the sheet P and the guide member (S6). Specifically, the CPU 60 determines the type and size of the sheet P acquired in S1, the paper moisture amount predicted in S2, the image printing rate of the front end of the sheet P calculated in S3, and the table shown in FIG. And determining the relative distance between the sheet P and the guide member.

  Next, the CPU 60 determines whether or not the relative distance between the sheet P and the guide member is equal to or greater than a predetermined reference value (S7). The reference value is exemplified here as 8 mm.

  When the relative distance between the sheet P and the guide member is not less than a predetermined reference value (not less than a predetermined distance) (S7: Yes), the CPU 60 biases the transfer bias variable portion 67 higher than the transfer bias. A switching signal for setting is output. Accordingly, the transfer bias power supply 68 applies a bias higher than the transfer bias before the sheet P is separated from the photosensitive drum 21 after the sheet P reaches the distance A and is separated from the photosensitive drum 21 to the transfer roller. (S8). Then, the CPU 60 ends the transfer conveyance process after the trailing end of the sheet P in the conveyance direction passes through the separation position. The bias higher than the transfer bias is exemplified here by a transfer bias of 40 μA.

  By applying a bias higher than the transfer bias to the transfer roller 25, the suction from the surface of the photosensitive drum 21 after passing through the transfer nip portion N1 becomes weaker. Accordingly, as shown in FIG. 7, the sheet P moves in the direction of the arrow e that moves away from the upper conveyance guide 54 after the sheet P is separated from the photosensitive drum 21. Thereby, it becomes difficult for the sheet P and the upper conveyance guide 54 to come into contact with each other, and the sheet P can be prevented from being caught or rubbed due to rubbing against the upper conveyance guide 54 and the lower conveyance guide 55.

  On the other hand, when the relative distance between the sheet P and the guide member is less than a predetermined reference value (less than a predetermined distance) (S8: No), the CPU 60 is lower than the transfer bias with respect to the transfer bias variable portion 67. A switching signal for setting the bias is output. Accordingly, the transfer bias power supply 68 applies a bias lower than the transfer bias before the sheet P is separated from the photosensitive drum 21 after the sheet P reaches the distance A and is separated from the photosensitive drum 21 to the transfer roller. (S10). Then, the CPU 60 ends the transfer conveyance process after the trailing end of the sheet P in the conveyance direction passes through the separation position. The bias lower than the transfer bias exemplifies a transfer bias of 10 μA here.

  Further, when the distance A is less than the reference value (S6: No), the CPU 60 does not output a switching signal to the transfer bias variable unit 67. Thereby, the transfer bias power supply 68 applies a transfer bias to the transfer roller 25 from the timing when the sheet P reaches the distance A to the timing when it reaches the distance B (S11). Then, the CPU 60 ends the transfer conveyance process when the trailing end of the sheet P in the conveyance direction passes the separation position. The transfer bias is exemplified here as 15 μA.

  As described above, in the present exemplary embodiment, the bias applied to the transfer roller 25 is changed, and the upper conveyance guide 54 and the lower conveyance guide 54 of the sheet P separated from the photosensitive drum 21 and sent to the upper conveyance guide 54 and the lower conveyance guide 55. The method of entering the side conveyance guide 55 is controlled. Accordingly, it is possible to provide an image forming apparatus that does not cause any inconvenience in the approach path to the fixing unit 30 that fixes the image on the sheet P.

  In the above-described embodiment, the rigidity of the sheet P is obtained, and based on the comparison result between the rigidity of the sheet P and the threshold value, the sheet P is prevented from being rubbed or scratched on the upper conveyance guide 54 and the lower conveyance guide 55. Alternatively, the bias applied to the transfer roller 25 may be controlled.

  Specifically, the CPU 60 acquires the paper moisture content, the image printing rate, and the type and size of the sheet P. The CPU 60 refers to the table in FIG. 8 to determine the rigidity of the sheet P associated with the acquired paper moisture content, the image printing rate, the type and size of the sheet P. The CPU 60 applies a bias higher than the transfer bias to the transfer roller 25 when the determined rigidity is less than a predetermined threshold. As a result, it is possible to prevent paper catching and rubbing failure. Further, the CPU 60 applies a transfer bias to the transfer roller 25 when the determined rigidity is equal to or greater than a predetermined threshold. Thereby, it is possible to avoid the occurrence of an abnormal image due to transfer omission or the like.

  In the above embodiment, the sheet P is rubbed against the upper conveyance guide 54 and the lower conveyance guide 55 based on the comparison result between the stiffness of the sheet P and the threshold and the comparison result between the image printing rate and the threshold. The bias applied to the transfer roller 25 may be controlled so as not to get caught.

  Specifically, the CPU 60 transfers the sheet P after the sheet P is separated from the photosensitive drum 21 and after the sheet P is separated from the photosensitive drum 21 when the stiffness of the sheet P is less than the threshold and the image printing rate is less than the threshold. A bias higher than this bias is applied to the transfer roller 25. As described above, when the sheet P has low rigidity and the image printing rate is low, a bias higher than the transfer bias before the sheet P is separated from the photosensitive drum 21 after the sheet P is separated from the photosensitive drum 21 is applied. Applied to the transfer roller 25.

  Accordingly, as shown in FIG. 9, the adsorption of the sheet P by the photosensitive drum 21 can be weakened, and the sheet P is not rubbed or caught on the upper conveyance guide 54 and the lower conveyance guide 55. Therefore, the sheet P can be stably conveyed.

  On the other hand, when the stiffness of the sheet P is less than the threshold value and the image printing rate is less than the threshold value, and the transfer bias is applied to the transfer roller 25, the CPU 60 attracts the sheet P to the photosensitive drum 21 and conveys the upper side. The guide 54 and the lower conveyance guide 55 are rubbed or caught. As described above, when the sheet P has a low rigidity and the image printing rate is low, in the conventional case where the transfer bias is applied to the transfer roller 25, the sheet P is moved to the upper conveyance guide 54 as shown in FIG. Rub on.

  Further, when the stiffness of the sheet P is equal to or greater than the threshold value and the image printing rate is equal to or greater than the threshold value, the CPU 60 easily separates the sheet P from the photosensitive drum 21 due to a developer interposed between the sheet P and the photosensitive drum 21. Become. Accordingly, in this case, even when a transfer bias is applied to the transfer roller 25, the sheet P does not rub against or catch on the upper conveyance guide 54 and the lower conveyance guide 55. On the other hand, when the stiffness of the sheet P is equal to or higher than the threshold value and the image printing rate is equal to or higher than the threshold value, the CPU 60 applies an abnormal image due to transfer omission when a bias higher than the transfer bias is applied to the transfer roller 25. Arise.

(Embodiment 2)
<Configuration of image forming apparatus>
The configuration of the image forming apparatus 100a according to Embodiment 2 of the present invention will be described in detail with reference to FIG.

  The image forming apparatus 100 a includes a sheet feeding unit 10, an image forming unit 20, a fixing unit 30, a sheet discharging unit 40, a sheet refeeding unit 50, an upper conveyance guide 54, and a lower conveyance guide 55. And a cleaning device 56. In addition, the image forming apparatus 100 a includes a CPU 60, a document reading device 61 a, an image data communication device 61 b, a dot counter 61 c, a RAM 62, a ROM 63, and an operation unit 64. Further, the image forming apparatus 100 a includes a cassette open / close detection unit 65, a temperature / humidity sensor 66, a transfer bias variable unit 67, a transfer bias power source 68, a separation claw 101, and a separation detection sensor 102. .

  10, parts having the same configuration as in FIGS. 1 to 3 are denoted by the same reference numerals and description thereof is omitted.

  A separation claw 101 as a separation unit is provided at a predetermined separation position, and separates the sheet P on which the developer image has been transferred by the transfer roller 25 from the photosensitive drum 21.

  The separation detection sensor 102 detects that the sheet P is in contact with the separation claw 101 and outputs the detection result to the CPU 60.

  In response to the contact of the sheet P with the separation claw 101, the CPU 60 changes the bias so that the sheet P is not rubbed or caught on the upper transport guide 54 and the lower transport guide 55.

  Specifically, when the detection result that the sheet P contacts the separation claw 101 is input from the separation detection sensor 102, the CPU 60 reads the image printing rate, the selection result, the paper moisture amount, and the table stored in the ROM 63. Based on the above, the relative distance between the sheet P and the guide member is determined. Here, the table stored in the ROM 63 is a table in which the paper moisture content, the image printing rate, the recording material type, the recording material size, and the relative distance between the sheet P and the guide member are associated with each other. It is.

  Based on the relative distance between the sheet P and the guide member, the CPU 60 causes the transfer bias variable unit 67 to set a bias that prevents the sheet P from rubbing or catching on the upper conveyance guide 54 and the lower conveyance guide 55. Since the configuration of the CPU 60 other than the above is the same as that of the CPU 60 of the first embodiment, description thereof is omitted.

  As described above, in the present embodiment, in response to the contact of the sheet P with the separation claw 101, the transfer roller 25 is prevented from being rubbed or caught by the upper conveyance guide 54 and the lower conveyance guide 55. Change the bias to be applied. Thereby, in addition to the effect of the first embodiment, the processing for obtaining the distance A can be made unnecessary, so that the processing load can be reduced.

  Although the embodiments of the invention have been specifically described, it is needless to say that the present invention is not limited to the above-described embodiments, and various modifications can be made without departing from the scope of the invention.

  Specifically, in the above embodiment, the reference value used in S6 or S8 in FIG. 4 can be set as appropriate in accordance with the apparatus conditions.

  In the above-described embodiment, the present invention is applied to laser beam printing, but it can also be applied to other image forming apparatuses such as copying machines and facsimiles.

N1 transfer nip part N2 fixing nip part PS1 feeding path PS2 refeeding path 10 sheet feeding part 11 feeding cassette 12 pickup roller 13 separation roller pair 14 feeding roller pair 15 registration roller pair 16 manual feed tray 17 feeding roller 18 separation Pad 19 Supply roller pair 20 Image forming portion 21 Photosensitive drum 22 Charging roller 23 Laser unit 24 Developing roller 25 Transfer roller 30 Fixing portion 31 Fixing roller 32 Pressure roller 40 Sheet discharge portion 41 Discharge roller pair 42 Discharge tray 50 Sheet refeed Portion 51 Re-feed roller pair 52 Re-feed roller pair 53 Re-feed roller pair 54 Upper transport guide 55 Lower transport guide 56 Cleaning device 60 CPU
61a Document reading device 61b Image data communication device 61c Dot counter 62 RAM
63 ROM
64 Operation Unit 65 Cassette Open / Close Detection Unit 66 Temperature / Humidity Sensor 67 Transfer Bias Variable Unit 68 Transfer Bias Power Supply 100 Image Forming Apparatus 100a Image Forming Apparatus 101 Separation Claw 102 Separation Detection Sensor

Claims (10)

An image carrier;
Transfer means for transferring a developer image formed on the image carrier to a recording material in a transfer nip portion formed by pressing the image carrier;
Fixing means for fixing the developer image transferred to the recording material by the transfer means to the recording material;
Guiding means for guiding the conveyance of the recording material to the fixing means;
A control means for changing a bias applied to the transfer means, and controlling a method of entering a recording material separated from the image carrier and sent to the guide means;
An image forming apparatus comprising: The control means includes
Changing the bias so that the recording material sent to the guide means is not rubbed or caught on the guide means;
The image forming apparatus according to claim 1. The control means includes
The guide means and the recording material when the recording material reaches the guide means based on at least one of the type of the recording material, the size of the recording material, the image printing rate, and the moisture content of the recording material And after the recording material is separated from the image carrier, the bias is changed so that the recording material is not rubbed or caught on the guide means according to the determination result of the distance.
The image forming apparatus according to claim 2. The transfer means includes
The developer image is transferred to the recording material by applying the bias,
The guiding means includes
An upper guide member provided on a side of the recording material on which the developer image is first transferred; and a lower guide member provided on a side of the recording material on which the developer image is not transferred first.
The control means includes
When the distance between the recording material and the lower guide member is a predetermined distance or more, after the recording material is separated from the image carrier, the bias is applied higher than before the separation, When the distance from the lower guide member is less than a predetermined distance, after the recording material is separated from the image carrier, the bias that is lower than before the separation is applied.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The control means includes
Based on the stiffness of the recording material, after the recording material is separated from the image carrier, the bias is changed so that the recording material is not rubbed or caught on the guide means.
The image forming apparatus according to claim 2. A storage unit that stores a table in which the moisture content of the recording material, the image printing rate, the type of the recording material, the size of the recording material, and the stiffness of the recording material are associated with each other;
The control means includes
According to the table, the recording material stiffness is determined based on the acquired moisture content of the recording material, the image printing rate, the type of the recording material, and the size of the recording material.
The image forming apparatus according to claim 5. Having measuring means for measuring the number of dots of the image formed on the recording material;
The control means includes
Obtain the image printing rate from the number of dots measured by the measuring means,
The image forming apparatus according to claim 3, wherein the image forming apparatus is an image forming apparatus. The control means includes
When the distance between the transfer nip portion and the position where the recording material is separated from the image carrier is a predetermined distance or more, the recording material rubs against the guide means after the recording material reaches the separation position. Or change the bias to avoid catching
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus. The control means includes
Based on at least one of the moisture content of the recording material, the image printing rate, the type of the recording material, and the size of the recording material, the position where the transfer nip and the recording material are separated from the image carrier, Determine the distance between,
The image forming apparatus according to claim 8. A separating unit that separates the recording material onto which the developer image has been transferred by the transfer unit from the image carrier;
The control means includes
In response to the recording material coming into contact with the separating means, the bias is changed so that the recording material is not rubbed or caught on the guide means.
The image forming apparatus according to claim 1, wherein the image forming apparatus is an image forming apparatus.

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