JP2021060504A - Image forming apparatus - Google Patents

Abstract

To provide an image forming apparatus that can appropriately determine the timing of movement to a position for transfer and arrange sheet detection means at a point relatively separated from a transfer position.SOLUTION: An image forming apparatus moves a transfer member 109 that brings a sheet into contact with an image carrier for transfer between a position for transfer and a retracted position, and the image forming apparatus performs the movement to the position for transfer based on an image formation start timing, and performs the movement to the retracted position based on a result of detection performed by sheet detection means 301 that is arranged on the upstream side of a transfer position in a sheet conveyance direction. The sheet detection means 301 may be arranged on the upstream side of a first sheet conveying member 111 that is arranged on the upstream side of the transfer position.SELECTED DRAWING: Figure 2

Description

The present invention relates to an image forming apparatus.

Conventionally, there is known an image forming apparatus that moves a transfer member that brings a sheet into contact with an image carrier for transfer between a transfer position and a retracted position retracted from the transfer position.
For example, in Patent Document 1, when the transfer member is brought into contact with and detached from the image carrier in order to prevent the back stain of the transfer paper (sheet) due to the stain on the transfer member, the timing of contact and separation is set on the sheet transport path. A technique for determining using a transfer paper sensor (sheet detecting means) provided in the above is described. It is also mentioned that the registration roll (resist transfer member) is arranged on the downstream side in the transfer direction or on the downstream side in the transfer direction.

The movement control of the transfer member with respect to the image carrier is not only for preventing the back surface of the sheet from being soiled, but also for the tip of the sheet to enter the transfer position, which is the contact portion between the image carrier and the transfer member, and for the sheet from the transfer position. There is also an image forming apparatus that is used to prevent image defects due to an impact caused by a rear end omission. In any case, as the image forming apparatus becomes smaller and faster, it becomes difficult to arrange the sheet detecting means in the vicinity of the transfer position.

In order to solve the above-mentioned problems, the present invention is an image forming apparatus for moving a transfer member that brings a sheet into contact with an image carrier for transfer between a transfer position and a retracted position retracted from the transfer position. In the above, the movement to the transfer position is performed based on the image formation start timing, and the movement to the retract position is performed based on the detection result of the sheet detection means arranged on the upstream side of the transfer position in the sheet transport direction. It is characterized by.

According to the present invention, the timing of movement to the transfer position can be appropriately determined, and the sheet detecting means can be arranged at a position relatively distant from the transfer position.

The schematic block diagram of the image forming apparatus which concerns on embodiment. Enlarged view of the vicinity of the secondary transfer roller. Explanatory drawing of the contact and separation mechanism between an intermediate transfer belt and a secondary transfer roller. The block diagram which shows a part of the electric circuit in the printer which concerns on embodiment. Explanatory drawing of the control example of the contact and separation mechanism which concerns on a comparative example. Flow chart of control of comparative example. The explanatory view of the control example of the contact / separation mechanism which concerns on embodiment. Flow chart of control of embodiment. Explanatory drawing of the deviation timing deviation due to the deviation of the paper length from the ideal length. Explanatory drawing of a factor which causes a shock jitter when the rear edge of a paper is pulled out.

FIG. 1 is a schematic configuration diagram of an image forming apparatus according to an embodiment. This image forming apparatus is an electrophotographic full-color printer using four color toners of yellow (Y), magenta (M), cyan (C), and black (K). Four image-forming units (image forming units) 101Y, 101M, 101C, and 101K that image with each color toner are arranged side by side in the upper part of the main body of the apparatus. Since the configurations and operations of the image-forming units 101Y, 101M, 101C, and 101K are substantially the same, the image-forming units will be described here by omitting the color symbols (Y, M, C, K). .. In the image forming unit 101, a charging device 103, a developing device 104, a cleaning device 105, and the like are arranged around the latent image carrier or the photoconductor drum 102 as the image carrier. Further, an optical writing device 107 as an image writing means is arranged above the photoconductor drum 102.

Below the four image forming units 101Y, 101M, 101C, and 101K, an intermediate transfer belt 108 as an intermediate transfer body, which is an endless belt member hung around a plurality of support rollers, is arranged. The intermediate transfer belt 108 is driven to travel in the direction of arrow A by rotationally driving one of the support rollers by a driving means. A transfer roller 106 as a primary transfer means is arranged so as to face the photoconductor drum 102 of each image forming unit with the intermediate transfer belt 108 interposed therebetween. The four image forming units 101Y, 101M, 101C, and 101K are arranged side by side so as to face the planar extension portion of the intermediate transfer belt 108.

In each image forming unit 101, the photoconductor drum 102 is rotationally driven counterclockwise in the drawing, and the photoconductor surface is uniformly charged to a predetermined polarity by the charger 103. Next, the charged surface is irradiated with a light-modulated laser beam emitted from the optical writing device 107, whereby an electrostatic latent image is formed on the photoconductor drum 102. The electrostatic latent image is developed by the toner applied from the developing device 104 and visualized as a toner image. The yellow, magenta, cyan, and black toner images formed by each image forming unit are sequentially superimposed and transferred on the intermediate transfer belt 108.

On the other hand, a feeding section 114 having accommodating trays 114a and 114b is provided in the lower part of the main body of the apparatus, and a sheet such as paper is fed from the feeding section 114. The fed sheet is conveyed as shown by an arrow B toward the resist roller 111, which is a resist conveying member.

The sheet that has been abutted against the resist roller 111 and temporarily stopped is sent out from the resist roller 111 at the timing of the toner image on the intermediate transfer belt 108, and the secondary transfer roller 109 and the intermediate transfer belt 108 are in contact with each other. It is sent to the next transfer section. A voltage opposite to the charging polarity of the toner is applied to the secondary transfer roller 109, whereby the layered toner image (full-color image) on the intermediate transfer belt 108 is transferred onto the sheet. The sheet after the toner image is transferred is conveyed to the fixing device 113 as a heating fixing means by the conveying belt 112, and the toner is fixed to the sheet by heat and pressure in the fixing device 113. The sheet after the toner image is fixed is discharged to the outside of the machine as shown by an arrow C, and is discharged onto the paper ejection tray 100.

The sheet discharge path after the toner image is fixed includes a straight paper discharge path and a reverse paper discharge path (switchback path). The straight paper ejection route is a route in which the paper ejection relay roller 130 conveys the paper in the direction indicated by the arrow E and the paper ejection roller 131 ejects the paper to the paper ejection tray 100 outside the machine as indicated by the arrow C. The reversing paper ejection path (switchback path) is conveyed to the sheet reversing portion 115 in the transport path direction as shown by the arrow D via the reversing inlet roller 120 and the paper ejection reversing roller 121, and then temporarily stopped, and then temporarily stopped. This is a path in which the 121 rotates in the reverse direction to switch back and feed the paper to the paper ejection roller 131, and as shown by the arrow C, the paper is ejected to the paper ejection tray 100 outside the machine. This reverse paper ejection route is used for single-sided printing and backside ejection (face-down ejection).

In the case of double-sided printing (double-sided function printing), the sheet after fixing is fed to the double-sided reversing portion 116 via the reversing inlet roller 120 and the paper ejection reversing roller 121, where the front end and the rear end of the transport are exchanged and the refeeding path It is sent to 117, and is re-delivered to the registration roller 111 via the re-feeding path 117. Then, the toner image is transferred from the intermediate transfer belt 108 to the back surface of the sheet. The sheet after the toner image transfer is fixed by the fixing device 113, and as shown by the arrow C from the fixing device 113 as in the case of single-sided printing, or as shown by the arrow C via the sheet reversing portion 115, the sheet is removed from the machine. It is ejected and is ejected onto the output tray 100. Switching claws 118 and 119 for switching the sheet transport direction are appropriately arranged.

In the illustrated example, a manual paper feed roller 212 and a paper feed roller 213 from a peripheral machine are also provided. Instead of applying a voltage opposite to the charging polarity of the toner to the secondary transfer roller 109, the toner is applied to the secondary transfer opposing roller 305 as a backup member around which the intermediate transfer belt 108 facing the secondary transfer roller 109 is wound. Repulsive transfer may be performed by applying a voltage having the same polarity as the charging polarity.

FIG. 2 is an enlarged view of the vicinity of the secondary transfer roller 109. A light-reflecting sheet detection sensor 301 as a sheet detection means is provided in a transport path on the upstream side of the secondary transfer nip, which is the transfer position. Specifically, the distance Y is separated from the relay roller 216 adjacent to the resist roller 111 from the upstream side in the transport direction. The sheet detection sensor 301 is provided at a position closer to the resist roller 111 side than the center of the distance Y.

In the example of FIG. 2, only one relay roller 216 is used between the paper feed roller pair 214 and the resist roller 111. Upstream from the relay roller 216, the sheet transfer path from the double-sided outlet roller 215 to the sheet and the sheet transfer path from the paper feed roller 213 from the peripheral device also merge. The transport path from the manual paper feed roller 212 joins between the relay roller 216 and the resist roller 111.

FIG. 3 is an explanatory view of a contact / detachment mechanism for bringing the intermediate transfer belt 108 and the secondary transfer roller 109 into contact with each other, and FIG. 3 (a) shows a position (evacuation position) where the secondary transfer roller 109 is separated from the intermediate transfer belt 108. ) Is shown. FIG. 3B shows a transfer state in which the secondary transfer roller 109 is in a transfer position where it is in pressure contact with the intermediate transfer belt 108 at a predetermined pressure. A urging force is applied to the secondary transfer roller 109 so as to be directed toward the secondary transfer opposed roller 305 by a spring 310b which is an urging means. A predetermined transfer pressure can be applied from the secondary transfer roller 109 to the paper S and the intermediate transfer belt 108 by the spring 310b.

The intermediate transfer belt 108 and the secondary transfer roller 109 can be freely brought into contact with each other within a certain range by a contact / detachment mechanism 302 composed of a stepping motor 303, an eccentric cam 309, and the like. Eccentric cams 309 are installed coaxially with the secondary transfer counter roller 305 at both ends of the secondary transfer counter roller 305 in the axial direction. As for the shape of the eccentric cam 309, the portion where the distance between the rotation center of the eccentric cam 309 and the outer shape portion is the shortest is shorter than the diameter of the secondary transfer opposing roller 305. Further, the portion where the distance between the rotation center of the eccentric cam 309 and the outer shape portion is the longest is longer than the diameter of the secondary transfer opposed roller 305.

When the cam shaft 309a to which the eccentric cam 309 is attached rotates by the rotational driving force from the stepping motor 303, the eccentric cam 309 and the cam shaft 309a are D-cut so that the eccentric cam 309 also rotates at the same timing and at the same angle. It is attached by being fitted with a groove or the like. The rotational driving force of the stepping motor 303 is transmitted to the cam shaft 309a via the timing belt 304. Opposing bearings 310 are attached to both ends of the secondary transfer roller 109 in the axial direction so as not to hinder the rotation of the secondary transfer roller 109, and the eccentric cam 309 is abutted against the opposing bearing 310. ing.

Here, "the distance connecting the center of rotation of the eccentric cam 309 to the closest point (which changes depending on the rotation angle) of the eccentric cam 309 to the facing bearing 310 + the radius of the facing bearing 310" is defined as the distance L1 (θ). Further, "the radius of the secondary transfer opposing roller 305 + the thickness of the intermediate transfer belt 108 + the radius of the secondary transfer roller 109" is defined as the distance L2. When the rotation angle of the eccentric cam 309 is a rotation angle that satisfies the relationship of distance L1 (θ)> distance L2, the eccentric cam 309 comes into contact with the facing bearing 310, and the secondary transfer roller 109 is urged from the spring 310b. It is pushed down in the direction away from the intermediate transfer belt 108. On the other hand, when the rotation angle of the eccentric cam 309 is a rotation angle that satisfies the relationship of distance L1 <distance L2, the intermediate transfer belt 108 and the secondary transfer roller 109 come into contact with each other, and a predetermined transfer pressure is applied to the paper S. Will be added.

FIG. 4 is a block diagram showing a part of an electric circuit in the printer according to the embodiment. The main control unit 300 controls the drive of each device in the printer, and includes a CPU (Central Processing Unit), a RAM (Random Access Memory) as a data storage means, a ROM (Read Only Memory) as a data storage means, and the like. Have. Then, based on the program stored in the ROM, the drive of various devices is controlled and a predetermined arithmetic process is executed.

A seat detection sensor 301, a process motor 321 and a development bias power supply 311, a primary transfer bias power supply 312, and a resist clutch 313 are connected to the main control unit 300. A motor drive circuit 320 for driving the stepping motor 303 of the contact / disconnection mechanism 302 and a secondary transfer bias power supply 314 are also connected. Further, an operation display unit 315, a charging power supply 316, a fixing heater power supply 317, an optical writing control unit 318, an image information receiving unit 319, and the like are also connected.

The image information receiving unit 319 receives the image information sent from the personal computer or scanner operated by the user and sends it to the main control unit 300 or the optical writing control unit 318. The optical writing control unit 318 controls the drive of the optical writing device 107 based on the image information sent from the image information receiving unit 319.

In the present embodiment, the paper enters the secondary transfer nip, which is the transfer position between the intermediate transfer belt 108 and the secondary transfer roller 109 backed up by the secondary transfer opposed roller 305, or the rear edge of the paper is pressed after transfer. The contact / detachment mechanism 302 is controlled as follows in order to suppress the occurrence of shock jitter when the secondary transfer nip is removed. That is, prior to the paper entering the secondary transfer nip, the secondary transfer roller 109 is forcibly moved so as to be separated from the intermediate transfer belt 108 by the rotational drive of the eccentric cam 309 included in the contact / detachment mechanism 302. As a result, a minute gap is formed between the secondary transfer roller and the intermediate transfer belt, and the generation of shock jitter when the paper enters the secondary transfer nip is suppressed. Immediately after the tip of the paper is brought into the minute gap, the forced movement of the secondary transfer roller 109 is released by the rotational drive of the eccentric cam 309, and the secondary transfer roller 109 is intermediately transferred by the urging force of the spring 310b. Press toward the belt 108. As a result, the secondary transfer roller 109 comes into contact with the intermediate transfer belt 108, and the secondary transfer nip exerts a sufficient transfer pressure during the transfer process. Then, the secondary transfer roller 109 is separated from the intermediate transfer belt 108 by the rotational drive of the eccentric cam 309 between the time when the image on the intermediate transfer belt 108 is transferred to the paper and the time when the paper passes through the secondary transfer nip. Forcibly move it. As a result, the occurrence of shock jitter when the rear edge of the paper is pulled out from the secondary transfer nip is suppressed.

FIG. 5 is a timing chart showing a control example of the contact / detachment mechanism 302 according to the comparative example, and FIG. 6 is a flowchart of the control. In this comparative example, both the start of the separation operation and the start of the contact operation are determined based on the image writing start timing as the image formation start timing. Specifically, it is as follows. First, before starting the image forming operation, the secondary transfer roller is kept away from the intermediate transfer belt 108 from the viewpoint of preventing stains. At the start of writing, ON of the write start signal is detected _{at T 50 (S61).}

Then, the contact start timing T ₅₁ (1 _{second after T 50} ) is calculated from the distance of the moving path of the peripheral surface of the photoconductor and the surface of the intermediate transfer belt from the writing position to the secondary transfer nip and the transport speed (S62). ). Subsequently, the separation start timing T _{52 (T 50} from t2 seconds after), calculates the distance to the same secondary transfer distance obtained by adding the theoretical length of the sheet from the conveying speed (S63). Then, when the calculated contact start timing T ₅₁ is reached, the contact is started by the rotational drive of the eccentric cam 309 (S64), and when the separation start timing T ₅₂ is reached after the transfer is completed, the separation is started by the rotational drive of the eccentric cam 309. Is started (S65).

In this comparative example, the separation start timing is calculated using the theoretical length of the paper, and if an error occurs in the actual paper length with respect to the theoretical paper length, the length of the error is the separation timing. If it is left as it is, it will lead to a problem. It will be described in detail later.

FIG. 7 is a timing chart showing a control example of the contact / detachment mechanism 302 according to the embodiment, and FIG. 8 is a flowchart of the control thereof. In this control example, the separation operation start timing in the comparative example is changed from the write start timing reference to the signal reference when the sensor installed in the transport path is pulled out from the rear edge of the paper. First, as in the timing of the comparative example, ON is detected _{at T 70 at the start of writing (S81).} After that, the contact start timing T ₇₁ (1 _{second after T 70} ) is calculated from the distance of the moving path of the peripheral surface of the photoconductor and the surface of the intermediate transfer belt from the writing position to the secondary transfer nip and the transport speed (S82). ). Next, when the calculated time T ₇₁ is reached, the paper rushes into the secondary transfer nip, so that the contact operation is started (S83).

Subsequently, the sheet detection sensor 301 installed in the transport path on the upstream side of the transfer nip detects the timing at which the paper comes out, and uses this signal to use this signal at the time T ₇₂ and the secondary transfer nip from the sheet detection sensor 301. from a distance and the conveying speed of up, to determine _(from the _{T 72} after t3 sec) spaced start timing _T 73 (S84). Finally, when the time T ₇₃ is reached, the separation start operation is performed (S85). _{The calculated time T 71} when the paper enters the secondary transfer nip and _{the timing T 72} when the sheet detection sensor 301 detects the paper coming out may be reversed depending on the paper type.

According to this control method, unlike the comparative example, the defect of the comparative example does not occur, and in addition, there are various advantages. It will be described in detail later.

FIG. 9 is an explanatory diagram of defects in the comparative example, and shows the deviation of the separation timing due to the deviation of the actual length of the paper from the ideal length. The horizontal axis is time, and the time when the upper and lower circles are in contact with the paper indicates the separation start timing, and the time when the upper and lower circles are separated from the paper indicates the separation completion time. In FIG. 9A, the timing T ₉₁ indicated by the broken line is the separation start timing, and the timing T ₉₂ indicated by the alternate long and short dash line is the separation completion timing. FIG. 9A shows a case where the paper length at which the separation completion timing T ₉₂ is immediately before the timing at which the rear end of the paper is pulled out is as per the theoretical value. Separation is started before the rear edge of the paper is pulled out, and when the rear edge of the paper is pulled out of the secondary transfer nip, it indicates that the secondary transfer roller 109 and the intermediate transfer belt 108 are completely separated from each other.

FIG. 9B is an explanatory diagram when the paper length is shorter than the ideal length. The upper part shows the separation start timing and the separation completion timing in the control of the comparative example (the separation start timing is determined based on the writing start timing). The lower part shows the separation start timing and the separation completion timing in the control of the embodiment (the separation start timing is determined based on the disconnection timing reference of the sheet detection sensor 301 on the transport path). In the control of the upper comparative example, the separation start position is determined based on the writing start timing, so if the paper becomes short, it will deviate from the ideal separation position, and the paper will come out in a state where it is not sufficiently separated. It ends up. As a result, the impact when the paper is pulled out becomes large.

On the other hand, in the control of the lower embodiment, the sheet detection sensor 301 on the transport path determines the separation start position using the signal when the rear edge of the paper is actually pulled out (based on the timing of pulling out). Even if the paper is shortened, the separation start timing is based on the time when the rear end of the paper passes through the sheet detection sensor 301, so that the change in length can be corrected. Therefore, when the paper comes out of the secondary transfer nip, the separation can be completed, and the impact when the paper comes out can be reduced.

Such an error in the paper length (ideal length deviation) may be present in the paper itself even before the image is formed, but in the paper toward the secondary transfer on the second side when performing double-sided printing. Can also occur. In double-sided printing, after passing through the fixing roller pair of the fixing device 113 once, it passes through the double-sided outlet roller 215 relay roller 216 and the resist roller 111, and then re-enters the secondary transfer roller 109. When passing through the fixing roller once, the fixing heat is high, so that the paper shrinks depending on the type of paper. As a result, the paper length may be shortened during double-sided printing. In the present embodiment, the sheet detection sensor 301 on the transport path detects the rear edge of the paper, and the separation start position is determined from the signal information, so that there is no error due to shrinkage.

As described above, in the present embodiment, the timing of moving to the transfer position and the timing of moving to the retracted position are determined by using different timing information. Specifically, the former uses information on the image formation start timing, and the latter uses the detection result of the sheet detecting means arranged on the upstream side of the transfer position in the transport direction to determine each. Obtain information for the timing of movement to the transfer position and the placement position of the sheet detection means for detecting the passing position information of the rear end position of the sheet for determining the timing of movement to the latter retracted position. It can be set without considering that. Therefore, it can be set at a location away from the transfer position, which makes it relatively easy to secure an installation space for the sheet detecting means. Since the distance to the transfer position can be secured, it is possible to cope with high speed. In particular, when the sheet detecting means is on the upstream side of the resist conveying member, the resist conveying member temporarily stops the sheet and then starts re-transporting at a predetermined timing, so that the sheet detecting means is arranged upstream of the resist conveying member. Where the information of the sheet tip detection by the sheet detecting means cannot be used for determining the timing of movement to the transfer position, the timing of movement to the transfer position can be determined by using the information of the image formation start timing.

Further, since the timing of moving to the retracted position is determined by detecting the rear edge of the actual paper, the shock jitter can be reduced even if the paper length varies.

Further, in the example of FIG. 2, a relay roller 216 is provided as a second sheet transport member on the upstream side of the resist roller 111 which is the first sheet transport member, and the sheet detection sensor 301 is connected to the resist roller 111 in the sheet transport path. Since it is arranged closer to the resist roller 111 than the center between the relay rollers 216, the rear end can be detected in a state where the resist roller 111 holds the portion in front of the rear end and the posture is stable. Therefore, it is possible to reduce the variation in the timing of pulling out the rear edge due to the variation in the posture of the paper.

As the image formation start timing when creating a full color, any one of the four image forming units (image forming units) 101Y, 101M, 101C, and 101K can be used.
Further, in the above example, the position where the secondary transfer roller 109 is separated from the intermediate transfer belt 108 is used as the evacuation position, but instead of this, the position where the secondary transfer roller 109 is in contact with the intermediate transfer belt 108 is set as the evacuation position. You may use it. In this case, if the pressure contact force is smaller in the retracted position than in the transfer position, the shock jitter can be reduced.

Further, although the example applied to the detachment operation in the secondary transfer apparatus in the image forming apparatus of the intermediate transfer method has been described, it can also be applied to the attachment / detachment operation of the direct transfer apparatus that directly transfers from the photoconductor to the paper. Various modifications are possible.

FIG. 10 shows an example of a factor that causes a shock jitter when the rear edge of the paper passes through the secondary transfer nip. In this example, the secondary transfer facing roller 305 includes an elastic layer. Therefore, it can be elastically deformed in the thickness direction. The paper is present during the transfer on the left side of the figure, and the elastic layer is deformed so as to be compressed by the thickness of the paper. After that, when the rear end of the paper passes through the secondary transfer nip, the thickness of the elastic layer is restored by the thickness of the paper, and at that time, the intermediate transfer belt 108 is pulled and the belt linear velocity increases in the vicinity A of the secondary transfer nip. To do. The increase in the linear velocity of the intermediate transfer belt 108 extends to the primary transfer region B, causing a shock jitter in the toner image. The cause of the occurrence of shock jitter is not limited to the use of a roller provided with an elastic layer in this way.

100: Paper ejection tray 101: Image forming unit 101C: Image forming unit 101K: Image forming unit 101M: Image forming unit 101Y: Image forming unit 102: Photoreceptor drum 103: Charger 104: Developing device 105: Cleaning device 106: Transfer Roller 107: Optical writing device 108: Intermediate transfer belt 109: Secondary transfer roller 111: Resist roller 112: Conveying belt 113: Fixing device 114: Feeding unit 114a: Accommodating tray 115: Sheet reversing unit 116: Double-sided reversing unit 117 : Refeed path 118: Switching claw 119: Switching claw 120: Inverting inlet roller 121: Paper ejection reversing roller 130: Paper ejection relay roller 131: Paper ejection roller 212: Manual paper feed roller 213: Paper feed roller 214: Paper feed Roller pair 215: Double-sided outlet roller 216: Relay roller 300: Main control unit 301: Sheet detection sensor 302: Contact / detachment mechanism 303: Stepping motor 304: Timing belt 305: Secondary transfer Opposing roller 309: Eccentric cam 309a: Cam shaft 310 : Opposed bearing 310b: Spring 311: Development bias power supply 312: Primary transfer bias power supply 313: Resist clutch 314: Secondary transfer bias power supply 315: Operation display unit 316: Charging power supply 317: Fixing heater power supply 318: Optical writing control unit 319 : Image information receiving unit 320: Motor drive circuit 321: Process motor L1: Distance L2: Distance S: Paper T50: Writing start T51: Contact start timing T52: Separation start timing T70: Writing start T71: Contact start timing T72: Timing T73: Separation start timing T91: Timing T92: Timing T92: Separation completion timing Y: Distance

Japanese Unexamined Patent Publication No. 8-314297

Claims (11)

In an image forming apparatus that moves a transfer member that brings a sheet into contact with an image carrier for transfer between a transfer position and a retracted position retracted from the transfer position.
The feature is that the movement to the transfer position is performed based on the image formation start timing, and the movement to the retract position is performed based on the detection result of the sheet detection means arranged on the upstream side of the transfer position in the sheet transport direction. Image forming apparatus. In the image forming apparatus of claim 1,
The image forming apparatus is characterized in that the sheet detecting means is arranged on the upstream side of the first sheet conveying member arranged on the upstream side. In the image forming apparatus of claim 2,
The second sheet conveying member is provided on the upstream side of the first sheet conveying member, and the sheet detecting means is said to be more than the center between the first sheet conveying member and the second sheet conveying member in the sheet conveying path. An image forming apparatus characterized in that it is arranged at a location close to the first sheet transport member. In the image forming apparatus of claim 2 or 3,
The image forming apparatus, wherein the first sheet transporting member is a resist transporting member. In the image forming apparatus according to any one of claims 1 to 4.
An image forming apparatus comprising a plurality of image forming portions for forming an image to be transferred to a common sheet, and using an image forming start timing in any of the plurality of image forming portions as the image forming start timing. In the image forming apparatus according to any one of claims 1 to 5,
An image forming apparatus characterized in that the detection result of the sheet detecting means is a detection result of passing through the rear end of the sheet. In the image forming apparatus according to any one of claims 1 to 6,
An image forming apparatus, wherein the image forming start timing is an image writing start timing by an image writing means. In the image forming apparatus according to any one of claims 1 to 7.
The toner image obtained by developing the latent image formed on the latent image carrier is transferred to the intermediate transfer body, and then the toner image is transferred from the intermediate transfer body to the sheet, and an endless belt member is used as the intermediate transfer body. An image forming apparatus characterized by. In the image forming apparatus of claim 8,
An image forming apparatus characterized in that at least one of a backup member that backs up the endless belt member at a transfer position and the transfer member is elastically deformable in the thickness direction. In the image forming apparatus according to any one of claims 1 to 9.
The image forming apparatus is characterized in that the transfer position is a position where the sheet is pressed against the image carrier at a predetermined pressure, and the retracted position is a position where the sheet comes into contact with the image carrier at a pressure smaller than the predetermined pressure. .. In the image forming apparatus according to any one of claims 1 to 10.
An image forming apparatus having a double-sided function of forming an image on one surface of a sheet, fixing the image on one surface of the sheet, and then fixing the image on the opposite surface of the sheet.

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