Sheet conveying device and image forming apparatus

Abstract

To reduce buckling of a sheet between a preregistration roller pair and a registration roller pair when a sheet is conveyed to a secondary transfer part.SOLUTION: A sheet conveyance device has a control part for controlling a preregistration roller pair and a registration roller pair on the downstream side in a sheet conveyance direction of the preregistration roller pair, in which the control part conveys the sheet by the preregistration roller pair, brings the tip of the sheet into contact with the registration roller pair stopping rotating, forms a first loop between the preregistration roller pair and the registration roller pair, sets the conveyance speed of the preregistration roller pair to be lower than that of the registration roller pair until the tip of the sheet reaches a transfer conveyance part after the registration roller pair starts rotating, and forms a second loop smaller than the first loop on the sheet between the preregistration roller pair and the registration roller pair.SELECTED DRAWING: Figure 4

Description

[Technical field]

[0001] The present invention relates to a sheet transport device that transports a sheet and an image forming apparatus including the same. [Background technology]

[0002] 2. Description of the Related Art Conventionally, in an image forming apparatus that forms an image on a sheet, a skew correction mechanism that corrects skew of a sheet being conveyed is provided in order to align the position of the sheet with that of the image formed on the sheet.

[0003] In Patent Document 1, a pre-registration roller pair provided upstream in the sheet conveying direction of a registration roller pair whose driving is stopped conveys the sheet a predetermined distance to correct the skew of the sheet. That is, the leading edge of the sheet conveyed by the pre-registration roller pair is abutted against the registration roller pair, and a loop is formed in the sheet between the pre-registration roller pair and the registration roller pair to correct the skew of the sheet. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-127753 Summary of the Invention [Problem to be solved by the invention]

[0005] In Patent Document 1, after the skew has been corrected, the registration roller pair and the pre-registration roller pair start conveying the sheet at the same speed by re-driving the registration roller pair in synchronization with an image signal, and the sheet is conveyed to the secondary transfer section. In this case, the sheet conveyed from the registration roller pair to the secondary transfer section is conveyed in a state in which the size of the loop formed between the pre-registration roller pair and the registration roller pair is maintained.

[0006] However, the optimum value of the size of the loop of the sheet for correcting the skew and the optimum value of the size of the loop of the sheet to be maintained during the transport from the registration roller pair to the secondary transfer portion are different.

[0007] In other words, the size of the loop of the sheet for skew correction is necessary to correct the skew of the sheet before it reaches the pair of registration rollers, and is determined in advance based on the largest amount of skew that should be assumed. Furthermore, when loop formation control is performed using a detection signal from a sensor immediately before the pair of registration rollers as a trigger, the skewed sheet is detected, and the size is determined by taking into account the variation in the amount of the detection signal.

[0008] On the other hand, if the size of the loop of the sheet maintained during conveyance is too large, the loop will buckle between the pre-registration roller pair and the registration roller pair, so it is preferable that the size of the loop of the sheet maintained during conveyance is small within a range in which the sheet is not pulled by the pre-registration roller pair and the registration roller pair.

[0009] Therefore, when the sheet is transported from the registration roller pair to the secondary transfer section while maintaining the size of the sheet loop for skew correction, there is a risk that the sheet will buckle due to the large size of the above-mentioned loop between the roller pair.

[0010] SUMMARY OF THE PRESENT EMBODIMENT An object of the present invention is to reduce buckling of a sheet between a pre-registration roller pair and a registration roller pair when the sheet is transported to a secondary transfer portion. [Means for solving the problem]

[0011] A representative configuration of the present invention for achieving the above object is a sheet transporting device that transports a sheet to a transfer transport section that transfers a toner image carried on an image carrier to the sheet, the sheet transporting device having a pair of pre-registration rollers that form a nip portion and transport the sheet by sandwiching it, a pair of registration rollers that are provided downstream in the sheet transport direction from the pair of pre-registration rollers and that form a nip portion and transport the sheet by sandwiching it, and a control unit that controls the pair of pre-registration rollers and the pair of registration rollers, the control unit controlling the pair of pre-registration rollers to transport the sheet by bringing a leading edge of the sheet into contact with the nip portion of the pair of registration rollers that have stopped rotating, and controlling the pair of pre-registration rollers and the pair of registration rollers to transfer the toner image carried on an image carrier to the sheet by sandwiching it between the pair of pre-registration rollers and the pair of registration rollers. a first loop is formed by bending the sheet toward one side between the pair of rollers, and a sheet conveying speed by the pair of pre-registration rollers is made substantially equal to a sheet conveying speed by the pair of registration rollers from when the pair of registration rollers starts conveying the sheet until the sheet is conveyed a predetermined distance, and further, a sheet conveying speed by the pair of pre-registration rollers is made slower than the sheet conveying speed by the pair of registration rollers from when the pair of registration rollers has conveyed the sheet the predetermined distance until the leading edge of the sheet reaches the transfer conveying section, thereby forming a second loop smaller than the first loop in the sheet between the pair of pre-registration rollers and the pair of registration rollers. and after conveying the sheet by a predetermined distance, the sheet conveying speed by the pre-registration roller pair is set to a conveying speed slower than the sheet conveying speed by the registration roller pair, and the sheet is accelerated to reach a conveying speed that is faster than the sheet conveying speed by the transfer conveying unit and is the same as the sheet conveying speed by the registration roller pair at a timing later than the timing of the registration roller pair. The present invention is characterized by the above. Effect of the Invention

[0012] According to the present invention, when the sheet is transported to the transfer transport section, the loop of the sheet between the pre-registration roller pair and the registration roller pair becomes small, so that buckling of the sheet therebetween can be reduced. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 illustrates a sheet conveying unit in an image forming apparatus. [Diagram 2] (a), (b), and (c) are diagrams showing the operation of correcting the skew of a sheet [Diagram 3] Block diagram of a control unit of an image forming apparatus [Figure 4] FIG. 1A is a diagram showing the positions of the leading and trailing ends of a sheet in a sheet conveying section, and FIG. 1B is a diagram showing the relationship between the conveying speed of a roller and time. [Diagram 5] 1A and 1B are diagrams showing the loop shape of a sheet caused by a sheet conveying unit; [Figure 6] FIG. 13 is a diagram showing a loop shape of a sheet produced by a sheet conveying unit of a comparative example; [Figure 7] (a) and (b) Graph showing the relationship between roller transport speed and time [Figure 8] FIG. 1A is a diagram showing the positions of the leading and trailing ends of a sheet in a sheet conveying section, and FIG. 1B is a diagram showing the relationship between the conveying speed of a roller and time. [Figure 9] FIG. 1 is a schematic cross-sectional view showing an overall configuration of an image forming apparatus. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] Hereinafter, preferred embodiments of the present invention will be described with reference to the drawings. However, the components described in the following description are merely examples, and the scope of the present invention is not limited to these.

[0015] First, a schematic configuration of an image forming apparatus to which a sheet conveying device is applied will be described with reference to Fig. 9. Fig. 9 is a schematic cross-sectional view showing a schematic configuration of a color digital printer as an example of an image forming apparatus.

[0016] The image forming apparatus 1 shown in FIG. 9 mainly includes an image forming section, a secondary transfer section, and a sheet conveying device (sheet conveying section).

[0017] The image forming section of the image forming apparatus 1 includes photoconductors 11 corresponding to the respective colors of yellow (Y), magenta (M), cyan (C), and black (Bk), a charging device 12, an exposure device 13 functioning as an image writing section, and a developing device 14. The image forming section also includes an intermediate transfer belt (image carrier) 21, a secondary transfer inner roller 22, and a primary transfer device 25.

[0018] In the image forming section, the surface of the photoconductor 11 is uniformly charged in advance by the charging device 12, and the exposure device 13 is driven based on the image information signal to form a latent image on the surface of the rotating photoconductor 11. The electrostatic latent image formed on the surface of the photoconductor 11 is developed by the developing device 14 with toner and becomes visible as a toner image. Thereafter, a predetermined pressure and electrostatic bias are applied by the primary transfer device 25, and the toner image is transferred onto the intermediate transfer belt 21.

[0019] Next, the intermediate transfer belt 21, which is an image carrier, will be described. The intermediate transfer belt 21 is stretched by rollers such as a drive roller 23, a tension roller 24, and a secondary transfer inner roller 22, and is driven to be transported in the direction of arrow B shown in FIG. 9. The image forming processes of each color Y, M, C, and Bk, which are processed in parallel, are performed at a timing to overlap the toner image of the upstream color that has been primarily transferred onto the intermediate transfer belt 21. As a result, a full-color toner image is finally formed on the intermediate transfer belt 21, and this toner image on the intermediate transfer belt 21 (on the image carrier) is transported to the secondary transfer section.

[0020] Here, the intermediate transfer belt 21 is exemplified as an image carrier that carries a toner image, but the image carrier is not limited to this. For example, in an image forming apparatus in which a toner image formed on a photoreceptor is directly transferred to a sheet, the photoreceptor is the image carrier.

[0021] The transfer material, that is, sheets P, are stored, for example, by size in paper feed cassettes (sheet storage cabinets) 31, 32 and manual feed section 33, and in order to record an image in an appropriate range, it is necessary to have the image forming device main body recognize the size of the stored sheets.

[0022] The paper feed cassettes 31 and 32 are provided with sheet size detection units 31d and 32d for detecting the size of the sheets stored therein. The sheet size detection units 31d and 32d are composed of a size detection lever (cam) and multiple sensors or switches. The detection lever (cam) is provided rotatably and is in sliding contact with and interlocked with a side regulation plate that regulates the position of the sheet in the width direction perpendicular to the conveying direction. The multiple sensors or switches are provided at positions corresponding to the size detection lever in the mounting section where the paper feed cassette is mounted.

[0023] Therefore, when the side regulating plate is moved to match the widthwise end of the sheet, the size detection lever rotates in tandem. When the paper feed cassette is attached to the attachment section in this state, the size detection lever selectively turns on / off the detection element of a sensor or switch arranged in the attachment section where the paper feed cassette is attached. This causes signals of different patterns to be sent from the sensor or switch to the image forming device main body. The image forming device main body can then recognize the size of the sheets stored in the paper feed cassette based on the signals.

[0024] The sheets P are fed one by one from, for example, the paper feed section 31a (or the paper feed section 32a). The sheet P has its skew corrected by the pre-registration roller pair 41 and the registration roller pair 42 that constitute the sheet feed section (sheet feeder), and is sent to the secondary transfer section. Specifically, the sheet P is transported by the pre-registration roller pair 41, and its leading edge is abutted against the nip portion of the registration roller pair 42 that is stopped. In this way, the pre-registration roller pair 41 loops the sheet P between itself and the stopped registration roller pair 42, and corrects the skew of the sheet.

[0025] The pair of registration rollers 42 transports the sheet P to the secondary transfer portion in accordance with the timing at which the toner image on the intermediate transfer belt 21 is transferred to the sheet P.

[0026] The secondary transfer section has a toner image transfer nip section formed by the secondary transfer inner roller 22 and the secondary transfer outer roller 44 which face each other via the intermediate transfer belt 21, and transfers a toner image onto the sheet P by applying a predetermined pressure and electrostatic bias. The secondary transfer outer roller 44 is a transfer conveyance section which transfers the toner image carried on the intermediate transfer belt 21, which is an image carrier, onto the sheet P, and is provided downstream in the sheet conveyance direction from the registration roller pair 42. The secondary transfer outer roller 44 faces the secondary transfer inner roller 22 via the intermediate transfer belt 21 in the secondary transfer section, and conveys the sheet by nipping it between it and the secondary transfer inner roller 22.

[0027] After the transfer, the sheet P is transported to the fixing device 50, where pressure and heat are applied to melt and fix the toner image to the sheet P. After the fixing, the sheet P is transported to the paper discharge transport section 60, and the transport path is switched by a flapper 64 depending on whether the paper passing mode is single-sided or double-sided. That is, in the single-sided mode, the flapper 64 switches to the paper discharge roller 62 side, and the sheet P is discharged by the paper discharge roller 62 toward the paper discharge tray 80 and stacked there.

[0028] On the other hand, in the case of the double-sided mode, the flapper 64 is switched to the discharge roller 63 side, and the sheet P is conveyed by the discharge roller 63 to the reversing roller 71, which then discharges the sheet P to the discharge tray 82. The sheet P is then stopped once with its rear end remaining a predetermined distance on the reversing roller 71. Thereafter, the sheet P is conveyed again to the registration roller pair 42 via the double-sided conveying section 70 by the reverse rotation of the reversing roller 71, and an image is formed and fixed on the second side in the image forming section.

[0029] After the fixing, the sheet P passes through a flapper 64 and is discharged by a discharge roller 62 or a reversing roller 71 toward a discharge tray 80 or a discharge tray 82, where it is stacked.

[0030] Next, a sheet conveying device applied to an image forming apparatus will be described with reference to Fig. 1. Fig. 1 is a cross-sectional view showing a sheet conveying device in an image forming apparatus. A sheet conveying section 40 shown in Fig. 1 is provided in a sheet conveying path connecting paper feed cassettes 31 and 32 and an image forming section in an image forming apparatus 1, and is a sheet conveying device that corrects skew of a sheet. Fig. 2 is a schematic diagram for explaining the skew correction operation of a sheet by the sheet conveying section 40.

[0031] The sheet conveying section 40 corrects skew of the sheet P and conveys the sheet P to a secondary transfer section where the toner image carried on the intermediate transfer belt 21 is transferred. The sheet conveying section 40 has a pre-registration roller pair 41 and a registration roller pair 42. The sheet conveying section 40 further has a registration sensor 43. The pre-registration roller pair 41 and the registration roller pair 42 are controlled by a control section 200 (CPU 201) described later.

[0032] The pre-registration roller pair 41 is a pair of conveying rollers that sandwich and convey the sheet P. The pre-registration roller pair 41 includes a driven roller 41a having a roller made of polyacetal (POM) and a driving roller 41b formed of a rubber roller. In the pre-registration roller pair 41, the driven roller 41a and the driving roller 41b are arranged facing each other. The driven roller 41a is pressed against the driving roller 41b by the elastic force of a spring (not shown), and rotates following the driving roller 41b, which is driven to rotate.

[0033] The registration roller pair 42 is a pair of conveying rollers that is provided downstream of the pre-registration roller pair 41 in the sheet conveying direction and conveys the sheet P by pinching it between them. The registration roller pair 42 includes a driven roller 42a having a roller made of polyacetal (POM) and a driving roller 42b formed of a rubber roller. The registration roller pair 42 is arranged such that the driven roller 42a and the driving roller 42b face each other. The driven roller 42a is pressed against the driving roller 42b by the elastic force of a spring (not shown), and rotates following the driving roller 42b, which is driven to rotate.

[0034] The sheet conveyed by the pre-registration roller pair 41 is brought into contact with a contact portion (nip portion) where the stopped driven roller 42a and driving roller 42b come into contact with each other so that the leading edge of the sheet is aligned with the contact portion, thereby correcting the skew.

[0035] The registration sensor 43 is a sheet detection unit for detecting the sheet P. The registration sensor 43 is disposed between the pre-registration roller pair 41 and the registration roller pair 42, immediately before the registration roller pair 42. The registration sensor 43 is also disposed in the center (dotted chain line shown in FIG. 2) in the width direction (thrust direction) perpendicular to the conveying direction of the sheet P (the direction of the arrow A in FIG. 2).

[0036] 1, reference numeral 101 denotes a pre-registration motor which is a first drive source for driving the drive roller 41b of the pre-registration roller pair 41. Reference numeral 102 denotes a registration motor which is a second drive source for driving the drive roller 42b of the registration roller pair 42.

[0037] Guides 45 and 46 for guiding the conveyed sheet are disposed between the pre-registration roller pair 41 and the registration roller pair 42. The guides 45 and 46 are partially spaced apart from each other and are configured to allow a loop to be formed by the sheet abutting against the nip portion of the registration roller pair 42.

[0038] Specifically, one guide 45 guides one surface side of the sheet being conveyed. The other guide 46 is provided opposite to the one guide 45 and guides the other surface side of the sheet being conveyed. Here, the one guide 45 is partially spaced apart from the other guide 46. Therefore, the sheet conveyed by the pre-registration roller pair 41 and having its leading edge abutted against the nip portion of the registration roller pair 42 forms a loop between the roller pairs 41 and 42, moving away from the other guide 46 and approaching the one guide 45. In other words, the sheet forms a loop between the roller pairs 41 and 42, bending the sheet toward one surface side.

[0039] Next, the operation of correcting skew of a sheet by the sheet conveying section 40 will be described with reference to the schematic diagram of FIG.

[0040] The skew of a sheet refers to a state in which one side of the leading edge of the sheet in the width direction leads the other side with respect to the center of the sheet in the width direction perpendicular to the conveying direction (the dashed line in the figure). Here, as shown in Fig. 2(a), the left side of the leading edge of the sheet in the width direction leads the right side, in other words, the sheet is skewed clockwise with respect to the conveying direction.

[0041] As shown in Fig. 2(a), the skew correction operation will be described when the sheet is skewed clockwise with respect to the conveying direction (the direction of the arrow A). Fig. 2(a) shows a state in which the leading edge of the sheet P has reached the registration sensor 43. When the pre-registration roller pair 41 further rotates and the sheet P is conveyed in the conveying direction, the leading edge on the left side of the sheet P abuts against the nip portion of the registration roller pair 42. At this time, the registration roller pair 42 stops rotating.

[0042] Furthermore, when the pre-registration roller pair 41 rotates and the sheet P is conveyed in the conveying direction, as shown in FIG. 2B, the entire leading edge of the sheet P comes into contact with the nip portion of the registration roller pair 42 across the width. At this time, the leading edge of the sheet P comes into contact with the nip portion of the registration roller pair 42 and is stopped, and a loop is gradually formed in the sheet, and the sheet turns so that the corner on the widthwise opposite side to the corner that comes into contact with the nip portion approaches the nip portion. Then, a loop is formed between the registration roller pair 42 and the pre-registration roller pair 41 by bending the sheet P to one side. As a result, the entire leading edge of the sheet P in the width direction comes into contact with the nip portion of the registration roller pair 42, and the skew is corrected.

[0043] Here, when correcting the skew of the sheet, the amount of loop formed in the sheet is appropriately set based on the size, basis weight, etc. of the sheet (hereinafter referred to as sheet information). The control unit 200 shown in Fig. 3 can determine the optimal amount of loop based on at least one of the sheet information designated by the user via the operation unit 203 and the sheet information detected by the sheet size detection units 31d and 32d, or a combination of these.

[0044] Thereafter, in the image forming section, a toner image is formed on the intermediate transfer belt 21. Then, in synchronization with this, the registration roller pair 42 and the pre-registration roller pair 41 rotate, and the sheet P is conveyed to the secondary transfer section in a state in which the skew of the sheet P is corrected, as shown in FIG.

[0045] Here, before the leading edge of the sheet reaches the secondary transfer section, the control section 200 sets the conveying speed of the pre-registration roller pair 41 to be slower than that of the registration roller pair 42, thereby reducing the loop of the sheet formed between the roller pairs 41 and 42. This is a characteristic operation of the present invention, and the reason for this will be described.

[0046] When the registration sensor 43 is disposed at the center in the thrust direction, the loop amount r of the sheet P at the same thrust position as the registration sensor 43 is 1C is a predetermined amount as described above. As shown in FIG. 2(b), the loop amount r 1R , the loop amount r at the delayed end 1F Then, the relationship of the loop amount is r 1R >r 1C >r 1F In other words, the amount of loop on the leading side of the sheet P is the amount of loop r 1C will be greater than.

[0047] A predetermined loop space is provided between the guide 45 and the guide 46, assuming the amount of loop of the sheet P formed between the pre-registration roller pair 41 and the registration roller pair 42. However, if the amount of skew of the sheet P before reaching the registration roller pair 42 is large, the amount of loop may be larger than expected. In this case, if the force received from the sheet P exceeds the force with which the registration roller pair 42 holds the sheet P, the sheet slips at the nip portion of the registration roller pair 42, causing the sheet to skew or to wrinkle. Alternatively, the loop of the sheet may buckle, generating a buckling sound. In addition, the load torque of the pre-registration motor may increase, causing the motor to stop.

[0048] In this embodiment, which has been made in consideration of such a situation, the loop amount r required for correcting the skew of the sheet is 1CAfter forming the registration roller pair 42, the amount of loop is r 2C In other words, the control unit 201 conveys the sheet by the pre-registration roller pair 41, and causes the leading edge of the sheet to come into contact with the registration roller pair 42, which is at rest. Then, the sheet is deflected toward one side between the pre-registration roller pair and the registration roller pair to form a first loop (loop amount r 1C ) is formed. Furthermore, the control unit 200 sets the conveying speed of the pre-registration roller pair 41 to be slower than that of the registration roller pair 42 during the period from when the registration roller pair 42 starts to rotate until the leading edge of the sheet reaches the secondary transfer unit. Then, a second loop (loop amount r 2C This makes it possible to keep r2R, which is the maximum loop amount on the leading side, within the allowable range, and eliminates the risk of problems such as the aforementioned buckling of the sheet loop.

[0049] Next, a control unit of the image forming apparatus 1 equipped with the sheet conveying unit 40 according to this embodiment and a flow of the sheet skew correction operation and loop amount control performed by this control unit will be described with reference to Figures 3 and 4. Figure 3 is a block diagram for explaining the control unit of the image forming apparatus 1. Figure 4(a) is a time series graph of the trailing edge position of the preceding sheet and the leading edge position of the succeeding sheet conveyed by the pre-registration roller pair 41 and the registration roller pair 42. Figure 4(b) shows a driving line diagram of the pre-registration motor 101 and the registration motor 102, with the horizontal axis indicating time and the vertical axis indicating the sheet conveying speed of the pre-registration roller and the registration roller driven by the motor.

[0050] 3, the control unit 200 has various functional units such as a CPU (Central Processing Unit) 201, a memory 202, an operation unit 203, an image formation control unit 205, and a sheet conveyance control unit 206. The CPU 201 executes a predetermined control program and the like to realize various processes performed by the image forming apparatus 1. The memory 202 is, for example, a RAM (Random Access Memory) or a ROM (Read Only Memory), and stores various programs and various data in a predetermined storage area. The operation unit 203 accepts various operations performed by the user, such as various information (such as size, basis weight, and surface property information) related to the sheet used by the user for printing, and an instruction to execute or interrupt printing.

[0051] The image formation control unit 205 issues instructions to the image forming unit including the exposure device 13 and controls image formation. The sheet transport control unit 206 issues instructions to the pre-registration motor 101, the registration motor 102, etc. that drive various transport rollers, and controls the transport of the sheet P. The sensor control unit 207 controls the start or stop of detection by the registration sensor 43 etc., and receives the detection results of each of these sensors.

[0052] It should be noted that it is also possible to configure the printer 200 so that it can receive various information relating to the sheets used for printing via a computer 204 connected via a network, for example.

[0053] The CPU 201 of the control unit 200 controls the driving of the motors 101, 102 to control the operation of the pre-registration roller and the registration roller as shown in Figures 4(a) and 4(b). Here, the pre-registration roller refers to the drive roller 41b of the pre-registration roller pair 41 that rotates by receiving a driving force from the pre-registration motor 101. Also, the registration roller refers to the drive roller 42b of the registration roller pair 42 that rotates by receiving a driving force from the registration motor 102.

[0054] The CPU 201 controls the driving of the pre-registration motor 101 to convey the succeeding sheet by the pre-registration roller pair 41. At this time, the pre-registration roller pair 41 conveys the succeeding sheet at a conveying speed v1, and the registration roller pair 42 stops rotating. The leading edge of the succeeding sheet conveyed by the pre-registration roller pair 41 at the conveying speed v1 is detected by the registration sensor 43 at time t1, and then contacted with the nip portion of the registration roller pair 42, which is stopped, at time t2. Then, the CPU 201 stops the driving of the pre-registration motor 101 at time t3 to stop the pre-registration roller pair 41. As a result, a first loop (loop amount Xs shown in FIG. 5A) is formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 by bending the sheet toward one surface side. This first loop is shown as loop amount Xs in FIG. 4B. The loop amount Xs is a predetermined loop amount of the sheet, and is in the area indicated by the lattice portion in FIG. 4(b).

[0055] Thereafter, at time t4, the CPU 201 simultaneously starts driving the pre-registration motor 101 and the registration motor 102 in accordance with the toner image formed on the intermediate transfer belt 21. Then, the pre-registration roller pair 41 and the registration roller pair 42 simultaneously start conveying the succeeding sheet.

[0056] Then, the CPU 201 sets the pre-registration roller pair 41 to a conveying speed v2 slower than that of the registration roller pair 42 at time t5 during the period from when the registration roller pair 42 starts to rotate until the leading edge of the succeeding sheet reaches the secondary transfer portion. As a result, the loop amount Xs formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 is reduced by the loop amount Xr. That is, a second loop (loop amount Xs-Xr shown in FIG. 5B) smaller than the first loop (loop amount Xs shown in FIG. 5A) is formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42. Here, the loop amount Xr is the area indicated by the hatched portion in FIG. 4B, and indicates the difference in conveying amount between the registration roller pair 42 and the pre-registration roller pair 41.

[0057] Regarding the magnitude relationship between this loop amount Xr and the loop amount Xs of the first loop described above, by making Xs>Xr, it is possible to reduce the loop amount Xs-Xr of the second loop while preventing it from becoming zero.

[0058] After the first loop is formed, the CPU 201 accelerates the registration roller pair 42 to a conveying speed v3 that is faster than the conveying speed of the outer secondary transfer roller 44 when the registration roller pair 42 starts to rotate.

[0059] On the other hand, the CPU 201 sets the pre-registration roller pair 41 to the same conveying speed as the registration roller pair 42 from when the registration roller pair 42 starts to rotate until the sheet is conveyed a predetermined distance. That is, the CPU 201 accelerates the pre-registration roller pair 41 at the same conveying speed as the registration roller pair 42 from when the registration roller pair 42 starts to rotate until the sheet is conveyed a predetermined distance (from time t4 to time t5).

[0060] 4B, after conveying the sheet a predetermined distance, the CPU 201 sets the pre-registration roller pair 41 to a constant conveying speed v2 slower than that of the registration roller pair 42 at time t5 as described above. Then, at time t6, the pre-registration roller pair 41 is set to a conveying speed v3 that is faster than that of the outer secondary transfer roller 44 and is the same as that of the registration roller pair. In this way, by setting the pre-registration roller pair 41 to a conveying speed v3 faster than that of the outer secondary transfer roller 44, the productivity of the device can be improved.

[0061] During the time until the leading edge of the succeeding sheet that has formed the second loop reaches the secondary transfer portion (between time t6 and time t7), the CPU 201 sets the pre-registration roller pair 41 and the registration roller pair 42 to the same conveying speed v4 as the secondary transfer outer roller 44. That is, after setting the pre-registration roller pair 41 and the registration roller pair 42 to a conveying speed v3 faster than the secondary transfer outer roller 44 and conveying the sheet a certain distance, the CPU 201 decelerates the conveying speed to reach the same conveying speed v4 as the secondary transfer outer roller 44 by time t7.

[0062] As described above, according to this embodiment, the loop amount formed for skew correction is reduced by the time the sheet reaches the secondary transfer unit. That is, the first loop of the sheet formed between the roller pair 41, 42 for skew correction is made into a second loop smaller than the first loop by the time the leading edge of the sheet reaches the secondary transfer unit. This makes it possible to reduce the buckling noise of the sheet during sheet transport, reduce the torque of the motor that drives the rollers, and improve image printing accuracy with a simple configuration.

[0063] 5(a) and 5(b) show the loop shape of the sheet between the roller pair 41, 42 when conveyed by the above-mentioned operation. Fig. 5(a) is a cross-sectional view showing the loop shape of the sheet at the timing of time t4 in Fig. 4. Fig. 5(b) is a cross-sectional view showing the loop shape of the sheet at the timing of time t7 in Fig. 4. Fig. 6 is a cross-sectional view showing the loop shape of the sheet at the timing corresponding to time t7 in a comparative example to which this embodiment is not applied.

[0064] The loop amount Xs, which is the size of the loop of the sheet formed between the roller pairs 41 and 42 when the leading edge of the sheet abuts against the registration roller pair 42, is determined from various viewpoints. One of the viewpoints is the distance for the delayed side of the skewed sheet to reach the registration roller pair 42, that is, the distance based on the skew amount. Another viewpoint is the variation in the conveyance amount from the detection by the registration sensor 43 to the arrival at the registration roller pair 42. Another viewpoint is the amount by which the leading edge position of the sheet drops due to gravity when the sheet is conveyed vertically upward as in this embodiment. In this way, the loop amount Xs of the sheet between the roller pairs 41 and 42 described above is determined from various viewpoints. Therefore, the required loop amount Xs of the sheet may be excessively large compared to the allowable loop space formed by the guides 45 and 46 between the roller pairs 41 and 42.

[0065] As a comparative example, FIG. 6 shows a case where the sheet is transported to the secondary transfer section while maintaining the loop amount Xs of the sheet formed between the roller pairs 41 and 42. In the comparative example shown in FIG. 6, the sheet between the pre-registration roller pair 41 and the registration roller pair 42 buckles, and the loop shape becomes distorted. In this case, buckling noise may occur during sheet transport, and buckling marks may appear on the sheet. In particular, this becomes more noticeable when the loop increases on the leading side of the skewed sheet as described above.

[0066] On the other hand, according to the operation of this embodiment, as shown in FIG. 5B, after the registration roller pair 42 starts to rotate, the loop amount Xs of the sheet is reduced by the loop amount Xr to set the loop amount to Xs-Xr. That is, the pre-registration roller pair 41 is made to have a conveying speed slower than that of the registration roller pair 42 during the period from after the registration roller pair 42 starts to rotate until the leading edge of the sheet reaches the secondary transfer portion. Then, a second loop (loop amount Xs-Xr) smaller than the first loop (loop amount Xs) is formed in the sheet between the roller pairs 41 and 42. This makes it possible to stably convey the sheet to the secondary transfer portion without buckling.

[0067] 6, a comparative example in which the basis weight, which is the thickness of the sheet, is relatively small has been described as an example, but the present invention is not limited to this. When the reaction force from the sheet becomes large, the load torque of the conveying motor may become excessive, or the sheet may slip during conveyance by the registration roller pair 42, causing a delay in the timing at which the sheet reaches the secondary transfer portion. Examples of cases in which the reaction force from the sheet becomes large include a case in which the basis weight of the sheet is relatively large, and a case in which the amount of loop of the sheet is relatively large with respect to the space of the guides 45, 46 between the roller pairs 41, 42.

[0068] It has been common in the past to have a table in which the sheet loop amount Xs is set to a different value depending on the sheet information, i.e., the size, basis weight, shape, and type of the sheet, but the sheet loop amount Xr can also be set to a value depending on the sheet information.

[0069] Specifically, the types of sheets are compared between plain paper (68 gsm), which is a type of copy paper commonly used in offices, cardboard (300 gsm), and thin paper (52 gsm). Here, cardboard is a sheet that is thicker and has higher stiffness than plain paper. Thin paper is a sheet that is thinner and has lower stiffness than plain paper.

[0070] In this embodiment, the loop amount Xs has the same value (Xsn) for plain paper and thin paper, and a value (Xsc) smaller than that for plain paper and thin paper for thick paper (Xsn > Xsc). This is because in the case of thick paper, even if the same loop as that for plain paper or thin paper is not formed when creating the loop, the sheet tip can obtain the force to abut against the nip portion of the registration roller pair by receiving the reaction force from the loop. That is, since the reaction force of the loop varies greatly depending on the type of sheet, the loop amount Xs to be formed also changes accordingly.

[0071] Also, regarding the loop amount Xr, it has the same value (Xrn) for plain paper and thick paper, and is set to a value (Xrt) larger than that for plain paper and thick paper for thin paper (Xrn < Xrt). That is, when the sheet is conveyed by the registration roller pair, the loop amount formed between the pre-registration roller pair and the registration roller pair is in the order of thick paper, plain paper > thin paper > 0. This is due to the stiffness of the sheet. In the case of thin paper, the smaller the formed loop, the less likely the sheet is to buckle.

[0072] The above has been described from the perspective of stiffness regarding the information of the sheet. However, the loop amount may be set in the same manner as above according to the size of the sheet, for example, the sheet width information in the width direction orthogonal to the conveyance direction of the sheet. That is, when the sheet width is large, the reaction force from the sheet becomes large, and it can be considered the same as a sheet with high stiffness. When the sheet width is small, the reaction force from the sheet becomes small, and it can be considered the same as a sheet with low stiffness.

[0073] Specifically, for example, when forming the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42, the size of the second loop may be changed according to the information of the sheet.

[0074] Here, let the basis weight of the sheet be the information of the sheet. In this case, as the basis weight of the sheet decreases, the CPU 201 decreases the size of the second loop formed on the sheet between the pre-registration roller pair and the registration roller pair.

[0075] Alternatively, the sheet information is the width in the direction perpendicular to the conveying direction of the sheet. In this case, the CPU 201 reduces the size of the second loop formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42 as the sheet width becomes smaller.

[0076] In this manner, when the second loop is formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42, the size of the second loop is changed according to the sheet information, thereby making it possible to prevent the sheet from buckling according to the sheet information.

[0077] Alternatively, it may be determined whether or not the second loop is formed on the sheet between the pre-registration roller pair 41 and the registration roller pair 42 depending on the information of the sheet.

[0078] Here, the sheet information is the basis weight of the sheet. In this case, when the basis weight of the sheet is equal to or less than a threshold, the CPU 201 forms the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair. On the other hand, when the basis weight of the sheet exceeds the threshold, the CPU 201 does not form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0079] Alternatively, the sheet information is the width in a direction perpendicular to the sheet conveying direction. In this case, when the sheet width is equal to or less than a threshold value, the CPU 201 forms the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42. On the other hand, when the sheet width exceeds the threshold value, the CPU 201 does not form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0080] In this manner, depending on the sheet information, it is determined whether or not to form the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42. This not only makes it possible to prevent the sheet from buckling depending on the sheet information, but also improves the productivity of the device.

[0081] As described above, by setting the loop amount Xn of the sheet formed between the roller pair 41, 42 for skew correction and the loop amount Xr by which the sheet leading edge is reduced before it reaches the secondary transfer section to values ​​according to the sheet information, buckling of the sheet can be prevented according to the sheet information.

[0082] In the above embodiment, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4. The reason for this will be described below. The registration roller pair 42 starts rotating at time t4 from a state in which the leading edge of the sheet is abutted against the pair and a loop is formed. At that time, for example, if the pre-registration roller pair 41 rises up slightly ahead of the pair of registration rollers 42, the loop amount increases according to the amount of deviation. This may result in an excessive loop amount of the sheet. On the other hand, if the pre-registration roller pair 41 rises up slightly behind the pair of registration rollers 42, no particular problem occurs if the reduction in the loop amount due to the amount of deviation is sufficiently small compared to the loop amount Xs. However, if this is not the case, if the loop amount is reduced before the sheet sufficiently enters the nip portion of the pair of registration rollers 42, the leading edge position of the sheet whose skew has been corrected may shift again, and the skew may worsen. For this reason, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4.

[0083] In the above embodiment, the motor control method for reducing the amount of loop of the sheet is described with reference to Fig. 4, but the present invention is not limited to this. For example, several means are possible, as shown in Figs. 7(a) and 7(b).

[0084] In the control shown in Figure 7(a), the registration roller and the pre-registration roller start up at the same time at time t4. Then, at time t8, the acceleration of the pre-registration roller is reduced compared to the registration roller. Then, at time t9, the pre-registration roller merges with the registration roller.

[0085] 7A, the CPU 201 makes the pre-registration roller slower in conveyance speed than the registration roller at time t8 after conveying the sheet a predetermined distance, and then accelerates the pre-registration roller so that the conveyance speed reaches v3, which is faster than the outer secondary transfer roller 44 and the same as the pair of registration rollers, at a timing (time t9) later than the registration roller.

[0086] As a result, the area indicated by the shaded area in Figure 7(a) is the loop reduction amount Xr2, and the loop amount of the sheet at the timing of reaching the secondary transfer section becomes the second loop (loop amount Xs-Xr2) which is smaller than the first loop (loop amount Xs).

[0087] 7B, the pre-registration rollers are rotated at a constant conveying speed v5 at time t10, and the pre-registration rollers reach the same speed as the registration rollers at time t11.

[0088] 7B, after conveying the sheet a predetermined distance, the CPU 201 sets the pre-registration roller to a constant conveying speed that is faster than the outer secondary transfer roller 44 and slower than the registration roller at time t10. After that, the pre-registration roller is set to the same conveying speed as the registration roller.

[0089] As a result, the area indicated by the shaded area in Figure 7(b) is the loop reduction amount Xr3, and the loop amount of the sheet at the timing of reaching the secondary transfer section becomes the second loop (loop amount Xs-Xr3) which is smaller than the first loop (loop amount Xs).

[0090] 7(a) and 7(b) are also examples, and are not limited thereto. It is sufficient to set the difference in the conveyance amount of the sheet from when the registration roller pair and the pre-registration roller pair rise simultaneously until the sheet reaches the secondary transfer portion so that the latter is smaller than the former.

[0091] Even when the control is performed as described above, the amount of loop formed for skew correction can be reduced by the time the sheet reaches the secondary transfer portion. This makes it possible to reduce the buckling noise of the sheet during sheet transport, reduce the torque of the motor that drives the rollers, and improve image printing accuracy with a simple configuration.

[0092] In the above embodiment, the sheet is stopped once after the sheet abuts against the pair of registration rollers 42 to form a loop, and then the sheet is resumed. However, the present invention is not limited to this. For example, as shown in FIG. 8, the sheet may be controlled to form a loop immediately before the pair of registration rollers resumes the sheet.

[0093] In the control shown in Fig. 8, when the pre-registration roller temporarily stops conveying at time t23, the leading edge of the sheet has not yet reached the pair of registration rollers 42. The conveying distance from when the pre-registration roller starts conveying at time t24 to time t25 is the first loop (loop amount XS2) of the sheet formed between the pair of rollers 41, 42. The control after the registration roller and the pre-registration roller rise at time t25 is as described above. When the leading edge of the sheet finally reaches the secondary transfer portion, the loop amount of the sheet between the pair of rollers 41, 42 is XS2-Xr.

[0094] 8, the CPU 201 stops the pre-registration roller at time t23, and then resumes rotation at time t24 before the registration rotation starts at time t23. Then, a first loop (loop amount Xs2) is formed on the sheet between the roller pair 41, 42. The first loop (loop amount Xs2) formed on the sheet between the roller pair 41, 42 may be formed in this manner.

[0095] In the above embodiment, a printer is used as an example of an image forming apparatus, but the present invention is not limited to this. For example, other image forming apparatuses such as a copier or facsimile machine, or other image forming apparatuses such as a multifunction machine combining the functions of these, may be used. In addition, an image forming apparatus that uses an intermediate transfer body, transfers toner images of each color to the intermediate transfer body in a sequentially overlapping manner, and transfers the toner images carried on the intermediate transfer body to a transfer material all at once is exemplified, but the present invention is not limited to this. For example, an image forming apparatus that uses a transfer material carrier, and transfers toner images of each color to the transfer material carried on the transfer material carrier in a sequentially overlapping manner may be used. By applying the present invention to a sheet conveying device applied to these image forming apparatuses, similar effects can be obtained.

[0096] In the above-described embodiment, the sheet conveying device applied to the image forming apparatus is an integral sheet conveying device provided in the image forming apparatus, but the present invention is not limited to this. formation A sheet that is removably attached to the device Delivery The present invention may be applied to a sheet conveying device, and the same effects can be obtained by applying the present invention to the sheet conveying device. [Explanation of symbols]

[0097] P … Sheet 1. Image forming device 11...photoreceptor 21 ... Intermediate transfer belt 22 ... Secondary transfer inner roller 23 ... Drive roller 24 ... Tension roller 25...Primary transfer device 31, 32 ... Paper feed cassette 31a...Paper feed section 31d, 32d … Sheet size detection section 33 …Manual feed section 40 …Sheet transport section 41 ... Pre-registration roller pair 41a ... driven roller 41b ... Drive roller 42 ... Registration roller pair 42a ... driven roller 42b ... Drive roller 43 ... Registration sensor 44 ... Secondary transfer outer roller 50... Fixing device 60 ... Paper ejection transport section 63,64 …Transport guide 80 ... Paper output tray 200 ... Control unit

Claims

1. A sheet conveying device that conveys a sheet to a transfer conveying section that transfers a toner image carried on an image carrier onto the sheet, A pair of pre-registration rollers that form a nip portion and sandwich and convey a sheet; a pair of registration rollers provided downstream of the pair of pre-registration rollers in a sheet conveying direction, the pair of registration rollers forming a nip portion and conveying the sheet by sandwiching the sheet; a control unit that controls the pre-registration roller pair and the registration roller pair, the control unit conveys the sheet by the pre-registration roller pair, and causes a leading edge of the sheet to abut against the nip portion of the registration roller pair, which is stopped from rotating, to form a first loop by bending the sheet toward one surface side between the pre-registration roller pair and the registration roller pair; a sheet conveying speed by the pre-registration roller pair is made substantially equal to a sheet conveying speed by the registration roller pair during a period from when the registration roller pair starts conveying the sheet to when the sheet is conveyed a predetermined distance; and forming a second loop on the sheet between the pre-registration roller pair and the registration roller pair, the second loop being smaller than the first loop, by setting a sheet conveying speed by the pre-registration roller pair to a conveying speed slower than a sheet conveying speed by the registration roller pair during a period from when the registration roller pair has conveyed the sheet by the predetermined distance until the leading edge of the sheet reaches the transfer conveying section; A sheet transporting device characterized in that, after transporting the sheet a predetermined distance, the sheet transport speed by the pre-registration roller pair is set to a transport speed slower than the sheet transport speed by the registration roller pair, and the sheet is accelerated to a transport speed faster than the sheet transport speed by the transfer transport section at a timing later than the registration roller pair, and to reach the same transport speed as the sheet transport speed by the registration roller pair.

2. A sheet conveying device that conveys a sheet to a transfer conveying section that transfers a toner image carried on an image carrier onto the sheet, A pair of pre-registration rollers that form a nip portion and sandwich and convey a sheet; a pair of registration rollers provided downstream of the pair of pre-registration rollers in a sheet conveying direction, the pair of registration rollers forming a nip portion and conveying the sheet by sandwiching the sheet; a control unit that controls the pre-registration roller pair and the registration roller pair, the control unit conveys the sheet by the pre-registration roller pair, and causes a leading edge of the sheet to abut against the nip portion of the registration roller pair, which is stopped from rotating, to form a first loop by bending the sheet toward one surface side between the pre-registration roller pair and the registration roller pair; a sheet conveying speed by the pre-registration roller pair is made substantially equal to a sheet conveying speed by the registration roller pair during a period from when the registration roller pair starts conveying the sheet to when the sheet is conveyed a predetermined distance; and forming a second loop on the sheet between the pre-registration roller pair and the registration roller pair, the second loop being smaller than the first loop, by setting a sheet conveying speed by the pre-registration roller pair to a conveying speed slower than a sheet conveying speed by the registration roller pair during a period from when the registration roller pair has conveyed the sheet by the predetermined distance until the leading edge of the sheet reaches the transfer conveying section; a sheet conveying device characterized in that, after conveying the sheet a predetermined distance, the sheet conveying speed by the pre-registration roller pair is set to a conveying speed that is faster than the sheet conveying speed by the transfer conveying section and slower than the sheet conveying speed by the registration roller pair, and then the conveying speed is set to the same as the sheet conveying speed by the registration roller pair.

3. 3. The sheet transport device of claim 1, wherein the control unit starts rotation of the registration roller pair after forming the first loop, sets the sheet transport speed by the registration roller pair to a transport speed faster than the sheet transport speed by the transfer transport unit, and after forming the second loop, sets the sheet transport speed by the pre-registration roller pair to a transport speed that is the same as the sheet transport speed by the registration roller pair and faster than the sheet transport speed by the transfer transport unit, and then sets the sheet transport speed by the pre-registration roller pair and the sheet transport speed by the registration roller pair to the same transport speed as the sheet transport speed by the transfer transport unit until the leading edge of the sheet in the state in which the second loop has been formed reaches the transfer transport unit.

4. 4. The sheet conveying device according to claim 1, wherein the control unit temporarily stops the pre-registration roller pair, and then resumes rotation before the registration roller pair starts to rotate, thereby forming the first loop between the pre-registration roller pair and the registration roller pair.

5. 3. The sheet conveying device according to claim 1, wherein the control unit determines whether or not to form the second loop on the sheet between the pair of pre-registration rollers and the pair of registration rollers in accordance with information about the sheet.

6. 6. The sheet conveying device of claim 5, wherein the sheet information is the basis weight of the sheet, and the control unit forms the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet is equal to or less than a threshold value, and does not form the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet exceeds the threshold value.

7. 6. The sheet conveying device of claim 5, wherein the sheet information is a width in a direction perpendicular to the sheet conveying direction, and the control unit forms the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the sheet width is equal to or less than a threshold value, and does not form the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the sheet width exceeds the threshold value.

8. 4. The sheet conveying device according to claim 1, wherein the control unit changes a size of the second loop in accordance with sheet information when forming the second loop on the sheet between the pre-registration roller pair and the registration roller pair.

9. The sheet information is the basis weight of the sheet, 9. The sheet conveying device according to claim 8, wherein the control unit reduces a size of the second loop formed in the sheet between the pre-registration roller pair and the registration roller pair in response to a decrease in basis weight of the sheet.

10. The sheet information is a width in a direction perpendicular to the sheet conveying direction, 9. The sheet conveying device according to claim 8, wherein the control unit reduces a size of the second loop formed on the sheet between the pre-registration roller pair and the registration roller pair as the width of the sheet decreases.

11. An image carrier; a transfer and transport section for transferring the toner image carried on the image carrier onto a sheet; A sheet conveying device according to any one of claims 1 to 10, An image forming apparatus comprising:

Images