Sheet conveying device and image forming apparatus

Abstract

To reduce buckling of a sheet between a pair of pre-registration rollers and a pair of registration rollers when the sheet is conveyed to a secondary transfer part.SOLUTION: A sheet conveying device according to the present invention has a control unit that controls a pair of pre-registration rollers and a pair of registration rollers on the downstream side of the pair of pre-registration rollers in a sheet conveying direction, and the control unit brings the leading edge of the sheet into contact with the pair of registration rollers that have stopped rotating, forms a first loop between the pair of pre-registration rollers and the pair of registration rollers, and further sets the pair of pre-registration rollers to have a conveying speed slower than that of the pair of registration rollers during the period from when the pair of registration rollers start rotating to the period when the leading edge of the sheet reaches a 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.SELECTED DRAWING: Figure 4

Description

Technical Field

[0001] The present invention relates to a sheet conveyance device for conveying a sheet and an image forming apparatus including the same.

Background Art

[0002] Conventionally, in an image forming apparatus that forms an image on a sheet, a skew correction mechanism for correcting the skew of the conveyed sheet is provided in order to align the position of the sheet and the image formed on the sheet.

[0003] In Patent Document 1, the sheet is conveyed by a predetermined amount by a pre-registration roller pair provided upstream of the registration roller pair in the sheet conveyance direction with respect to the registration roller pair whose driving is stopped, and the skew of the sheet is corrected. That is, the leading end 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

Summary of the Invention

Problems to be Solved by the Invention

[0005] In Patent Document 1, after the skew is corrected, the sheet is conveyed by the re-driving of the registration roller pair synchronized with the image signal, and the registration roller pair and the pre-registration roller pair start conveying at the same speed and are conveyed to the secondary transfer unit. In this case, the sheet conveyed from the registration roller pair to the secondary transfer unit is conveyed while maintaining the size of the loop formed between the pre-registration roller pair and the registration roller pair.

[0006] However, the optimal values of the size of the sheet loop for skew correction and the size of the sheet loop to be maintained during conveyance from the registration roller pair to the secondary transfer section are different.

[0007] That is, the size of the sheet loop for skew correction is necessary to correct the skew of the sheet before reaching the registration roller pair and is determined in advance based on the largest skew amount to be assumed. Further, when loop formation control is performed triggered by the detection signal at the sensor immediately before the registration roller pair, the skewed sheet will be detected, so the size will be one that takes into account the variation.

[0008] On the other hand, if the size of the sheet loop to be maintained during conveyance is too large, the loop will buckle between the pre-registration roller pair and the registration roller pair. Therefore, the size of the sheet loop to be maintained during conveyance is preferably small within a range where no tension occurs in the sheet between the pre-registration roller pair and the registration roller pair.

[0009] Therefore, when the sheet is conveyed 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 loop between the roller pairs as described above.

[0010] Therefore, an object of the present invention is to reduce the buckling of the sheet between the pre-registration roller pair and the registration roller pair when the sheet is conveyed to the secondary transfer section.

Means for Solving the Problem

[0011] In order to achieve the above object, the present invention is a sheet conveying device that conveys a sheet to a transfer conveying unit that transfers a toner image carried on an image carrier to a sheet, and includes a preregistration roller pair that sandwiches and conveys the sheet, and a registration roller pair that is provided on the downstream side in the sheet conveying direction from the preregistration roller pair and sandwiches and conveys the sheet, and a control unit that controls the preregistration roller pair and the registration roller pair. The control unit conveys the sheet by the preregistration roller pair, abuts the leading end of the sheet against the registration roller pair that has stopped rotating, and forms a first loop in which the sheet is deflected to one surface side between the preregistration roller pair and the registration roller pair. Further, after the registration roller pair starts rotating and until the leading end of the sheet reaches the transfer conveying unit, the preregistration roller pair is set to a conveyance speed slower than that of the registration roller pair, and a second loop smaller than the first loop is formed in the sheet between the preregistration roller pair and the registration roller pair.

Advantages of the Invention

[0012] According to the present invention, when the sheet is conveyed to the transfer conveying unit, the loop of the sheet between the preregistration roller pair and the registration roller pair becomes small, so that the buckling of the sheet during that period can be reduced.

Brief Description of the Drawings

[0013]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Figure 9

Embodiments for Carrying Out the Invention

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

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

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

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

[0018] In the image forming unit, the surface of the photoreceptor 11 is uniformly charged by the charging device 12 in advance, the exposure device 13 is driven based on the signal of the image information, and a latent image is formed on the rotating surface of the photoreceptor 11. The electrostatic latent image formed on the surface of the photoreceptor 11 is developed into a toner image through toner development by the developing device 14. Then, a predetermined pressing force and an 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 the driving roller 23, the tension roller 24, and the secondary transfer inner roller 22, and is conveyed and driven in the direction of arrow B shown in FIG. 9. The image forming processes for the Y, M, C, and Bk colors that are processed in parallel are performed at the timing of overlapping on the toner image of the upstream color primarily transferred onto the intermediate transfer belt 21. As a result, finally, a full-color toner image is formed on the intermediate transfer belt 21, and the toner image on this intermediate transfer belt 21 (on the image carrier) is conveyed to the secondary transfer unit.

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

[0021] The sheet P, which is the material to be transferred, is loaded and stored in, for example, the paper feed cassettes (sheet storage) 31 and 32 and the manual feed unit 33 for each size. In order to record an image within an appropriate range, it is necessary to let the image forming apparatus main body recognize the size of the stored sheet.

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

[0023] Therefore, when the side regulation plate is moved in accordance with the end portion in the width direction of the sheet, the size detection lever rotates in conjunction therewith. When the paper feed cassette is mounted on the mounting portion in this state, the size detection lever selectively turns on / off the detection element of the sensor or switch arranged in the mounting portion where the paper feed cassette is mounted. As a result, signals of different patterns are sent from the sensor or switch to the image forming apparatus main body. Then, the image forming apparatus main body can recognize the size of the sheet stored in the paper feed cassette based on the signal.

[0024] The sheet P is fed one by one from, for example, the paper feed unit 31a (or the paper feed unit 32a). The skew is corrected by the pre-registration roller pair 41 and the registration roller pair 42 that constitute the sheet feeding unit (sheet feeding device), and the sheet is sent to the secondary transfer unit. Specifically, the sheet P is conveyed by the pre-registration roller pair 41 and is abutted against the nip portion of the registration roller pair 42 whose tip is stopped. In this way, the pre-registration roller pair 41 loops the sheet P between itself and the stopped registration roller pair 42 to correct the skew of the sheet.

[0025] The registration roller pair 42 conveys the sheet P to the secondary transfer unit in accordance with the timing when the toner image on the intermediate transfer belt 21 is transferred to the sheet P.

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

[0027] The sheet P after transfer is conveyed to a fixing device 50, and the toner image is melted and fixed onto the sheet P by applying a pressure and a heating effect. The sheet P after fixing is conveyed to a paper discharge conveyance unit 60, and the conveyance path is switched by a flapper 64 depending on whether the paper passing mode is single-sided or double-sided. That is, in the case of the single-sided mode, it is switched to the paper discharge roller 62 side by the flapper 64, and the sheet P is discharged toward the paper discharge tray 80 by the paper discharge roller 62 and stacked.

[0028] On the other hand, in the case of the double-sided mode, it is switched to the paper discharge roller 63 side by the flapper 64, and the sheet P is conveyed to a reverse roller 71 by the paper discharge roller 63 and discharged toward the paper discharge tray 82 by the reverse roller 71. Then, the sheet P is temporarily stopped with its rear end left at a predetermined distance from the reverse roller 71. After that, the sheet P is conveyed again to the registration roller pair 42 via the double-sided conveyance unit 70 by the reverse rotation of the reverse roller 71, and the second image formation and fixing are performed in the image forming unit.

[0029] The sheet P after fixing is discharged toward the paper discharge tray 80 or the paper discharge tray 82 by the paper discharge roller 62 or the reverse roller 71 via the flapper 64 and stacked.

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

[0031] The sheet conveyance unit 40 corrects the skew of the sheet P and conveys the sheet P to the secondary transfer unit that transfers the toner image carried on the intermediate transfer belt 21. The sheet conveyance unit 40 includes a pre-registration roller pair 41 and a registration roller pair 42. Further, the sheet conveyance unit 40 includes a registration sensor 43. The pre-registration roller pair 41 and the registration roller pair 42 are controlled by a control unit 200 (CPU 201) described later.

[0032] The pre-registration roller pair 41 is a pair of conveyance 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 to face 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 that is rotationally driven.

[0033] The registration roller pair 42 is provided on the downstream side in the sheet conveyance direction from the pre-registration roller pair 41, and is a conveyance roller pair that sandwiches and conveys the sheet P. 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. In the registration roller pair 42, the driven roller 42a and the driving roller 42b are arranged to face each other. And 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 that is rotationally driven.

[0034] Then, the sheet conveyed by the pre-registration roller pair 41 is abutted so that the leading end of the sheet follows the abutting portion (nip portion) where the stationary driven roller 42a and the driving roller 42b abut, whereby the skew is corrected.

[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, and is disposed immediately before the registration roller pair 42. Also, the registration sensor 43 is disposed at the center (the dashed-dotted line shown in FIG. 2) in the width direction (thrust direction) orthogonal to the conveyance direction of the sheet P (the direction of arrow A in FIG. 2).

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

[0037] Between the pre-registration roller pair 41 and the registration roller pair 42, guides 45 and 46 for guiding the conveyed sheet are arranged. The guides 45 and 46 are partially spaced apart and are configured to allow a loop formed by the sheet abutted against the nip portion of the registration roller pair 42.

[0038] Specifically, one guide 45 guides one surface side of the conveyed sheet. The other guide 46 is provided opposite to the one guide 45 and guides the other surface side of the conveyed sheet. 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 the leading end of the sheet abutted against the nip portion of the registration roller pair 42 forms a loop that moves away from the other guide 46 and approaches the one guide 45 between the roller pairs 41 and 42. In other words, the sheet forms a loop that deflects the sheet to one surface side between the roller pairs 41 and 42.

[0039] Next, the skew correction operation of the sheet by the sheet conveyance unit 40 will be described with reference to the schematic diagram of FIG. 2.

[0040] The skew of the sheet is a state in which one side in the width direction of the leading end of the sheet precedes the other side with respect to the center in the width direction (the dashed-dotted line shown in the figure) orthogonal to the conveyance direction of the sheet. Here, as shown in FIG. 2(a), a state in which the left side in the width direction of the leading end of the sheet precedes the right side, in other words, a state in which the sheet is skewed clockwise with respect to the conveyance direction is illustrated.

[0041] As shown in Fig. 2(a), the skew correction operation when the sheet is skewed clockwise with respect to the conveyance direction (the direction of arrow A) will be described. Fig. 2(a) shows a state where the leading edge of the sheet P has reached the registration sensor 43. When the preregistration roller pair 41 further rotates and the sheet P is conveyed in the conveyance 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 rotation of the registration roller pair 42 has stopped.

[0042] Furthermore, when the preregistration roller pair 41 rotates and the sheet P is conveyed in the conveyance direction, as shown in Fig. 2(b), all of the leading edge of the sheet P abuts against the nip portion of the registration roller pair 42 over the width direction. At this time, the leading edge of the sheet P abuts against and stops at the nip portion with the corner portion on the leading side, and gradually a loop is formed in the sheet and the corner portion on the side opposite to the width direction of the corner portion that abuts against the nip portion turns so as to approach the nip portion. Then, a loop that deflects the sheet P to one surface side is formed between the registration roller pair 42 and the preregistration roller pair 41. Thereby, the entire width direction of the leading edge of the sheet P is abutted against 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 the 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 optimum amount of the loop based on at least one of the sheet information specified by the user at the operation unit 203 or the sheet information detected by the sheet size detection units 31d and 32d, or a combination thereof.

[0044] Thereafter, in the image forming unit, a toner image is formed on the intermediate transfer belt 21. Then, in synchronization with this, the registration roller pair 42 and the preregistration roller pair 41 rotate, and as shown in Fig. 2(c), the sheet P is conveyed to the secondary transfer unit in a state where the skew is corrected.

[0045] Here, before the leading edge of the sheet reaches the secondary transfer unit, the control unit 200 sets the pre-registration roller pair 41 to a slower conveyance speed than the registration roller pair 42 to reduce the loop of the sheet formed between the roller pairs 41 and 42. This is a characteristic operation in the present invention, and the reason will be explained.

[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 this registration sensor 43 1C is an amount determined in advance as described above. As shown in FIG. 2(b), the loop amount r of the leading end portion on the leading side of the sheet P 1R , the loop amount r of the trailing end portion on the trailing side 1F If we denote them as such, the relationship of the loop amounts is r 1R > r 1C > r 1F That is, the loop amount on the leading side of the sheet P is larger than the preset loop amount r by the amount that has advanced. 1C

[0047] Assuming the loop amount of the sheet P formed between the pre-registration roller pair 41 and the registration roller pair 42, a predetermined loop space is secured between the guide 45 and the guide 46. However, if the skew amount of the sheet P before reaching the registration roller pair 42 is large, the loop amount may become larger than expected. In this case, if the force received from the sheet P exceeds the force with which the registration roller pair 42 sandwiches the sheet P, the sheet may slip at the nip portion of the registration roller pair 42, causing the sheet to skew or wrinkles to occur on the sheet. Alternatively, there may be a case where a buckling sound is generated due to buckling of the loop of the sheet. Also, there is a risk that the load torque of the pre-registration motor increases and the motor stops.

[0048] In the present embodiment made in view of such a situation, the loop amount r necessary for skew correction of the sheet 1C ​After 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] As shown in FIG. 3, the control unit 200 includes 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 realizes various processes performed by the image forming apparatus 1 by executing a predetermined control program and the like. The memory 202 is, for example, a RAM (Random Access Memory), a ROM (Read only memory), etc., and stores various programs and various data in a predetermined storage area. The operation unit 203 receives various operations performed by the user, such as various information (size, basis weight, surface property information, etc.) regarding the sheet used for printing by the user, and instructions to execute printing or interrupt it.

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

[0052] Note that it is also possible to configure to be able to receive various information regarding the sheet 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 and 102, and controls the operations of the pre-registration roller and the registration roller as shown in FIGS. 4(a) and 4(b). Here, the pre-registration roller is the driving roller 41b that rotates by receiving the driving force from the pre-registration motor 101 among the pre-registration roller pair 41. The registration roller is the driving roller 42b that rotates by receiving the driving force from the registration motor 102 among the registration roller pair 42.

[0054] The CPU 201 controls the drive of the pre-registration motor 101 to convey the subsequent sheet by the pre-registration roller pair 41. At this time, the pre-registration roller pair 41 conveys the subsequent sheet at the conveyance speed v1, and the registration roller pair 42 is stopped. After the leading edge of the subsequent sheet conveyed at the conveyance speed v1 by the pre-registration roller pair 41 is detected by the registration sensor 43 at time t1, it abuts on the nip portion of the stopped registration roller pair 42 at time t2. Then, the CPU 201 stops the drive 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. 5(a)) in which the sheet between the pre-registration roller pair 41 and the registration roller pair 42 is bent to one surface side is formed. This first loop is shown as the loop amount Xs in FIG. 4(b). The loop amount Xs is a predetermined loop amount of the sheet and is the area shown by the grid portion in FIG. 4(b).

[0055] Thereafter, the CPU 201 simultaneously starts the drives of the pre-registration motor 101 and the registration motor 102 at time t4 in accordance with the toner image formed on the intermediate transfer belt 21. Then, the conveyance of the subsequent sheet is simultaneously started by the pre-registration roller pair 41 and the registration roller pair 42.

[0056] Then, after the registration roller pair 42 starts rotating and until the leading edge of the subsequent sheet reaches the secondary transfer section, the CPU 201 sets the pre-registration roller pair 41 to a conveyance speed v2 that is slower than that of the registration roller pair 42 at time t5. As a result, the loop amount Xs formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 decreases by the loop amount Xr. That is, a second loop (loop amount Xs - Xr shown in FIG. 5(b)) smaller than the first loop (loop amount Xs shown in FIG. 5(a)) 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 region indicated by the hatched portion in FIG. 4(b) and represents the difference in the conveyance amounts 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 which is the first loop described above, by setting Xs > Xr, the loop amount Xs - Xr which is the second loop can be reduced while preventing it from becoming zero.

[0058] Then, after forming the first loop, when the registration roller pair 42 starts rotating, the CPU 201 accelerates the registration roller pair 42 to a conveyance speed v3 that is faster than that of the secondary transfer outer roller 44.

[0059] On the other hand, after the registration roller pair 42 starts rotating and until the sheet is conveyed a predetermined distance, the CPU 201 sets the pre-registration roller pair 41 to the same conveyance speed as that of the registration roller pair 42. That is, the CPU 201 accelerates the pre-registration roller pair 41 to the same conveyance speed as that of the registration roller pair 42 after the registration roller pair 42 starts rotating and until the sheet is conveyed a predetermined distance (between time t4 and time t5).

[0060] In the operation example shown in FIG. 4(b), after the CPU 201 conveys the sheet by a predetermined distance, as described above, at time t5, the pre-registration roller pair 41 is set to a constant conveyance speed v2 that is slower than the registration roller pair 42. At time t6 thereafter, the pre-registration roller pair 41 is set to a conveyance speed v3 that is faster than the secondary transfer outer roller 44 and the same as the conveyance speed of the registration roller pair. By setting the pre-registration roller pair 41 to a conveyance speed v3 that is faster than the secondary transfer outer roller 44 in this way, the productivity of the apparatus can be improved.

[0061] Until the leading edge of the subsequent sheet that forms the second loop reaches the secondary transfer section (from time t6 to time t7), the CPU 201 sets the pre-registration roller pair 41 and the registration roller pair 42 to the same conveyance speed v4 as the secondary transfer outer roller 44. That is, after the pre-registration roller pair 41 and the registration roller pair 42 are set to a conveyance speed v3 that is faster than the secondary transfer outer roller 44 and the sheet is conveyed a certain distance, the speed is reduced so as to reach the same conveyance speed v4 as the secondary transfer outer roller 44 by time t7.

[0062] As described above, according to the present embodiment, the loop amount is reduced before reaching the secondary transfer section with respect to the loop amount formed for skew correction. That is, the first loop of the sheet formed between the roller pairs 41 and 42 for skew correction is changed to a second loop that is smaller than the first loop before the leading edge of the sheet reaches the secondary transfer section. Thereby, with a simple configuration, it is possible to reduce the buckling sound of the sheet during sheet conveyance, reduce the torque of the motor that drives the roller, and improve the image printing accuracy.

[0063] Also, the loop shape of the sheet between the roller pairs 41 and 42 when conveyed by the above-described operation is shown in FIGS. 5(a) and 5(b). 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. Further, FIG. 6 is a cross-sectional view showing the loop shape of the sheet at the timing corresponding to time t7 in the comparative example where 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 by abutting the leading end of the sheet against the registration roller pair 42, is determined from the following various viewpoints. One of the viewpoints is the distance for the side of the skewed sheet that is delayed to reach the registration roller pair 42, that is, the distance based on the skew amount. Also, one of the viewpoints is the conveyance amount variation from when it is detected by the registration sensor 43 until it reaches the registration roller pair 42. Also, one of the viewpoints is the amount by which the leading end position of the sheet drops due to gravity when the sheet is conveyed vertically upward as in this embodiment. Thus, 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 become excessively large with respect 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 it is conveyed 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, a buckling sound may occur during sheet conveyance, or a buckling mark may occur on the sheet. In particular, as described above, it becomes more prominent due to an increase in the loop on the leading side of the skewed sheet.

[0066] On the other hand, according to the operation of this embodiment, as shown in FIG. 5(b), after starting the rotation of the registration roller pair 42, the loop amount Xs of the sheet is reduced by the loop amount Xr so that the loop amount becomes Xs - Xr. That is, after the registration roller pair 42 starts rotating and until the leading end of the sheet reaches the secondary transfer section, the pre-registration roller pair 41 is set to a conveyance speed slower than that of the registration roller pair 42. 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. Thereby, it becomes possible to stably convey the sheet to the secondary transfer section without buckling.

[0067] In the comparative example shown in FIG. 6, the case where the basis weight, which is the thickness of the sheet, is relatively small was illustrated and described, but the present invention is not limited thereto. When the reaction force from the sheet increases, the load torque of the conveyance motor may become excessive, or the sheet may slip during conveyance by the registration roller pair 42, resulting in a deviation in the timing at which the sheet reaches the secondary transfer section. Examples of cases where the reaction force from the sheet increases include when the basis weight of the sheet is relatively large, and when the loop amount of the sheet is relatively large with respect to the space between the guides 45 and 46 between the roller pairs 41 and 42.

[0068] A configuration having a table in which the loop amount Xs of the sheet is set to different values according to sheet information, that is, the size, basis weight, shape, and type of the sheet, has been common in the past. However, the loop amount Xr of the sheet can also be set to a value according to the sheet information.

[0069] Specifically, the types of sheets are compared: plain paper (68 gsm), which is commonly used for copying in offices; cardboard (300 gsm); and thin paper (52 gsm). Here, the cardboard is a sheet whose thickness is greater than that of plain paper and whose stiffness is higher than that of plain paper. The thin paper is a sheet whose thickness is smaller than that of plain paper and whose stiffness is lower than that of plain paper.

[0070] In this embodiment, the loop amount Xs has the same value (Xsn) for plain paper and thin paper, and has 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 leading edge of the sheet receives the reaction force of the loop to obtain the force to abut against the nip portion of the registration roller pair. 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, a smaller loop makes the sheet less likely 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, which 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, which 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, the information of the sheet is the basis weight 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 and the registration roller pair as the basis weight of the sheet decreases.

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

[0076] When forming the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 in this way, by changing the size of the second loop according to the information of the sheet, it is possible to prevent buckling of the sheet according to the information of the sheet.

[0077] Alternatively, it may be 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 according to the information of the sheet.

[0078] Here, the information of the sheet is taken as the basis weight of the sheet. In this case, when the basis weight of the sheet is equal to or less than the threshold value, the CPU 201 forms the second loop in 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 value, the second loop is not formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0079] Alternatively, the information of the sheet is taken as the width in the direction orthogonal to the conveyance direction of the sheet. In this case, when the width of the sheet is equal to or less than the threshold value, the CPU 201 forms the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42. On the other hand, when the width of the sheet exceeds the threshold value, the second loop is not formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0080] According to the information of the sheet in this way, it is determined whether or not to form the second loop on the sheet between the pre-registration roller pair 41 and the registration roller pair 42. Thereby, not only can the buckling of the sheet be prevented according to the information of the sheet, but also the productivity of the apparatus can be improved.

[0081] As described above, regarding the loop amount Xn of the sheet formed between the roller pairs 41 and 42 for skew correction and the loop amount Xr to be decreased before the leading end of the sheet reaches the secondary transfer portion, by setting values according to the information of the sheet, the buckling of the sheet can be prevented according to the information of the sheet.

[0082] In addition, in the above-described embodiment, it has been described that the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4. The reason therefor will be described below. The registration roller pair 42 starts rotating at time t4 from the state where the leading end of the sheet is abutted to form a loop. At that time, for example, if the pre-registration roller pair 41 starts up slightly ahead of the registration roller pair 42, the loop amount will increase according to the deviation amount. As a result, there is a risk that the loop amount of the sheet becomes excessive. On the other hand, if the pre-registration roller pair 41 starts up slightly later than the registration roller pair 42, there will be no particular problem as long as the amount of loop reduction due to the deviation amount is sufficiently smaller than 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 registration roller pair 42, the skew may deteriorate because the leading end position of the sheet with the skew corrected is displaced again. For this reason, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4.

[0083] In addition, in the above-described embodiment, the motor control method for reducing the loop amount of the sheet has been described by exemplifying FIG. 4, but the present invention is not limited thereto. For example, as exemplified in FIGS. 7(a) and 7(b), several means can be considered.

[0084] In the control shown in Fig. 7(a), at time t4, the registration roller and the pre-registration roller start up simultaneously. After that, at time t8, the acceleration of the pre-registration roller is reduced compared to that of the registration roller. And at time t9, the pre-registration roller merges with the speed of the registration roller.

[0085] That is, in the control shown in Fig. 7(a), after the CPU 201 conveys the sheet by a predetermined distance, at time t8, the pre-registration roller is set to a conveyance speed slower than that of the registration roller. Then, the pre-registration roller is accelerated so that it has a conveyance speed faster than that of the secondary transfer outer roller 44 at a timing (time t9) slower than that of the registration roller and reaches the same conveyance speed v3 as the registration roller pair.

[0086] As a result, the area shown by the hatched portion in Fig. 7(a) becomes the loop reduction amount Xr2, and the loop amount of the sheet at the timing of reaching the secondary transfer portion becomes a second loop (loop amount Xs - Xr2) smaller than the first loop (loop amount Xs).

[0087] Also, in the control shown in Fig. 7(b), at the timing of time t10, the pre-registration roller is set to a constant conveyance speed v5. And at time t11, the pre-registration roller merges with the speed of the registration roller.

[0088] That is, in the control shown in Fig. 7(b), after the CPU 201 conveys the sheet by a predetermined distance, at time t10, the pre-registration roller is set to a conveyance speed faster than that of the secondary transfer outer roller 44 and slower than that of the registration roller. Then, the pre-registration roller is set to the same conveyance speed as the registration roller.

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

[0090] Note that the control shown in Figs. 7(a) and 7(b) is also an example and is not limited thereto. It may be set such that the difference in the conveyance amount of the sheet from when the registration roller pair and the pre-registration roller pair start up simultaneously until reaching the secondary transfer unit is smaller for the latter than for the former.

[0091] Even if controlled as described above, the loop amount can be reduced before reaching the secondary transfer unit with respect to the loop amount formed for skew correction. Thereby, with a simple configuration, it is possible to reduce the buckling sound of the sheet during sheet conveyance, reduce the torque of the motor driving the roller, and improve the image printing accuracy.

[0092] In the above-described embodiment, the control in which the sheet is abutted against the registration roller pair 42 to form a loop and then temporarily stopped and then conveyance is restarted has been described, but it is not limited thereto. For example, as shown in Fig. 8, control in which a loop is formed immediately before restarting the conveyance of the registration roller pair may be used.

[0093] In the control shown in Fig. 8, when the pre-registration roller temporarily stops conveyance at time t23, the leading end of the sheet has not yet reached the registration roller pair 42. Then, the conveyance distance from when the pre-registration roller starts conveyance at time t24 until time t25 is the first loop (loop amount XS2) of the sheet formed between the roller pairs 41 and 42. And the control after the registration roller and the pre-registration roller start up at time t25 is as described above. Finally, when the leading end of the sheet reaches the secondary transfer unit, the loop amount of the sheet between the roller pairs 41 and 42 becomes XS2 - Xr.

[0094] That is, in the control shown in FIG. 8, the CPU 201 restarts the rotation of the pre-registration roller at time t24 after once stopping the pre-registration roller at time t23 and before starting the rotation of the registration at time t23. Then, a first loop (loop amount Xs2) is formed in the sheet between the roller pairs 41 and 42. The first loop (loop amount Xs2) formed in the sheet between the roller pairs 41 and 42 may be formed in this way.

[0095] In the above-described embodiment, a printer is exemplified as the image forming apparatus, but the present invention is not limited thereto. For example, other image forming apparatuses such as a copying machine, a facsimile apparatus, or a multi-functional apparatus combining these functions may be used. Further, an image forming apparatus that uses an intermediate transfer member, sequentially superimposes toner images of respective colors on the intermediate transfer member and transfers them, and collectively transfers the toner image carried on the intermediate transfer member to a transfer material has been exemplified, but the present invention is not limited thereto. For example, an image forming apparatus that uses a transfer material carrier and sequentially superimposes toner images of respective colors on the transfer material carried on the transfer material carrier may be used. By applying the present invention to the sheet conveyance apparatus applied to these image forming apparatuses, the same effect can be obtained.

[0096] In the above-described embodiment, as the sheet conveyance apparatus applied to the image forming apparatus, a sheet conveyance apparatus integrally provided in the image forming apparatus has been exemplified, but the present invention is not limited thereto. For example, a sheet conveyance apparatus detachably attached to the image forming apparatus may be used, and the same effect can be obtained by applying the present invention to the sheet conveyance apparatus.

Explanation of Reference Numerals

[0097] P... Sheet 1... Image forming apparatus 11... Photoconductor 21... Intermediate transfer belt 22... Secondary transfer inner roller 23... Driving roller 24... Tension roller 25... Primary transfer device 31, 32... Sheet feeding cassette 31a... Sheet feeding unit 31d, 32d... Sheet size detection unit 33... Manual feed unit 40... Sheet conveyance unit 41... Preregistration roller pair 41a... Driven roller 41b... Driving roller 42... Registration roller pair 42a... Driven roller 42b... Driving roller 43... Registration sensor 44... Secondary transfer outer roller 50... Fixing device 60... Paper discharge conveyance unit 63, 64... Conveyance guide 80... Paper discharge 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 for sandwiching and transporting 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 sandwiching and conveying the sheet; a control unit that controls the pre-registration roller pair and the registration roller pair; having The control unit is a first loop is formed by conveying the sheet by the pre-registration roller pair and bringing a leading end of the sheet into contact with the registration roller pair that is stopped from rotating, and bending the sheet toward one surface side between the pre-registration roller pair and the registration roller pair; The sheet transport device is further characterized in that, between the time when the registration roller pair starts to rotate and the time when the leading edge of the sheet reaches the transfer transport section, the pre-registration roller pair is set to a transport speed slower than that of the registration roller pair, thereby forming a second loop smaller than the first loop on the sheet between the pre-registration roller pair and the registration roller pair.

2. The control unit is After the first loop is formed, the registration roller pair is started to rotate, and the registration roller pair is set to a conveying speed faster than that of the transfer conveying section; After forming the second loop, the pre-registration roller pair is set to a conveying speed that is the same as that of the registration roller pair and is faster than that of the transfer conveying section, 2. The sheet transport device according to claim 1, wherein the pre-registration roller pair and the registration roller pair are set to the same transport speed as the transfer transport section until the leading edge of the sheet that has formed the second loop reaches the transfer transport section.

3. 3. The sheet transport device according to claim 1, wherein the pre-registration roller pair is set to have the same transport speed as the registration roller pair from after the registration roller pair starts transporting the sheet until the sheet is transported a predetermined distance.

4. The control unit is 4. The sheet transport device according to claim 3, wherein after the sheet has been transported a predetermined distance, the pre-registration roller pair is set to a constant transport speed slower than that of the registration roller pair, and then set to a transport speed faster than that of the transfer transport section and the same as that of the registration roller pair.

5. The control unit is 4. The sheet transport device according to claim 3, wherein after transporting the sheet a predetermined distance, the pre-registration roller pair is set to a transport speed slower than that of the registration roller pair, and accelerated to reach a transport speed faster than that of the transfer transport section at a timing later than that of the registration roller pair, and the same transport speed as that of the registration roller pair.

6. The control unit is 4. The sheet transport device according to claim 3, wherein after the sheet has been transported a predetermined distance, the pre-registration roller pair is set to a transport speed that is faster than the transfer transport section and slower than the registration roller pair, and then set to the same transport speed as the registration roller pair.

7. The control unit is 7. The sheet conveying device according to claim 1, wherein the pre-registration roller pair is stopped once 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.

8. 5. 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 pre-registration roller pair and the registration roller pair depending on information about the sheet.

9. The sheet information is the basis weight of the sheet, 9. The sheet conveying device of claim 8, wherein 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.

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 forms the second loop on the sheet between the pre-registration roller pair and the registration roller pair when the width 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 width of the sheet exceeds the threshold value.

11. 9. 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.

12. The sheet information is the basis weight of the sheet, 12. The sheet conveying device according to claim 11, 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.

13. The sheet information is a width in a direction perpendicular to the sheet conveying direction, 12. The sheet transport device according to claim 11, wherein the control unit reduces a size of the second loop formed on the sheet between the pair of pre-registration rollers and the pair of registration rollers as the width of the sheet decreases.

14. 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 13, An image forming apparatus comprising:

Images