Sheet transport 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 conveying device for conveying sheets and an image forming apparatus equipped therewith. [Background technology]

[0002] Conventionally, in image forming apparatuses that form images on sheets, a skew correction mechanism is provided to correct the skew of the conveyed sheet in order to align the sheet with the image formed on the sheet.

[0003] In Patent Document 1, a pair of pre-registration rollers, located upstream of a pair of registration rollers that are not driven, are used to transport a predetermined amount of the sheet and correct the sheet's skewness. Specifically, the leading edge of the sheet being transported by the pre-registration roller pair abuts against the registration roller pair, forming a loop in the sheet between the pre-registration roller pair and the registration roller pair, thereby correcting the sheet's skewness. [Prior art documents] [Patent Documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 6-127753 [Overview of the project] [Problems that the invention aims to solve]

[0005] In Patent Document 1, after the skew correction is performed, the registration roller pair and the pre-registration roller pair begin transporting the sheet at the same speed by restarting the registration roller pair in synchronization with the image signal, and the sheet is transported to the secondary transfer section. In this case, the sheet transported from the registration roller pair to the secondary transfer section is transported while maintaining the size of the loop formed between the pre-registration roller pair and the registration roller pair.

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

[0007] In other words, the size of the sheet loop for skew correction is necessary to correct the skew of the sheet before it reaches the registration roller pair, and is predetermined based on the largest expected amount of skew. Furthermore, when loop formation control is triggered by a detection signal from a sensor immediately before the registration roller pair, the size of the loop will take into account the variation in the detection of a skewed sheet.

[0008] On the other hand, if the size of the sheet loop maintained during transport is too large, the loop will buckle between the pre-registration roller pair and the registration roller pair. Therefore, it is preferable that the size of the sheet loop maintained during transport be small enough so that there is no tensile tension on the sheet between the pre-registration roller pair and the registration roller pair.

[0009] Therefore, if 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 buckling of the sheet may occur between the roller pair due to the large size of the loop mentioned above.

[0010] Therefore, the object of the present invention is to reduce buckling of the sheet between the pre-registration roller pair and the registration roller pair when the sheet is transported to the secondary transfer section. [Means for solving the problem]

[0011] A typical configuration of the present invention is a sheet transport device for transporting a sheet to a transfer transport unit that transfers a toner image supported on an image carrier to a sheet, comprising: a pair of pre-registration rollers that grip and transport the sheet; a pair of registration rollers provided downstream of the pair of pre-registration rollers in the sheet transport direction and gripping and transporting the sheet; and a control unit that controls the pair of pre-registration rollers and the pair of registration rollers, wherein the control unit transports the sheet with the pair of pre-registration rollers, brings the leading edge of the sheet into contact with the nip portion of the stopped-rotating pair of registration rollers, forms a first loop between the pair of pre-registration rollers and the pair of registration rollers with the sheet bent to one side, and further transports the sheet by driving the pair of pre-registration rollers and the pair of registration rollers, after the pair of registration rollers has started to rotate. The sheet enters the nip portion of the registration roller pair. Until then teeth , Without changing the conveying speed by the pre-registration roller pair and the registration roller pair, after the sheet enters the nip portion of the registration roller pair, The pre-registration roller pair is transported at a slower speed than the registration roller pair, forming a second loop smaller than the first loop in the sheet between the pre-registration roller pair and the registration roller pair. Then, after forming the second loop in the sheet, the pre-registration roller pair is transported at a faster speed than the transfer transport unit together with the registration roller pair, so that the loop of the sheet does not become larger than the first loop by the time the sheet reaches the transfer transport unit. It is characterized by doing so. [Effects of the Invention]

[0012] According to the present invention, when a 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 smaller, thereby reducing buckling of the sheet in that area. [Brief explanation of the drawing]

[0013] [Figure 1] Diagram showing the sheet transport section in an image forming apparatus. [Figure 2] (a), (b), (c) Diagrams showing the sheet's skew correction operation. [Figure 3] Block diagram of the control unit of the image forming apparatus. [Figure 4] (a) A diagram showing positions of a leading end and a trailing end of a sheet in a sheet conveying unit, (b) A diagram showing a relationship between a conveyance speed of a roller and time [Figure 5] (a), (b) Diagrams showing a loop shape of a sheet formed by a sheet conveying unit [Figure 6] A diagram showing a loop shape of a sheet formed by a sheet conveying unit of a comparative example [Figure 7] (a), (b) Diagrams showing a relationship between a conveyance speed of a roller and time [Figure 8] (a) A diagram showing positions of a leading end and a trailing end of a sheet in a sheet conveying unit, (b) A diagram showing a relationship between a conveyance speed of a roller and time [Figure 9] A schematic cross-sectional view showing the overall configuration of an image forming apparatus [Description of Embodiments]

[0014] Hereinafter, preferred embodiments of the present invention will be illustratively 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 thereto.

[0015] First, with reference to Figure 9, a schematic configuration of an image forming apparatus to which a sheet conveying apparatus is applied will be described. Figure 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 Figure 9 is mainly configured to include an image forming unit, a secondary transfer unit, and a sheet conveying apparatus (sheet conveying unit).

[0017] The image forming unit of the image forming apparatus 1 is configured to include a photoconductor 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. The image forming unit further 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 unit, the surface of the photoreceptor 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 rotating surface of the photoreceptor 11. The electrostatic latent image formed on the surface of the photoreceptor 11 becomes apparent as a toner image after toner development by the developing device 14. Subsequently, 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 serves as the image carrier, will be described. The intermediate transfer belt 21 is stretched by rollers such as the drive roller 23, tension roller 24, and secondary transfer inner roller 22, and is transported in the direction of arrow B shown in Figure 9. The parallel image formation processes for each color, Y, M, C, and Bk, are performed at the timing when they are superimposed on the toner image of the upstream color that has been primary transferred onto the intermediate transfer belt 21. As a result, a full-color toner image is ultimately 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, an intermediate transfer belt 21 is given as an example of an image carrier that holds a toner image, but the image carrier is not limited to this. For example, in an image forming apparatus that directly transfers a toner image formed on a photoreceptor to a sheet, the photoreceptor is the image carrier.

[0021] The transfer material, sheets P, are stored in paper feed cassettes (sheet storage units) 31, 32 and manual feed unit 33, for example, according to size. In order to record an image within the appropriate range, the image forming apparatus must recognize the size of the stored sheets.

[0022] The paper feed cassettes 31 and 32 are each equipped with sheet size detection units 31d and 32d for detecting the size of the sheets stored inside. The sheet size detection units 31d and 32d consist of a size detection lever (cam) and a plurality of sensors or switches. The detection lever (cam) is rotatably mounted and slides in conjunction with a side regulating plate that regulates the position in the width direction perpendicular to the sheet transport direction. The plurality of sensors or switches are provided in the mounting section where the paper feed cassette is mounted, at positions corresponding to the size detection lever.

[0023] Therefore, when the side regulating plate is moved to align with the width edge of the sheet, the size detection lever rotates in conjunction. When the paper feed cassette is then installed in the mounting section, the size detection lever selectively turns ON / OFF the detection element of the sensor or switch located in the mounting section where the paper feed cassette is installed. As a result, different patterns of signals are sent from the sensor or switch to the image forming apparatus main unit. The image forming apparatus main unit can then recognize the size of the sheet stored in the paper feed cassette based on these signals.

[0024] Sheets P are fed one by one, for example, from the paper feeding section 31a (or paper feeding section 32a). The sheet feeding section (sheet feeding device) consists of a pre-registration roller pair 41 and a registration roller pair 42, which correct the sheet's skew and then send it to the secondary transfer section. Specifically, sheet P is transported by the pre-registration roller pair 41, and its leading edge is brought against the nip of the stationary registration roller pair 42. In this way, the pre-registration roller pair 41 loops sheet P between itself and the stationary registration roller pair 42, correcting the sheet's skew.

[0025] The registration roller pair 42 transports the sheet P to the secondary transfer section in time with the transfer of the toner image on the intermediate transfer belt 21 to the sheet P.

[0026] The secondary transfer section has a toner image transfer nip section formed by a secondary transfer inner roller 22 and a secondary transfer outer roller 44 facing each other via an 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 transport section that transfers the toner image supported on the intermediate transfer belt 21, which is the image carrier, to the sheet P, and is provided downstream of the registration roller pair 42 in the sheet transport direction. In the secondary transfer section, the secondary transfer outer roller 44 faces the secondary transfer inner roller 22 via the intermediate transfer belt 21, and transports the sheet while sandwiching it between the secondary transfer inner roller 22 and the secondary transfer outer roller 44.

[0027] After the transfer, the sheet P is transported to the fixing unit 50, where pressure and heating are applied to melt and fix the toner image to the sheet P. After fixing, the sheet P is transported to the paper discharge unit 60, where the transport path is switched by the flapper 64 depending on whether the paper feeding mode is single-sided or double-sided. In other words, in single-sided mode, the flapper 64 switches the transport path towards the paper discharge roller 62, and the sheet P is discharged by the paper discharge roller 62 toward the paper discharge tray 80 and loaded.

[0028] On the other hand, in double-sided mode, the flapper 64 switches the sheet P towards the paper output roller 63, and the sheet P is transported by the paper output roller 63 to the reversing roller 71, which then discharges it toward the paper output tray 82. The sheet P is then stopped with its rear end remaining on the reversing roller 71 for a predetermined distance. After that, the sheet P is transported again to the registration roller pair 42 via the double-sided transport section 70 by the reversing of the reversing roller 71, and the second image is formed and fixed in the image forming section.

[0029] After fixing, the sheet P is discharged via the flapper 64 and then by the output roller 62 or reversing roller 71 towards the output tray 80 or output tray 82, where it is loaded.

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

[0031] The sheet transport unit 40 corrects the skew of the sheet P and transports the sheet P to the secondary transfer unit, which transfers the toner image carried on the intermediate transfer belt 21. The sheet transport unit 40 has a pre-registration roller pair 41 and a registration roller pair 42. Furthermore, the sheet transport unit 40 has 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), which will be described later.

[0032] The pre-registration roller pair 41 is a conveying roller pair that grips and conveys the sheet P. The pre-registration roller pair 41 comprises a driven roller 41a having a polyacetal (POM) roller and a drive roller 41b formed of a rubber roller. The driven roller pair 41 and the drive roller 41b are arranged facing each other. The driven roller 41a is pressed against the drive roller 41b by the elastic force of a spring (not shown) and rotates in conjunction with the rotationally driven drive roller 41b.

[0033] The registration roller pair 42 is located downstream of the pre-registration roller pair 41 in the sheet transport direction and is a transport roller pair that grips and transports the sheet P. The registration roller pair 42 comprises a driven roller 42a having a polyacetal (POM) roller and a drive roller 42b formed of a rubber roller. The registration roller pair 42 has the driven roller 42a and the drive roller 42b facing each other. The driven roller 42a is pressed against the drive roller 42b by the elastic force of a spring (not shown) and rotates in conjunction with the rotationally driven drive roller 42b.

[0034] Then, the sheet being transported by the pre-registration roller pair 41 is corrected for skewness by ensuring that the leading edge of the sheet comes into contact with the contact portion (nip portion) where the stationary driven roller 42a and the drive roller 42b come into contact.

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

[0036] In Figure 1, 101 is a pre-registration motor, which is a first drive source for driving the drive roller 41b of the pre-registration roller pair 41. 102 is 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 are positioned between the pre-registration roller pair 41 and the registration roller pair 42 to guide the sheet being conveyed. The guides 45 and 46 are partially spaced apart and configured to allow for loops formed by the sheet when it abuts against the nip portion of the registration roller pair 42.

[0038] Specifically, one guide 45 guides one side of the sheet being conveyed. The other guide 46 is provided opposite to the one guide 45 and guides the other side of the sheet being conveyed. Here, one guide 45 is partially spaced wider than the other guide 46. Therefore, the sheet, conveyed by the pre-registration roller pair 41 and with its leading edge in contact with 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 closer to the one guide 45. In other words, the sheet forms a loop between the roller pairs 41 and 42, with the sheet flexing to one side.

[0039] Next, the sheet skew correction operation by the sheet transport unit 40 will be explained using the schematic diagram in Figure 2.

[0040] Sheet skew refers to a state where, with respect to the center of the sheet's width direction perpendicular to the conveying direction (indicated by the dashed line in the figure), one side of the sheet's leading edge in the width direction leads the other side. Here, as shown in Figure 2(a), we illustrate a state where the left side of the sheet's leading edge in the width direction leads the right side, in other words, the sheet is skewed clockwise relative to the conveying direction.

[0041] As shown in Figure 2(a), the diagonal correction operation when the sheet is moving diagonally clockwise with respect to the transport direction (direction of arrow A) will be explained. Figure 2(a) shows the state when the leading edge of the sheet P has reached the registration sensor 43. As the pre-registration roller pair 41 rotates further and the sheet P is transported in the transport direction, the left leading edge of the sheet P comes into contact with the nip portion of the registration roller pair 42. At this time, the registration roller pair 42 has stopped rotating.

[0042] Furthermore, as the pre-registration roller pair 41 rotates and the sheet P is transported in the transport direction, the entire leading edge of the sheet P comes into contact with the nip portion of the registration roller pair 42 across its width, as shown in Figure 2(b). At this time, the leading edge of the sheet P stops when the leading corner comes into contact with the nip portion of the registration roller pair 42, and gradually a loop is formed in the sheet as it rotates so that the corner on the opposite side in the width direction from the corner that came 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, causing the sheet P to bend to one side. As a result, the entire width of the leading edge of the sheet P abuts against the nip portion of the registration roller pair 42, correcting any skew.

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

[0044] Subsequently, a toner image is formed on the intermediate transfer belt 21 in the image forming section. In sync with this, the registration roller pair 42 and the pre-registration roller pair 41 rotate, and as shown in Figure 2(c), the sheet P is transported to the secondary transfer section with its skew corrected.

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

[0046] When the registration sensor 43 is located in the center of the thrust direction, the loop amount r of the sheet P at the same thrust position as the registration sensor 43 is... 1C As mentioned above, this is a predetermined quantity. As shown in Figure 2(b), the loop amount r at the leading end of sheet P. 1R , Loop amount r at the delaying end 1F Therefore, the relationship of the loop length is r 1R >r 1C >r 1F This means that the loop amount on the preceding side of sheet P is equal to the pre-set loop amount r for the preceding portion. 1C It will become larger than that.

[0047] A predetermined loop space is secured between guides 45 and 46, taking into account the amount of loop formed by the sheet P 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 loop amount may exceed the expected amount. In this case, if the force received from the sheet P exceeds the force with which the registration roller pair 42 grips the sheet P, the sheet may slip at the nip portion of the registration roller pair 42, causing the sheet to skew or wrinkle. Alternatively, a buckling noise may occur due to buckling of the sheet loop. Furthermore, the load torque of the pre-registration motor may increase, potentially causing the motor to stop.

[0048] In this embodiment, which was made in consideration of these circumstances, the amount of loop r required for skew correction on the sheet is 1CAfter forming, the loop amount is set to r during the period from when the conveyance by the registration roller pair 42 is started until the sheet is conveyed to the secondary transfer portion 2C is decreased. In other words, the control unit 201 conveys the sheet by the pre-registration roller pair 41, and brings the leading edge of the sheet into contact with the registration roller pair 42 that is stopped from rotating. Then, a first loop in which the sheet is deflected to one surface side between the pre-registration roller pair and the registration roller pair (loop amount r 1C ) is formed. Further, after the registration roller pair 42 starts rotating and before the leading edge of the sheet reaches the secondary transfer portion, the control unit 200 sets the conveyance speed of the pre-registration roller pair 41 to be lower than that of the registration roller pair 42. Then, a second loop smaller than the first loop (the loop amount is r 2C ) is formed on the sheet between the pre-registration roller pair and the registration roller pair. 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 buckling of the sheet loop described above.

[0049] Next, with reference to FIG. 3 and FIG. 4, the control unit of the image forming apparatus 1 including the sheet conveying unit 40 according to the present embodiment, and the flow of the sheet skew correction operation and loop amount control by this control unit will be described. FIG. 3 is a block diagram for explaining the control unit of the image forming apparatus 1. FIG. 4(a) is a time-series graph of the rear end position of the leading sheet and the leading end position of the succeeding sheet conveyed by the pre-registration roller pair 41 and the registration roller pair 42. FIG. 4(b) shows a drive diagram by the pre-registration motor 101 and the registration motor 102, in which the horizontal axis represents time, and the vertical axis represents the sheet conveyance speed of the pre-registration roller and the registration roller driven by the motors.

[0050] As shown in Figure 3, the control unit 200 has various functional units such as a CPU (Central Processing Unit) 201, memory 202, operation unit 203, image forming control unit 205, and sheet transport control unit 206. The CPU 201 realizes various processes performed by the image forming apparatus 1 by executing predetermined control programs. The memory 202 is, for example, RAM (Random Access Memory) or ROM (Read-only memory) and stores various programs and data in a predetermined storage area. The operation unit 203 receives various information about the sheet to be used for printing (size, basis weight, surface properties, etc.), and various operations performed by the user, such as instructions to start or interrupt printing.

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

[0052] Furthermore, it is also possible to configure the system to receive various information about 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 drive of each motor 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 is the drive roller 41b of the pre-registration roller pair 41 that rotates by receiving the driving force from the pre-registration motor 101. The registration roller is the drive roller 42b of the registration roller pair 42 that rotates by receiving the driving force from the registration motor 102.

[0054] The CPU 201 controls the drive of the pre-registration motor 101 to transport the subsequent sheet by the pre-registration roller pair 41. At this time, the pre-registration roller pair 41 transports the subsequent sheet at a transport speed v1, and the registration roller pair 42 stops rotating. The leading edge of the subsequent sheet being transported at a transport speed v1 by the pre-registration roller pair 41 is detected by the registration sensor 43 at time t1, and then at time t2, it comes into contact with the nip portion of the stopped registration roller pair 42. Then, at time t3, the CPU 201 stops the drive of the pre-registration motor 101 and stops the pre-registration roller pair 41. As a result, a first loop (loop amount Xs shown in Figure 5(a)) is formed in the sheet between the pre-registration roller pair 41 and the registration roller pair 42, causing the sheet to bend to one side. This first loop is shown as loop amount Xs in Figure 4(b). The loop amount Xs is the predetermined loop amount of the sheet, and is the region shown in the grid area of ​​Figure 4(b).

[0055] Subsequently, the CPU 201 simultaneously starts driving 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 pre-registration roller pair 41 and the registration roller pair 42 simultaneously start transporting the subsequent sheet.

[0056] Then, at time t5, between the start of rotation of the registration roller pair 42 and the arrival of the leading edge of the subsequent sheet at the secondary transfer section, the CPU 201 sets the transport speed of the pre-registration roller pair 41 to a slower transport speed v2 than that of the registration roller pair 42. 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 Figure 5(b)) smaller than the first loop (loop amount Xs shown in Figure 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 shown by the shaded area in Figure 4(b), and represents the difference in transport amount between the registration roller pair 42 and the pre-registration roller pair 41.

[0057] Regarding the relationship between this loop amount Xr and the aforementioned first loop, loop amount Xs, by setting Xs > Xr, it is possible to reduce the second loop, loop amount Xs-Xr, while ensuring that it does not become zero.

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

[0059] On the other hand, the CPU 201 sets the pre-registration roller pair 41 to the same transport speed as the registration roller pair 42 from the time the registration roller pair 42 starts rotating until the sheet is transported a predetermined distance. In other words, the CPU 201 accelerates the pre-registration roller pair 41 to the same transport speed as the registration roller pair 42 from the time the registration roller pair 42 starts rotating until the sheet is transported a predetermined distance (from time t4 to time t5).

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

[0061] The CPU 201 sets the pre-registration roller pair 41 and the registration roller pair 42 to the same transport speed v4 as the secondary transfer outer roller 44 while the leading edge of the subsequent sheet forming the second loop reaches the secondary transfer section (between time t6 and time t7). That is, the pre-registration roller pair 41 and the registration roller pair 42 transport the sheet a certain distance at a faster transport speed v3 than the secondary transfer outer roller 44, and then decelerate to reach the same transport speed v4 as the secondary transfer outer roller 44 by time t7.

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

[0063] Furthermore, Figures 5(a) and 5(b) show the loop shape of the sheet between roller pairs 41 and 42 when conveyed using the operation described above. Figure 5(a) is a cross-sectional view showing the loop shape of the sheet at time t4 in Figure 4. Figure 5(b) is a cross-sectional view showing the loop shape of the sheet at time t7 in Figure 4. Figure 6 is a cross-sectional view showing the loop shape of the sheet at 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 formed between the registration roller pair 41 and 42 when the leading edge of the sheet abuts against the registration roller pair 42, is determined from various perspectives. One perspective is the distance required for the lagging side of the skewed sheet to reach the registration roller pair 42, i.e., the distance based on the amount of skew. Another perspective is the variation in the amount of conveyed sheet from detection by the registration sensor 43 until it reaches the registration roller pair 42. Another perspective is the amount by which the leading edge of the sheet is lowered 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 pair 41 and 42 is determined from various perspectives. Therefore, the required loop amount Xs of the sheet may become excessively large compared to the allowable loop space formed by the guides 45 and 46 between the roller pair 41 and 42.

[0065] As a comparative example, Figure 6 shows a case in which the sheet was transported to the secondary transfer section while maintaining the loop amount Xs of the sheet formed between roller pairs 41 and 42. In the comparative example shown in Figure 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. This is particularly noticeable as the loop increases on the leading side of the skewed sheet, as mentioned above.

[0066] On the other hand, according to the operation of this embodiment, as shown in Figure 5(b), after the registration roller pair 42 starts rotating, the loop amount Xs of the sheet is reduced by a loop amount Xr to Xs-Xr. That is, from the time the registration roller pair 42 starts rotating until the leading edge of the sheet reaches the secondary transfer section, the pre-registration roller pair 41 is transported at a slower speed than 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. This makes it possible to transport the sheet to the secondary transfer section stably without buckling.

[0067] In the comparative example shown in Figure 6, the case where the sheet thickness (basis weight) is relatively small was used as an example for explanation, but it is not limited to this. If the reaction force from the sheet becomes large, the load torque of the conveyor motor may become excessive, or the sheet may slip during conveyance by the registration roller pair 42, causing a delay in the timing of the sheet reaching the secondary transfer section. Cases in which the reaction force from the sheet becomes large include when the sheet basis weight is relatively large, or when the loop amount of the sheet is relatively large relative to the space created by the guides 45 and 46 between the roller pairs 41 and 42.

[0068] While it has been common practice to have a table that assigns different values ​​to the sheet loop amount Xs depending on the sheet information, i.e., the sheet size, basis weight, shape, and type, the sheet loop amount Xr can similarly be assigned a value according to the sheet information.

[0069] Specifically, we will compare three types of paper commonly used in offices: plain paper (68gsm), thick paper (300gsm), and thin paper (52gsm). Here, thick paper is thicker than plain paper and has higher rigidity. Thin paper is thinner than plain paper and has lower rigidity.

[0070] In the present embodiment, the loop amount Xs is an equivalent value (Xsn) for plain paper and thin paper, and is set to 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 without forming a loop similar to that for plain paper or thin paper during loop formation, the leading edge of the sheet receives a reaction force from the loop and obtains a 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 formed loop amount Xs also changes accordingly.

[0071] Further, regarding the loop amount Xr, it is an equivalent 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 a sheet is conveyed by the registration roller pair, the amount of loop formed between the pre-registration roller pair and the registration roller pair follows the order of thick paper, plain paper > thin paper > 0. This is due to the rigidity of the sheet. As for the formed loop, a smaller loop makes the thin sheet less likely to buckle.

[0072] While the above description has been given from the viewpoint of rigidity with respect to sheet information, the loop amount may be set in the same manner as described above based on sheet size, for example, sheet width information in the width direction orthogonal to the sheet conveyance direction. That is, when the sheet width is large, the reaction force from the sheet increases, which is considered similar to a sheet with high rigidity, and when the sheet width is small, the reaction force from the sheet decreases, which is considered similar to a sheet with low rigidity.

[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 sheet information is defined as 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 sheet information may be defined as the width in a direction perpendicular to the sheet transport direction. In this case, 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 as the sheet width decreases.

[0076] When forming the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42 in this manner, buckling of the sheet can be prevented according to the sheet information by changing the size of the second loop according to the sheet information.

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

[0078] Here, the sheet information is defined as the sheet's basis weight. In this case, if the sheet's basis weight is below a threshold, 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, if the sheet's basis weight exceeds a threshold, the CPU 201 does not form the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0079] Alternatively, the sheet information may be defined as the width in a direction perpendicular to the sheet's transport direction. In this case, if the sheet width is less than or equal to a threshold, 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, if the sheet width exceeds the threshold, the CPU 201 does not form the second loop in the sheet between the pre-registration roller pair 41 and the registration roller pair 42.

[0080] In this way, based on the information of the sheet, 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 prevents buckling of the sheet according to the information of the sheet, but also improves the productivity of the device.

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

[0082] In the previously described embodiment, it was explained that the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4. The reason for this is explained below. The registration roller pair 42 starts rotating at time t4 from a state where the leading edge of the sheet is abutting against it and forming a loop. In this case, for example, if the pre-registration roller pair 41 rises slightly ahead of the registration roller pair 42, the amount of loop will increase according to the amount of the misalignment. This may result in an excessive amount of loop on the sheet. On the other hand, if the pre-registration roller pair 41 rises slightly behind the registration roller pair 42, there is no particular problem if the reduction in loop due to the misalignment is sufficiently small compared to the loop amount Xs. However, if this is not the case, reducing the amount of loop before the sheet has fully entered the nip of the registration roller pair 42 may cause the corrected leading edge position of the sheet to shift again, potentially worsening the skew. For this reason, the pre-registration roller pair 41 and the registration roller pair 42 start rotating simultaneously at time t4.

[0083] Furthermore, while the motor control method for reducing the sheet loop amount was illustrated in Figure 4 in the above-described embodiment, the method is not limited to this. For example, several methods can be considered, as illustrated in Figures 7(a) and 7(b).

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

[0085] In other words, in the control shown in Figure 7(a), after transporting the sheet a predetermined distance, the CPU 201 sets the transport speed of the pre-registration roller to a slower speed than the registration roller at time t8. Then, at a later timing than the registration roller (time t9), the pre-registration roller is accelerated to a transport speed faster than the secondary transfer outer roller 44, and to reach the same transport speed v3 as the registration roller pair.

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

[0087] Furthermore, with the control shown in Figure 7(b), the pre-registration roller is set to a constant transport speed v5 at time t10. Then, at time t11, the pre-registration roller merges with the speed of the registration roller.

[0088] In other words, in the control shown in Figure 7(b), after transporting the sheet a predetermined distance, the CPU 201 sets the pre-registration roller to a constant transport speed that is faster than the secondary transfer outer roller 44 but slower than the registration roller at time t10. Subsequently, the pre-registration roller is set to the same transport speed as the registration roller.

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

[0090] Note that the control shown in Figures 7(a) and 7(b) is illustrative and not limiting. The setting should be such that the difference in the amount of sheet transported from the time the registration roller pair and the pre-registration roller pair are raised simultaneously until they reach the secondary transfer section is smaller for the latter than for the former.

[0091] Even with the control described above, the amount of loop formed for skew correction can be reduced before reaching the secondary transfer section. This makes it possible to reduce buckling noise of the sheet during sheet transport, reduce the torque of the motor driving the rollers, and improve image printing accuracy with a simple configuration.

[0092] Furthermore, in the embodiment described above, control was described in which the sheet is brought against the registration roller pair 42 to form a loop, then the system is stopped, and then transport is resumed. However, the system is not limited to this. For example, as shown in Figure 8, control may be used in which the registration roller pair forms a loop just before transport is resumed.

[0093] In the control shown in Figure 8, when the pre-registration roller temporarily stops transporting at time t23, the leading edge of the sheet has not yet reached the registration roller pair 42. The transport distance from time t24, when the pre-registration roller starts transporting, until time t25 is the first loop of the sheet formed between roller pairs 41 and 42 (loop amount XS2). The control after the registration roller and pre-registration roller stand up at time t25 is as described above. Finally, when the leading edge of the sheet reaches the secondary transfer section, the loop amount of the sheet between roller pairs 41 and 42 is XS2 - Xr.

[0094] In other words, in the control shown in Figure 8, the CPU 201 stops the pre-registration roller at time t23, and then restarts its rotation at time t24 before the registration rotation starts at time t23. Then, it forms a first loop (loop amount Xs2) in the sheet between roller pairs 41 and 42. The first loop (loop amount Xs2) formed in the sheet between roller pairs 41 and 42 may be formed in this manner.

[0095] Furthermore, although a printer was used as an example of an image forming apparatus in the embodiments described above, the present invention is not limited thereto. For example, other image forming apparatuses such as photocopiers and facsimile machines, or other image forming apparatuses such as multifunction devices that combine these functions, may also be used. Furthermore, although an image forming apparatus that uses an intermediate transfer body and sequentially transfers toner images of each color onto the intermediate transfer body, and then transfers the toner images supported on the intermediate transfer body to the transfer material all at once was given as an example, the present invention is not limited thereto. For example, an image forming apparatus that uses a transfer material carrier and sequentially transfers toner images of each color onto the transfer material supported on the transfer material carrier may also be used. Similar effects can be obtained by applying the present invention to a sheet transport device applied to these image forming apparatuses.

[0096] Furthermore, in the embodiments described above, a sheet transport device integrated with the image forming apparatus was exemplified as the sheet transport device applied to the image forming apparatus, but the present invention is not limited thereto. For example, it may be a sheet transport device that is detachably attached to the image forming apparatus, and similar effects can be obtained by applying the present invention to such a sheet transport device. [Explanation of Symbols]

[0097] P...sheet 1 ...Image forming apparatus 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 unit 33 ... Manual feeding unit 40 ... Sheet transport section 41 ... Pre-registration roller vs 41a ... Driven roller 41b ... Drive roller 42...Registration Roller vs 42a ... Driven roller 42b ... Drive roller 43...Registration Sensor 44... Secondary transfer outer roller 50 ... Fixing device 60 ... Paper output and transport section 63, 64 ... Transport guide 80 ... Paper output tray 200 ... Control Unit

Claims

1. A sheet transport device that transports a sheet to a transfer transport unit that transfers a toner image supported on an image carrier to a sheet, A pair of pre-registration rollers that grip and transport the sheet, A pair of registration rollers is provided downstream of the aforementioned pair of pre-registration rollers in the sheet transport direction, and which grips and transports the sheet. The pre-registration roller pair and the control unit that controls the registration roller pair, It has, The control unit transports the sheet using the pre-registration roller pair, bringing the leading edge of the sheet into contact with the nip portion of the registration roller pair, which is stopped rotating, thereby forming a first loop between the pre-registration roller pair and the registration roller pair, with the sheet flexed to one side. Furthermore, the sheet is transported by driving the pre-registration roller pair and the registration roller pair, and the transport speed of the pre-registration roller pair and the registration roller pair is not changed from the time the registration roller pair starts rotating until the sheet enters the nip portion of the registration roller pair, and after the sheet enters the nip portion of the registration roller pair, the transport speed of the pre-registration roller pair is set to be slower than that of the registration roller pair, thereby forming a second loop smaller than the first loop in the sheet between the pre-registration roller pair and the registration roller pair. A sheet conveying device characterized in that, after forming the second loop in the sheet, the pre-registration roller pair is conveyed at a faster speed than the transfer conveying unit together with the registration roller pair, so that the loop of the sheet does not become larger than the first loop by the time the sheet reaches the transfer conveying unit.

2. The control unit, after forming the first loop, starts the rotation of the registration roller pair and sets the transfer speed of the registration roller pair to a speed faster than that of the transfer transfer unit. The sheet conveying device according to claim 1, characterized in that, after forming the second loop, the pre-registration roller pair is set to the same conveying speed as the registration roller pair and faster than the transfer conveying section, and then, while the leading edge of the sheet forming the second loop is reaching the transfer conveying section, the pre-registration roller pair and the registration roller pair are set to the same conveying speed as the transfer conveying section.

3. The sheet conveying device according to claim 1 or 2, wherein the control unit further drives the pre-registration roller pair and the registration roller pair to convey the sheet, and from the time the registration roller pair starts rotating until the sheet is conveyed a predetermined distance, the pre-registration roller pair is conveyed at the same speed as the registration roller pair, and from the time the registration roller pair has conveyed the sheet a predetermined distance until the leading edge of the sheet reaches the transfer conveying unit, the pre-registration roller pair is conveyed at a slower speed than the registration roller pair, thereby forming a second loop smaller than the first loop in the sheet between the pre-registration roller pair and the registration roller pair.

4. The sheet conveying apparatus according to claim 3, characterized in that the control unit, after conveying the sheet a predetermined distance, sets the pre-registration roller pair to a constant conveying speed slower than the registration roller pair, and then sets the conveying speed to a speed faster than the transfer conveying unit and the same as the registration roller pair.

5. The sheet conveying apparatus according to claim 3, characterized in that the control unit, after conveying the sheet a predetermined distance, sets the conveying speed of the pre-registration roller pair to a slower speed than the registration roller pair, and accelerates it to a conveying speed faster than the transfer conveying unit, at a later timing than the registration roller pair, so as to reach the same conveying speed as the registration roller pair.

6. The sheet conveying apparatus according to claim 3, characterized in that the control unit conveys the sheet a predetermined distance, then sets the pre-registration roller pair to a conveying speed faster than the transfer conveying unit but slower than the registration roller pair, and then sets it to the same conveying speed as the registration roller pair.

7. The sheet conveying device according to any one of claims 1 to 6, characterized in that the control unit stops the pre-registration roller pair once, and then restarts the rotation of the registration roller pair before the registration roller pair starts rotating, thereby forming the first loop between itself and the registration roller pair.

8. The sheet conveying device according to any one of claims 1 to 4, characterized in that the control unit determines whether or not to form the second loop in the sheet between the pre-registration roller pair and the registration roller pair, according to information about the sheet.

9. The information on the aforementioned sheet refers to the basis weight of the sheet. The sheet conveying device according to claim 8, characterized in that the control unit forms the second loop in the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet is less than or equal to a threshold, and does not form the second loop in the sheet between the pre-registration roller pair and the registration roller pair when the basis weight of the sheet exceeds a threshold.

10. The information on the sheet refers to the width in the direction perpendicular to the sheet's transport direction. The sheet conveying device according to claim 8, characterized in that the control unit forms the second loop in the sheet between the pre-registration roller pair and the registration roller pair when the sheet width is less than or equal to a threshold, and does not form the second loop in the sheet between the pre-registration roller pair and the registration roller pair when the sheet width exceeds a threshold.

11. The sheet conveying device according to any one of claims 1 to 4, 8, characterized in that the control unit changes the size of the second loop in accordance with the sheet information when forming the second loop in the sheet between the pre-registration roller pair and the registration roller pair.

12. The information on the aforementioned sheet refers to the basis weight of the sheet. The sheet conveying device according to claim 11, characterized in that the control unit reduces the size of the second loop formed in the sheet between the pre-registration roller pair and the registration roller pair in accordance with the decrease in the basis weight of the sheet.

13. The information on the sheet refers to the width in the direction perpendicular to the sheet's transport direction. The sheet conveying device according to claim 11, characterized in that the control unit reduces the size of the second loop formed in the sheet between the pre-registration roller pair and the registration roller pair in accordance with the reduction in sheet width.

14. Image carrier and, A transfer transport unit that transfers the toner image supported on the image carrier to a sheet, A sheet conveying device according to any one of claims 1 to 13, An image forming apparatus characterized by having the following features.

Images