JP2011063373A - Sheet carrying device and image forming device - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a sheet carrying device capable of carrying a sheet without pulling the sheet. <P>SOLUTION: The sheet carrying device includes a carrying passage, a plurality of roller pairs Ru and Rd arranged along the carrying passage, and a control part for controlling the drive of the roller pairs Ru and Rd. The control part performs temporary stopping control for driving at a higher speed than the upstream side roller pairs Ru positioned immediately before the upstream side of the one roller pairs Rd after temporarily stopping the drive until a specified stop time passes after the sheet front end reaches a nip position of the roller pairs Rd on at least the one roller pairs Rd among the plurality of roller pairs Ru and Rd. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a sheet conveying apparatus that conveys a sheet by a plurality of roller pairs arranged along a guide path, and an image forming apparatus equipped with the sheet conveying apparatus.

  2. Description of the Related Art Conventionally, an image forming apparatus including a sheet conveying device that conveys a sheet (for example, a sheet, a cloth sheet, an OHP sheet, etc.) and a printing unit that performs a printing process on the conveyed sheet is widely known.

  For example, Patent Document 1 discloses an image forming apparatus that forms a monochrome image or a color image by passing a sheet onto which a toner image has been transferred through a transfer unit and a fixing unit a plurality of times and overlapping the toner image on the sheet. Is disclosed. This image forming apparatus incorporates a circulation path that goes around the photosensitive drum.

  Patent Document 2 discloses an image forming apparatus equipped with a sheet conveying device that can appropriately correct the posture of the conveyed paper. In the sheet conveying apparatus described in Patent Document 2, while the registration roller pair provided on the conveyance path (guide path) is temporarily stopped, the conveyance roller pair positioned immediately upstream of the registration roller is rotationally driven. Thus, the paper is temporarily bent (bent), and then the rotation of the transport roller pair is temporarily stopped to naturally correct the posture of the paper.

JP 2006-349701 A Japanese Patent Laid-Open No. 7-309476

  By the way, in such a sheet conveying apparatus, a conveying path (guide path) for guiding the conveyance of the sheet is installed at an interval on the conveying path, and the sheet is rotated while sandwiching the sheet to convey the sheet downstream. And a plurality of roller pairs. In order to prevent pulling of the conveyed sheet, speed control is often performed so that a roller pair positioned on the upstream side of a pair of rollers positioned on the downstream side among adjacent roller pairs is rotated at a high speed. In other words, in order to prevent the sheet from being pulled, the speed of the roller pair is often decreased as it goes downstream.

  However, when the speed of the roller pair is gradually decreased as approaching the downstream side, the sheet transport speed near the downstream end becomes excessively slow when the transport path is long and a large number of roller pairs are provided. There was a problem. This problem can be solved by making the downstream side faster than the upstream side. In this case, however, there arises a problem that the sheet is pulled.

  Some sheet conveying apparatuses include a circulation conveyance path for repeatedly circulating and conveying one sheet to the same location. In such a sheet conveying apparatus, one roller pair may be on the upstream side as well as on the downstream side as viewed from another roller pair, and the roller pair positioned on the upstream side of the roller pair on the downstream side may be at a higher speed. There was a case that I could not do.

  Accordingly, an object of the present invention is to provide a sheet conveying apparatus and an image forming apparatus that can prevent the sheet from being pulled.

  The sheet conveying apparatus according to the present invention includes a conveying path that guides conveyance of a sheet, and a plurality of pairs of rollers disposed on the conveying path at intervals, while sandwiching the sheet conveyed from the upstream side. A plurality of roller pairs that rotate to convey the sheet downstream, and a control unit that controls at least driving of the plurality of roller pairs, and the control unit includes at least one of the plurality of roller pairs. For one roller pair, after temporarily stopping driving until a specified stop time elapses after the front end of the sheet reaches the nip position of the roller pair, an upstream roller positioned immediately before the upstream side of the one roller pair Temporary stop control is performed to drive at higher speed than the pair.

  In a preferred aspect, the stop time is T, the sheet conveyance speed when the roller pair that is temporarily stopped sandwiches and conveys the sheet is Sd, and the sheet conveyance speed when the upstream roller pair sandwiches and conveys the sheet is Su, When the distance between the nips of the temporarily stopped roller pair and the upstream roller pair is X, and the sheet length, which is a predetermined reference sheet conveyance direction length, is L, T ≧ (Sd−Su) × (L −X) / (Su × Sd). In this case, it is desirable that the reference sheet is a sheet having the longest sheet length among sheets that are actually conveyed or sheets that can be conveyed by the sheet conveying apparatus.

  In another preferred aspect, the conveyance path constitutes a circulation path so that one sheet is repeatedly circulated and conveyed. In another preferred aspect, the pause control is performed for two or more roller pairs.

  The control unit performs the pause control on a roller pair positioned immediately upstream of a detection unit (for example, a paper leading edge detection sensor) for controlling at least sheet conveyance timing among the plurality of roller pairs. It is desirable to do. In addition, it is preferable that the control unit performs the pause control for at least a roller pair positioned immediately upstream of the roller pair for correcting the posture of the sheet among the plurality of roller pairs. In addition, it is preferable that the control unit performs the pause control for at least a roller pair positioned immediately upstream of a size detecting unit that detects a sheet size among the plurality of roller pairs. In addition, it is preferable that the control unit does not perform the pause control for a roller pair positioned immediately downstream of the size detection unit that detects the size of the sheet. Further, the control unit performs the pause control for at least a roller pair positioned immediately downstream of a fixing device that fixes the toner image transferred to the sheet to the sheet, among the plurality of roller pairs. It is desirable.

  According to the first and eleventh aspects of the present invention, since the sheet is temporarily slackened by performing the temporary stop control, the sheet is not pulled even if it is conveyed at a high speed thereafter.

  According to the second and third aspects of the invention, since a sufficient stop time is secured for preventing the sheet from being pulled, the sheet is prevented from being pulled more reliably.

  According to the fourth aspect of the present invention, even when the sheet is circulated and conveyed, the sheet is prevented from being pulled. According to the invention described in claim 5, it is possible to prevent an excessive decrease in the sheet conveying speed.

  According to the sixth aspect of the invention, since the sheet is sent to the roller pair that controls the timing in a state where there is no tension, the timing error can be reduced. According to the seventh aspect of the invention, since the sheet is sent to the roller pair that corrects the posture in a state where there is no tension, the posture correction effect can be improved. According to the eighth and ninth aspects of the invention, the sheet size detection accuracy can be improved. According to the invention of claim 10, sheet wrinkles and image defects can be reduced.

1 is a schematic configuration diagram of an image forming apparatus according to an embodiment of the present invention. It is a figure which shows the example of the speed relationship in the case of carrying out circulation conveyance of a sheet. It is a schematic diagram which shows the mode of a temporary stop process. It is an image figure in the case of having many roller pairs.

  Embodiments of the present invention will be described below with reference to the drawings. FIG. 1 shows an image forming apparatus 10 according to an embodiment of the present invention. The image forming apparatus 10 is a device that executes a printing process for transferring and fixing a toner image on a sheet conveyed along a conveyance path. It has a configuration suitable for forming one image by overlapping images. In the following, only the configuration of the image forming apparatus 10 alone is disclosed. However, the image forming apparatus 10 may be used as a single printer, or incorporated in a FAX, a copier, a multifunction peripheral, or the like. Also good. Hereinafter, the image forming apparatus 10 will be described in detail.

  The image forming apparatus 10 is broadly classified into a printing unit that performs a printing process on a conveyed sheet, a conveyance mechanism that conveys the sheet, and a control unit (not shown) that controls these.

  The printing unit includes a developing rack 12, a photosensitive drum 14, a transfer roller 16, a fixing roller pair 18, and the like. The developing rack 12 is driven to rotate in the direction of arrow A, and developing devices Y, M, C, and K corresponding to toners of four colors of yellow, magenta, cyan, and black are mounted on the outer periphery thereof. Yes.

  Each of the developing units Y, M, C, and K is opposed to a photosensitive drum 14 (photosensitive member) that is rotationally driven in the direction of arrow F in accordance with the rotational driving of the developing rack 12. A transfer roller 16 (transfer device) is disposed in contact with the photosensitive drum 14. The photosensitive drum 14 and the transfer roller 16 rotate while sandwiching the conveyed sheet so that the toner image is transferred onto the surface of the sheet. In other words, the photosensitive drum 14 and the transfer roller 16 constitute a transfer unit 20 that transfers a toner image to a sheet. In this embodiment, the toner image is directly transferred from the photosensitive drum 14 to the sheet. However, an intermediate transfer member may be interposed between the photosensitive drum 14 and the sheet. Although not shown, a charging device for uniformly charging the surface of the photosensitive drum 14 is provided around the photosensitive drum 14, and the surface of the uniformly charged photosensitive drum 14 is exposed according to image data to be static. An exposure device for forming an electrostatic latent image, and a toner image formed by developing the electrostatic latent image by each of the developing units Y, M, C, and K is transferred onto a sheet and then remains on the surface of the photosensitive drum 14 A cleaning device is provided for collecting the toner to be recovered.

  The fixing roller pair 18 is a roller pair for fixing the transferred toner image on the sheet, and a heater is provided therein. The fixing roller pair 18 is heated by a heater, and rotates while sandwiching (pressing) the conveyed sheet, thereby feeding the sheet downstream while heating and pressing the sheet. At this time, the toner on the sheet is melted by the heat received from the fixing roller pair 18, whereby the toner image is fixed on the sheet. In the following, the sheet sandwiching position by the fixing roller pair 18 is referred to as a “fixing portion”.

  The conveyance mechanism is a mechanism for conveying the sheet. The conveyance mechanism according to the present embodiment is configured such that one sheet can be repeatedly conveyed to the transfer unit 20 and the fixing unit 22 and circulated. More specifically, the conveyance mechanism includes a conveyance path 30 that guides conveyance of the sheet, a plurality of conveyance roller pairs 38 arranged along the conveyance path 30, and the like.

  The conveyance path 30 is further divided into a main conveyance path 32, a paper discharge conveyance path 36, and a circulation conveyance path 34. The main conveyance path 32 is a conveyance path that guides the sheet received from the sheet feeding unit 40 to the transfer unit 20 and the fixing unit 22. The downstream end of the main conveyance path 32 is branched into a paper discharge conveyance path 36 and a circulation conveyance path 34. The paper discharge conveyance path 36 is a conveyance path that guides the sheet output from the main conveyance path 32 to a paper discharge unit (not shown). The sheet for which the printing process has been completed is conveyed along the paper discharge conveyance path 36.

  The circulation conveyance path 34 is a conveyance path for returning the sheet output from the main conveyance path 32 to the main conveyance path 32 again. The downstream end of the circulation conveyance path 34 is connected to the upstream position of the posture correction roller pair 38a described later in the main conveyance path 32. By being transported along the circulation transport path 34 and retransmitted to the main transport path 32, and eventually to the transfer section 20 and the fixing section 22, a single sheet is subjected to a plurality of printing processes.

  A switch 42 is provided near the downstream end of the main conveyance path 32 to switch the conveyance destination of the sheet output from the main conveyance path 32 to one of the discharge conveyance path 36 and the circulation conveyance path 34. The control unit drives the switch 42 in accordance with the progress of printing to switch the sheet conveyance destination.

  The pair of conveying rollers 38 is composed of two rollers that are opposed to each other and rotates while sandwiching the sheet, thereby sending the sheet to the downstream side. A plurality of the conveyance roller pairs 38 are arranged along the conveyance path 30.

  Among the plurality of transport roller pairs 38, an attitude correction roller pair 38c and a timing roller pair 38b are included. The attitude correction roller pair 38c is a conveyance roller pair 38 that corrects the attitude of the conveyed sheet, that is, eliminates the skew of the sheet. The posture correction roller pair 38 c is located upstream of the transfer unit 20 and downstream of the joining position of the main conveyance path 32 and the circulation conveyance path 34. Therefore, the circulated and conveyed sheet always passes through the posture correction roller pair 38c. When correcting the posture of the sheet, the pair of posture correction rollers 38c is arranged obliquely in the side guide direction so that the sheet is placed along the side guide for aligning the sheet posture on the side surface. It is transported downstream while being along.

  The timing roller pair 38b is disposed on the downstream side of the posture correction roller pair 38c and on the upstream side of the transfer unit 20, and detects the timing at which the conveyed sheet passes the timing sensor 45, thereby conveying the sheet. Control timing. That is, in order to transfer the toner image formed on the photosensitive drum 14 to a specified position on the conveyed sheet, the rotational speed of the photosensitive drum 14, the formation position of the toner image on the photosensitive drum 14, It is necessary to accurately control the sheet conveyance timing. The timing roller pair 38 b accurately detects the passage of the sheet by the timing sensor 45 and sends the sheet downstream so that the sheet is sent to the transfer unit 20. That is, the timing roller pair 38 can be said to be a roller pair positioned immediately upstream of the timing sensor 45 that is a detection unit for controlling the sheet conveyance timing.

  Incidentally, as is apparent from the above description, not only the conveying roller pair 38 but also a roller pair composed of the photosensitive drum 14 and the transfer roller 16 and a roller pair composed of two fixing rollers are conveyed. It functions as a member that conveys the sheet to the downstream side by rotating while sandwiching the incoming sheet. In the following description, a member that conveys a sheet downstream by rotating the sheet conveyed in this manner is referred to as a “roller pair”.

  Further, among the adjacent roller pairs, the roller pair positioned on the upstream side is referred to as “upstream roller pair”, and the roller pair positioned on the downstream side is referred to as “downstream roller pair”. Furthermore, the sheet conveyance speed when the upstream roller pair actually conveys the sheet is “upstream speed”, and the sheet conveyance speed when the downstream roller pair actually conveys the sheet is “downstream speed”. "

  A sheet length detection sensor 44 that is the length in the sheet conveyance direction is further provided on the conveyance path 30. The sheet length detection sensor 44 is a sensor that detects the arrival of the front and rear ends of the conveyed sheet. The control unit acquires the sheet length of the sheet being conveyed based on the difference between the front end arrival timing and the rear end arrival timing detected by the sheet length detection sensor 44 and the sheet conveyance speed.

  The control unit controls driving of the printing unit and the transport mechanism described above. The control unit of the present embodiment also controls the conveyance speed of the roller pair. Hereinafter, the speed control of the roller pair will be described in detail.

  The control unit according to the present exemplary embodiment is configured so that a specified stop time elapses after the front end of the sheet reaches the nip position of the roller pair of at least one of the plurality of roller pairs provided along the conveyance path 30. Then, the drive is temporarily stopped until it is stopped, and then the pause control is performed to drive at a higher speed than the roller pair positioned immediately upstream of the one roller pair. The reason for performing such control is as follows.

  Originally, when sheets are sequentially conveyed downstream by a plurality of roller pairs along the conveyance path 30, the sheet conveyance of the upstream roller pair is greater than the downstream speed Su that is the sheet conveyance speed of the downstream roller pair. It is desirable to control the conveying speed of each roller pair so that the upstream speed Sd, which is the speed, increases (Sd> Su). This is to prevent the sheet from being pulled during the conveyance process.

  That is, in order to sequentially convey the sheet to the downstream side, a plurality of roller pairs are arranged at intervals less than the sheet length, and the sheet nipping conveyance by the upstream roller pair ends (the sheet trailing edge is the upstream roller pair). Before reaching the nip of the sheet), the sheet needs to be transferred to the downstream roller pair (the front end of the sheet reaches the nip of the downstream roller pair). In other words, it is necessary that the conveyed sheet is always sandwiched and conveyed by two or more adjacent roller pairs. In this way, in the case of being conveyed while being sandwiched between two adjacent roller pairs, the speed of the downstream roller pair that sandwiches and conveys the front of the sheet is greater than the speed of the upstream roller pair that sandwiches and conveys the rear of the sheet. The front of the seat is pulled downstream as compared to the rear of the seat. Such pulling may lead to sheet wrinkling, kinking, and image defects. Therefore, originally, it is desirable to set [upstream speed> downstream speed].

  Here, the condition of [upstream speed> downstream speed] is that the sheet is transported along a path without feedback, for example, transported from the paper feed unit 40 to the paper discharge unit through the transfer unit 20 and the fixing unit 22. Then it can be realized without problems. However, the sheet is fed back and circulated, for example, after passing from the sheet feeding unit 40 through the transfer unit 20 and the fixing unit 22, the sheet is again transferred to the transfer unit 20 and the fixing unit 22 through the circulation conveyance path 34. In transport such as sending, it is difficult to establish the condition [upstream speed> downstream speed].

  This will be described with reference to FIG. FIG. 2 is a schematic diagram illustrating how the sheet is circulated and conveyed. Four roller pairs A, B, C, and D are provided along the annular conveyance path 30, and the sheets are circulated and conveyed in the order of the roller pairs A, B, C, D, A, B,. And In this case, in order to establish [upstream speed> downstream speed], it is assumed that the speed of the roller pair positioned downstream from the roller pair A is decreased by 0.01. That is, when the speed of the roller pair A is Sa = 1, the speed of the roller pair B is Sa = 0.99, the speed of the roller pair C is Sc = 0.98, and the speed of the roller pair D is Sd = 0.97. Suppose that In this case, the condition of [upstream speed> downstream speed] is satisfied in the flow up to the roller pair A, B, C, D, and there is no problem. However, when transporting from the roller pair D to the roller pair A, the condition [upstream speed> downstream speed] is not satisfied. That is, when conveying from the roller pair D to the roller pair A, the roller pair A corresponds to the “downstream roller pair” as viewed from the roller pair D, but the speed Sa = 1 of the roller pair A is the roller pair. The speed Sd of D exceeds 0.97. As a result, the sheet is pulled when being conveyed from the roller pair D to the roller pair A.

  Therefore, in the present embodiment, for at least one of the plurality of roller pairs provided along the conveyance path 30, a specified stop time elapses after the front end of the sheet reaches the nip position of the roller pair. After the drive is temporarily stopped, temporary stop control is performed to drive the roller pair at a higher speed than the roller pair located immediately upstream of the one roller pair. By performing such temporary stop control, in some cases, the sheet is not pulled even if [upstream speed <downstream speed]. This will be described with reference to FIG.

  FIG. 3 is a diagram illustrating a state in which temporary stop control for preventing pulling is performed. In FIG. 2, the sheet is conveyed from the right side of the drawing to the left side. Further, the upstream roller positioned on the upstream side (right side of the drawing) than the sheet conveyance speed (downstream speed Sd) when the downstream roller pair Rd positioned on the downstream side (left side of the drawing) actually sandwiches and conveys the sheet. The sheet conveyance speed (upstream speed Su) when the pair Ru actually conveys the sheet is small (Sd> Su).

  In this state, if the conveyance of the sheet is continued without temporarily stopping the downstream roller pair Rd, the front of the sheet is naturally pulled by the high-speed downstream roller pair Rd. In order to prevent this pulling, in the present embodiment, first, the downstream roller pair until the front end of the sheet fed by the upstream roller pair Ru reaches the nip position of the downstream roller pair Rd. Rd is stopped rotating.

  Even if the rotation of the downstream roller pair Rd is stopped, since the upstream roller pair Ru is driven to rotate, naturally, the front end of the sheet gradually approaches the downstream roller pair Rd, and finally the downstream roller pair Rd It contacts the nip of the side roller pair Rd (time 0 state). Thus, even after the front end of the sheet comes into contact with the nip of the downstream roller pair Rd, the rotation of the downstream roller pair Rd is stopped until a stop time T described later elapses. On the other hand, since the upstream roller pair Ru continues to rotate, the sheet feeding is continued by the upstream roller pair Ru. As a result, as shown at time t1 in FIG. 3, the sheet that cannot pass through the nip of the downstream roller pair Rd is gradually loosened and bent.

  Thereafter, when the predetermined stop time T has elapsed and time t2 = t1 + T is reached, the driving of the downstream roller pair Rd is resumed at the downstream speed Sd (> upstream speed Su). At this time, the front part of the sheet is fed out at a higher speed than the rear part of the sheet. However, since the downstream roller pair Rd is temporarily stopped to loosen the sheet in advance, even if the front of the sheet is fed at a high speed at this time, the sheet is not pulled. That is, by temporarily stopping the downstream roller pair Rd, even if the upstream speed Su is smaller than the downstream speed Sd (Sd> Su), the sheet is prevented from being pulled. As a result, even when the sheet is repeatedly circulated and conveyed, the sheet is prevented from being pulled. In order to reliably prevent the sheet from being pulled, it is necessary to set the stop time T to an appropriate value.

  Therefore, a stop time T, which is a time from when the front end of the sheet reaches the nip of the downstream roller pair Rd whose rotation is stopped to when the rotation of the downstream roller pair Rd is restarted, will be described with reference to FIG. To do.

  Naturally, if the stop time T is excessively short, the amount of looseness of the sheet is reduced, and the sheet may be pulled after the driving of the downstream roller pair Rd is resumed. Therefore, in the present embodiment, the stop time T is set as T ≧ (Sd−Su) × (L−X) / (Su × Sd). Here, L is the sheet length of the sheet being conveyed, and X is the distance between the nip between the upstream roller pair Ru and the downstream roller pair Rd. As described above, Sd is the sheet conveyance speed when the downstream roller pair Rd nipping and conveying the sheet, Su is the sheet conveyance speed when the upstream roller pair Ru nipping and conveying the sheet, and Sd> Su.

  The reason why T ≧ (Sd−Su) × (L−X) / (Su × Sd) is as follows. It is assumed that the front end of the sheet has reached the nip position of the downstream roller pair Rd at time 0. Thereafter, the rotational driving of the downstream roller pair Rd is stopped only for the stop time T, and the downstream roller pair Rd resumes driving at time t1 when the stop time T has elapsed, and finally the trailing edge of the sheet is on the upstream side at time t2. It is assumed that the nip position of the roller pair Ru has been reached.

  In this case, in order to prevent the sheet from being pulled, [the sheet distance Au sent by the upstream roller pair Ru from time 0 to time t2] ≧ [downstream roller pair Rd from time 0 to time t2. Sheet distance Ad] to be sent out. The sheet distance Au sent by the upstream roller pair Ru between time 0 and time t2 is Au = L−X, and [time from time 0 to time t2] is (L−X) / Su.

  On the other hand, the sheet distance Ad sent by the downstream roller pair Rd from time 0 to time t2 is Ad = [time from time t1 to time t2] × Sd. Then, [time from time t1 to time t2] = [time from time 0 to time t2 minus stop time T], which is (L−X) / Su−T. Therefore, Ad = {(L−X) / Su−T} × Sd. Since Au ≧ Ad, L−X ≧ {(L−X) / Su−T} × Sd, and T ≧ (Sd−Su) × (L−X) / (Su × Sd).

  In the present embodiment, the stop time T is calculated based on the sheet length of the sheet being conveyed. However, instead of the sheet being conveyed, the stop time T may be calculated based on the maximum sheet length among the sheet lengths that can be handled by the image forming apparatus 10. By calculating the stop time T based on the maximum sheet length in this way, the stop time T is sufficiently ensured regardless of which sheet is conveyed. As a result, there is no need to change the stop time T according to the type of sheet being conveyed (difference in sheet length), and the drive control of the roller pair is simplified.

  By the way, the pause process as described above may be executed for a plurality of roller pairs. By executing the temporary stop process for the plurality of roller pairs, the conveyance speed of some of the roller pairs does not become excessively small. Further, variation in the conveyance speed of the roller pair is reduced. That is, as shown in FIG. 4, a case is considered in which a sheet is circulated and conveyed by 100 roller pairs R1 to R100. In this case, if the temporary stop control described above is executed only for one roller pair R1, it is necessary to gradually decrease the conveying speed from the roller pair R1 to the roller pair R100, and finally the roller pair R100. There is a possibility that the conveyance speed of the machine becomes very low. In this case, of course, the time required for sheet conveyance increases. Further, the variation in the conveyance speed between the roller pairs becomes large, and it becomes difficult to convey the sheet at a conveyance speed suitable for the sheet type. That is, it is conventionally known that it is desirable to change the conveyance speed depending on the sheet type (sheet thickness or the like). However, when only a single roller pair is temporarily stopped, the difference in sheet conveyance speed between the roller pair R100 and the roller pair R1 also increases, and either the roller pair R100 or the roller pair R1 is suitable for the sheet type. The transfer speed must be significantly different from the transfer speed.

  On the other hand, when the pause process is executed for a plurality of roller pairs, the condition of [upstream speed> downstream speed] is not restricted. Therefore, the degree of freedom in setting the conveyance speed of each roller pair is improved, and it becomes easy to select a conveyance speed suitable for the sheet type.

  Here, the roller pair on which the temporary stop process is executed may be any of a plurality of roller pairs. However, in order to reduce the sheet timing error, it is desirable to perform a pause process on the roller pair 38b immediately upstream of the timing sensor 45 used for sheet timing control. With this configuration, the timing sensor 45 transports the sheet in a state where there is no delay in the sheet transport speed due to pulling, and the timing error is reduced. In order to improve the effect of correcting the posture of the sheet, it is desirable to perform a pause process on the roller a immediately before the upstream of the posture correction roller pair 38c. With this configuration, the sheet is conveyed to the posture correction roller pair 38c without being skewed due to the tension, and the adverse effect of the sheet tension on the posture correction is reduced. That is, if a pull occurs between the roller pair 38, sheet conveyance by the roller pair 38 on the downstream side becomes unstable and skew is likely to occur. However, such a skew occurs by preventing the pull in advance. It becomes difficult.

  Furthermore, in order to improve the accuracy of sheet length detection, it is desirable to perform a pause process on the roller pair 38e immediately upstream of the sheet length detection sensor 44. With such a configuration, the sheet passes through the sheet length detection sensor 44 without being pulled, and the accuracy of sheet length detection is improved. Of course, if the sheet is slackened in the process of passing through the sheet length detection sensor 44, the sheet length cannot be accurately detected. Therefore, it is desirable not to execute the temporary stop process for the roller pair 38 f immediately after the downstream side of the sheet length detection sensor 44.

  Further, in order to reduce sheet wrinkles and image defects, it is desirable to perform a pause process on the roller pair 38e immediately after the fixing roller pair 18 on the downstream side. That is, the sheet immediately after passing through the fixing roller pair 18 is fragile due to the influence of heat or the like, and is easily creased and image defects are easily caused by being pulled. By executing the temporary stop process in the roller pair 38e immediately after the downstream side of the fixing roller pair 18, the fragile sheet is not pulled, and consequently wrinkles and image defects are reduced.

  As is apparent from the above description, according to the present embodiment, the temporary stop process is executed on some of the plurality of roller pairs that circulate and convey the sheet. This prevents the sheet from being pulled even when the sheet is circulated and conveyed.

  In the above description, the image forming apparatus 10 that circulates and conveys a sheet is described as an example. However, if a sheet conveying apparatus that conveys a sheet using a plurality of pairs of rollers is incorporated, other types are used. You may apply to an apparatus. For example, it may be incorporated in an image forming apparatus that is not circulated and conveyed. Further, the present invention is not limited to the image forming apparatus, and may be applied to, for example, a coating apparatus that circulates and conveys the plastic sheet in order to apply a plurality of types of coatings on the surface of the plastic sheet.

  DESCRIPTION OF SYMBOLS 10 Image forming apparatus, 12 Developing rack, 14 Photosensitive drum, 16 Transfer roller, 18 Fixing roller pair, 20 Transfer part, 22 Fixing part, 30 Transport path, 32 Main transport path, 34 Circulation transport path, 36 Paper discharge transport path , 38 Conveying roller pair, 40 Paper feeding unit, 42 Switching unit, 44 Sheet length detection sensor.

Claims (11)

A guide path for guiding the conveyance of the sheet;
A plurality of roller pairs disposed on the guide path at intervals, and a plurality of roller pairs that convey the sheet downstream by rotating while sandwiching the sheet conveyed from the upstream side; and
A control unit that controls driving of the plurality of roller pairs;
With
The control unit temporarily stops driving of at least one of the plurality of roller pairs until a predetermined stop time elapses after the front end of the sheet reaches the nip position of the roller pair. , Performing a pause control to drive at a higher speed than the upstream roller pair located immediately before the upstream side of the one roller pair,
A sheet conveying apparatus. The sheet conveying apparatus according to claim 1,
T is the stop time, Sd is the sheet conveyance speed when the pair of rollers to be paused nipping and conveying the sheet, Su is the sheet conveyance speed at which the pair of upstream rollers nipping and conveying the sheet, and the roller that is temporarily stopped T ≧ (Sd−Su) × (L−X) / (L) where X is the distance between the nips of the pair of rollers and the upstream roller pair, and L is the sheet length that is a predetermined reference sheet conveyance direction length. (Su × Sd). The sheet conveying apparatus according to claim 2,
The sheet conveying apparatus, wherein the reference sheet is a sheet that is actually conveyed or a sheet having a maximum sheet length among sheets that can be conveyed by the sheet conveying apparatus. The sheet conveying apparatus according to any one of claims 1 to 3,
The sheet conveying apparatus is characterized in that the guide path constitutes a route that is circulated so that one sheet is repeatedly circulated and conveyed. The sheet conveying device according to any one of claims 1 to 4,
The control unit performs the temporary stop control for two or more pairs of rollers. The sheet conveying device according to any one of claims 1 to 5,
The control unit performs the suspension control on at least a roller pair positioned immediately upstream of a detection unit for controlling sheet conveyance timing among the plurality of roller pairs. Conveying device. The sheet conveying device according to any one of claims 1 to 6,
The control unit performs the suspension control on at least a roller pair positioned immediately upstream of a roller pair that corrects the posture of the sheet among the plurality of roller pairs. . The sheet conveying apparatus according to any one of claims 1 to 7,
The control unit performs the pause control on at least a roller pair positioned immediately upstream of a size detection unit that detects the size of the sheet among the plurality of roller pairs. apparatus. The sheet conveying device according to any one of claims 1 to 8,
The sheet conveying apparatus, wherein the control unit does not perform the pause control for a roller pair positioned immediately downstream of a size detecting unit that detects a size of a sheet. The sheet conveying device according to any one of claims 1 to 9,
The control unit performs the pause control for at least a roller pair positioned immediately downstream of a fixing device that fixes the toner image transferred to the sheet to the sheet among the plurality of roller pairs. A sheet transporting device. A guide path for guiding the conveyance of the sheet;
A plurality of roller pairs including a roller pair for transferring a toner image to a sheet and a roller pair for fixing the transferred toner image to the sheet, which are arranged at intervals on the guide path, and are conveyed from the upstream side. A plurality of pairs of rollers for conveying the sheet to the downstream side by rotating while sandwiching the sheet,
A control unit that controls driving of the plurality of roller pairs;
With
The control unit temporarily stops driving of at least one of the plurality of roller pairs until a predetermined stop time elapses after the front end of the sheet reaches the nip position of the roller pair. , Performing a pause control to drive at a higher speed than the upstream roller pair located immediately before the upstream side of the one roller pair,
An image forming apparatus.

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