JP2012051699A - Image forming apparatus - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming apparatus by which, in conveyance after the skew of a sheet has been corrected by hitting the leading edge of the sheet against a registration roller, stable conveyance can be performed by preventing conveyance failure due to influence of the difference in the shape across the width of the loop formed at the time of skew correction.SOLUTION: The image forming apparatus includes the registration roller for correcting the skew of the sheet and conveying the sheet, an upstream side roller which is disposed upstream rather than the registration roller, and forms a loop to the sheet, and a control section for controlling the rotations of the registration roller and the upstream side roller, and the contact/release of the nip of the upstream side roller. The control section controls so that the sheet with a loop formed between the registration roller and the upstream side roller is conveyed in the downstream direction in the sheet conveyance direction, and the nip of the upstream side roller is contacted/released so as to release the nip of the upstream side roller during the conveyance, and further controls so that the total number of rotations of the registration roller from the start of sheet conveyance till releasing of the sheet from nip of the upstream side roller is kept at a prescribed total number of rotations preset in conformance to thickness of the sheet.

Description

  The present invention relates to a copying machine, a printer, a facsimile machine, and an image forming apparatus having a composite function thereof.

  Conventionally, a toner image is formed on a photosensitive drum, which is an image carrier, using an electrophotographic method, and then the toner image is transferred onto a transfer material (hereinafter also referred to as paper) and fixed by a fixing unit. An image forming apparatus configured to process is known.

  In this image forming apparatus, when the image forming operation is started, the paper stored in the paper storage unit is sequentially fed into the paper transport path by the paper supply unit.

  A registration roller that nips and transports the paper and an upstream roller (hereinafter referred to as a loop forming roller) that nips and transports the paper are disposed in the paper transport path. .

  The registration roller and the loop forming roller are each composed of a pair of rollers including a driving roller and a driven roller.

  The sheet fed from the sheet feeding unit by the sheet feeding unit is transported by a plurality of transport rollers including a loop forming roller arranged on the upstream side of the registration roller, and the leading end of the sheet abuts against the registration roller whose rotation is stopped. It is done. Thereafter, a loop is formed on the sheet by the rotation of the loop forming roller. As a result, the entire leading edge of the sheet is abutted against the registration roller, and the skew of the sheet is corrected.

  After correcting the skew of the sheet, the registration roller starts rotating in synchronization with the image formation on the photosensitive drum, and the sheet conveyance is resumed (re-feeding). In the re-feeding of the paper, the paper is nipped and conveyed by the registration roller while the loop is maintained by the rotating loop forming roller.

  In the re-feeding process, the deviation in the paper width direction is corrected.

  With regard to such skew correction and offset correction, after the leading edge of the conveyed sheet material (paper) is abutted against the registration roller, the sheet material is moved downstream of the conveyance path as the registration roller starts rotating. A guide member having a loop shape extending in a direction perpendicular to the conveying direction of the sheet material is rotatably provided at a position near the upstream side of the registration roller, and has a structure for feeding toward the next process. Is disclosed (for example, see Patent Document 1).

JP 2008-265974 A

  In Patent Document 1, the leading edge of a sheet conveyed by a conveying roller including a loop forming roller arranged on the upstream side of the registration roller is abutted against the registration roller whose rotation is stopped, and further, by rotating the loop forming roller, the sheet A loop is formed to correct the skew of the paper.

  After correcting the skew of the sheet, the registration roller and the loop forming roller are rotated at a substantially constant speed in synchronization with the image formation on the photosensitive drum, and the sheet is fed again while maintaining the loop.

  In the process of refeeding the sheet, the registration roller is corrected by moving the registration roller in the width direction. When the registration roller moves, the above-described guide member is rotated in a direction that promotes the displacement of the sheet that is displaced along with the movement of the registration roller, thereby suppressing distortion generated in the sheet.

  Here, when the leading edge of the skewed paper abuts against the registration roller and the skew feeding is corrected, the loop size is different at both ends in the width direction orthogonal to the paper transport direction, that is, the loop shape is different. The difference in the loop shape increases as the skew increases.

  Hereinafter, this differently shaped loop is also referred to as a deformed loop.

  FIG. 2 is a schematic diagram illustrating skew feeding and skew correction of a sheet. FIG. 2A is a diagram illustrating a state in which the sheet P is skewed and conveyed by the loop forming roller 22D. FIG. 2B shows a state where the leading edge of the paper P is abutted against the registration roller 23 whose rotation is stopped, and a loop is formed on the paper by the rotation of the loop forming roller 22D to correct the skew of the paper. FIG.

  As shown in FIG. 2B, when the sheet P is skewed, if the leading edge of the sheet P is abutted against the registration roller 23 to form a loop and the skew is corrected, the width of the sheet P in the width direction perpendicular to the sheet conveyance direction is corrected. The size of the loop differs between the lower side in the figure (hereinafter also referred to as the front side) and the upper side in the figure (hereinafter also referred to as the back side). FIG. 2B is an example in which the near loop L1 is larger than the far loop L2.

  After correcting the skew by abutting the leading edge of the paper with the registration roller, when the rotation of the registration roller is started, the paper is ejected by the urging force in the paper conveyance direction generated in the loop by the stiffness of the paper P, and the registration Nipped by a roller. Further, while the loop is maintained by the rotating loop forming roller, it is sent out toward the downstream side.

  This urging force varies depending on the loop shape, and is larger as the loop is smaller. For this reason, this urging force increases as the difference in loop shape (the size of the loop) increases.

  For this reason, when the rotation of the registration roller is started and the paper is fed by the registration roller and the loop forming roller, it is influenced by the difference in the biasing force of the deformed loop, that is, the conveyance on the side where the loop shape is small (FIG. 2, loop L2) The amount becomes larger than the conveyance amount on the side where the loop shape is large (FIG. 2, loop L1), and the leading edge of the sheet is conveyed in a fan shape.

  FIG. 3 is a schematic diagram illustrating a state where the leading edge of the paper P is conveyed in a fan shape. A difference in distance d1 occurs between the left and right ends of the leading edge of the paper P, and a fan shape as shown by hatching is generated.

  When the paper is conveyed in such a fan shape, the front and back image position accuracy is lowered particularly in the case of double-sided printing in which the front and back of the paper P are reversed and printed on the back side after printing on the front side.

  FIG. 4 is a diagram showing image positions when a fan shape as shown in FIG. 3 is generated on both sides during double-sided printing. In the figure, the solid line indicates the image position on the front surface, and the broken line indicates the image position on the back surface.

  In Patent Document 1, as described above, in the skew correction and deviation correction of a sheet, a rotatable guide member is used to suppress distortion generated in the sheet. The difference, that is, the suppression of the fan shape is not dealt with, and it has been difficult to improve the printing position accuracy, particularly the double-sided printing position accuracy.

  In order to suppress such image displacement, it is preferable that the nip of the loop forming roller 22D is released as soon as possible, and the sheet is conveyed only by the registration roller 23 so as not to be affected by the deformed loop. .

  Here, when the leading edge of the sheet is brought into contact with the registration roller in the skew correction, the position of the leading edge with respect to the registration roller varies depending on the sheet thickness.

  FIG. 5 is a diagram comparing the leading end positions when the leading ends of the thin paper and the thick paper abut against the registration roller 23. Note that the diameters of the registration rollers, the thicknesses of the thin paper and the thick paper, and the like in the figure do not indicate actual dimensional relationships but are exaggerated. In the figure, the solid line indicates the thin paper and the broken line indicates the leading edge of the thick paper.

  As shown in FIG. 5, when the leading edge of the paper abuts against the registration roller 23, the leading edge of the thin paper is closer to the nip portion N formed by the driving roller 23a and the driven roller 23b than the leading edge of the thick paper, and the difference in the distance d2 is large. Arise.

  For this reason, after the rotation of the registration roller 23 is started, the paper is conveyed in the downstream direction while maintaining the loop, and the nip of the loop forming roller 22D is released after a certain time regardless of the paper thickness. The transported amount of thin paper is larger than that of thick paper. That is, the distance from the center of the nip of the registration roller to the leading edge of the paper is greater for thin paper than for thick paper. For this reason, a fan-shaped part becomes large at the time of thin paper conveyance. That is, the influence of the irregular loop becomes large.

  In order to avoid this, if the time until the nip of the loop forming roller 22D is released is shortened, the transport amount becomes insufficient when transporting thick paper, and the nip at the registration roller 23 becomes insufficient. There is a risk of damage.

  The present invention has been made in view of the above situation, and a loop formed at the time of skew correction in transporting a sheet after the leading edge of the sheet is abutted against a registration roller to form a loop and correct the skew of the sheet. An object of the present invention is to provide an image forming apparatus capable of preventing conveyance failure due to the influence of the difference in shape in the width direction and performing stable conveyance.

  The above object is achieved by the following configuration.

1. A registration roller having a pair of rollers for correcting and conveying the skew of the paper;
An upstream roller provided with a pair of rollers disposed upstream of the registration roller in the paper transport direction and forming a loop on the paper by abutting the leading edge of the paper against the registration roller;
A controller that controls rotation of the registration roller and the upstream roller and contact / separation of the nip of the upstream roller,
The control unit rotates the upstream roller so that the leading edge of the paper abuts against the registration roller, and then feeds the paper in which a loop is formed between the registration roller and the upstream roller in the paper conveyance direction. To rotate the registration roller and the upstream roller in order to convey in the downstream direction, and to control the nip of the upstream roller so as to release the nip of the upstream roller during the conveyance, In addition, the total number of rotations of the registration roller from the start of the conveyance of the sheet by the rotation of the registration roller and the upstream roller to the release of the nip of the upstream roller according to the sheet thickness of the sheet An image forming apparatus that controls to a set predetermined total number of revolutions.

  2. 2. The image forming apparatus according to 1 above, wherein the predetermined total number of rotations is set to be larger when the paper thickness is thinner than when the paper thickness is thin.

  3. The predetermined total number of revolutions is set so that a distance between a front end of the sheet and the center of the nip portion of the registration roller when releasing the nip of the upstream roller is constant regardless of the thickness of the sheet. The image forming apparatus as described in 1 or 2 above.

  4). The control unit performs the control of the total number of rotations of the registration rollers by changing at least one of a rotation speed and a rotation time of the registration rollers. Image forming apparatus.

  5. The control unit performs the control of the total number of rotations of the registration rollers by changing the rotation time in accordance with the paper thickness while keeping the rotation speed constant regardless of the paper thickness. 5. The image forming apparatus according to 4.

  6). The control unit performs the control of the total number of rotations of the registration rollers by changing the rotation speed according to the paper thickness while keeping the rotation time constant regardless of the paper thickness. 5. The image forming apparatus according to 4.

  7). The control unit corrects misalignment of the sheet by releasing a nip of the upstream roller and then moving a registration roller in a width direction of the sheet perpendicular to the sheet conveyance direction. The image forming apparatus according to any one of 1 to 6.

  As described above, regardless of the paper thickness, it is possible to prevent the generation of a fan shape due to loop deformation (irregular loop), and it is possible to stably carry after registration. Thereby, image position shift can be suppressed.

1 is a diagram illustrating an example of an image forming apparatus according to the present invention. FIG. 5 is a schematic diagram illustrating skew feeding and skew correction of a sheet. It is a schematic diagram which shows the state in which the front-end | tip of paper is conveyed in fan shape. It is a figure which shows the image position when a fan shape arises on both surfaces at the time of duplex printing. It is a figure which shows the state which contact | abutted the front end of the thin paper and the thick paper to the registration roller. It is side surface sectional drawing of the principal part of a registration roller and the loop formation roller vicinity. FIG. 6 is a diagram illustrating a paper conveyance process by a registration roller and a loop forming roller. FIG. 8 is a timing chart of the paper conveyance process shown in FIG. 7. FIG. FIG. 10 is a diagram illustrating misalignment correction in the paper width direction.

  Embodiments relating to the present invention will be described below with reference to the drawings, but are not limited thereto.

  First, an example of an image forming apparatus according to the present invention will be described with reference to the block diagram of FIG.

  The image forming apparatus includes an image forming apparatus main body GH and an image reading apparatus YS. The image forming apparatus main body GH is called a tandem type color image forming apparatus, and includes a plurality of sets of image forming units 10Y, 10M, 10C, and 10K, a belt-like intermediate transfer belt 5, a sheet feeding / conveying means and a fixing device 8. A reverse discharge unit 600, an ADU (Auto Duplex Unit) 700 that performs reverse conveyance, and the like are included.

  Moreover, it has control part AS which controls each part.

  An image reading device YS including an automatic document feeder 201 and a document image scanning exposure device 202 is installed on the upper part of the image forming apparatus main body GH. The document d placed on the document table of the automatic document feeder 201 is transported by a transport unit, and one or both images of the document are scanned and exposed by the optical system of the document image scanning exposure device 202 and read into the line image sensor CCD. It is.

  A signal formed by photoelectric conversion by the line image sensor CCD is subjected to analog processing, A / D conversion, shading correction, image compression processing, and the like in an image processing unit, and then exposure means 3Y, 3M, 3C, 3K. Sent to.

  In the image forming unit 10Y that forms a yellow (Y) image, a charging unit 2Y, an exposure unit 3Y, a developing unit 4Y, and a cleaning unit 7Y are arranged around the photosensitive drum 1Y. In the image forming unit 10M that forms a magenta (M) color image, a charging unit 2M, an exposure unit 3M, a developing unit 4M, and a cleaning unit 7M are arranged around the photosensitive drum 1M. In the image forming unit 10C that forms a cyan (C) color image, a charging unit 2C, an exposure unit 3C, a developing unit 4C, and a cleaning unit 7C are arranged around the photosensitive drum 1C. In the image forming unit 10K that forms a black (K) image, a charging unit 2K, an exposure unit 3K, a developing unit 4K, and a cleaning unit 7K are arranged around the photosensitive drum 1K. The charging unit 2Y and the exposure unit 3Y, the charging unit 2M and the exposure unit 3M, the charging unit 2C and the exposure unit 3C, and the charging unit 2K and the exposure unit 3K constitute a latent image forming unit.

  The developing units 4Y, 4M, 4C, and 4K include a two-component developer composed of a toner having a small particle size of yellow (Y), magenta (M), cyan (C), and black (K) and a carrier. The toner is composed of a pigment or dye serving as a color former, a wax that helps the toner peel from the fixing member after fixing, and a binder resin that holds these.

  The intermediate transfer belt 5 is wound around a plurality of rollers and is rotatably supported.

  The fixing device 8 heats and presses the toner image on the paper P at a nip portion formed between a fixing belt 81 as a heated fixing member and a pressure roller 83 as a pressure member to fix the toner image.

  Thus, each color image formed by the image forming units 10Y, 10M, 10C, and 10K is sequentially transferred to the rotating intermediate transfer belt 5 by the transfer means 6Y, 6M, 6C, and 6K (primary transfer), and color. A combined toner image is formed.

  The paper P accommodated in the paper feed cassette 20 is fed by the paper feed means 21 and conveyed to the transfer means 6A through the paper feed rollers 22A, 22B, 22C, the loop forming roller 22D, the registration roller 23, and the like. A color image is transferred to the paper P (secondary transfer).

  The loop forming roller 22 </ b> D and the registration roller 23 perform skew correction and misalignment correction of the paper P. Details of the skew correction and the offset correction will be described later.

  The sheet P on which the color image has been transferred is heated and pressurized by the fixing device 8 and the color toner image on the sheet P is fixed. Thereafter, the paper is nipped and conveyed by the paper discharge roller 24 and placed on a paper discharge tray 25 outside the apparatus.

  Further, by switching the position of the path switching member 601 of the reverse paper discharge unit 600, the paper P can be discharged with its upper and lower surfaces reversed.

  For example, in the reverse discharge of the paper P, the position of the path switching member 601 is switched so that the paper P is guided downward along the right side of the path switching member 601, and the paper P is conveyed in the direction of the roller pair 602. . Next, after the rear end is nipped by the roller pair 602, the roller pair 602 is rotated in the reverse direction to raise the paper P. Thereafter, the sheet P passes through the left side of the path switching member 601, reaches the sheet discharge roller 24, is nipped and conveyed by the sheet discharge roller 24, and is placed on the sheet discharge tray 25 outside the apparatus.

  In the double-sided printing mode using the ADU 700, the sheet P after the fixing process on which an image is formed on one side (first side) is guided downward along the right side of the path switching member 601, and the rear end of the sheet P Is stopped with the roller pair 605 sandwiched therebetween.

  Next, the roller pair 605 is rotated in the reverse direction, is raised along the guide plate G1, and is guided to the ADU 700 having a plurality of roller pairs 701, 703, 705, and the paper P is reversed.

  On the other hand, after the color image is transferred to the paper P by the transfer means 6A, the residual toner is removed by the cleaning means 7A from the intermediate transfer belt 5 from which the paper P is curved and peeled.

  The image forming apparatus for forming a color image has been described above. However, an image forming apparatus for forming a monochrome image may be used, or an intermediate transfer belt may or may not be used.

  The fixing device 8 may be a heat roller fixing method using a roller having a fixing member provided with heating means.

  Next, the conveyance of the paper P by the registration roller 23 and the loop forming roller 22D will be described. The driving of these rollers, the contact and separation of the nip, and the like are controlled by the control unit AS.

  FIG. 6 is a side sectional view of a main part in the vicinity of the registration roller 23 and the loop forming roller 22D.

  The registration roller 23 includes a driving roller 23a and a driven roller 23b, and the loop forming roller 22D includes a driving roller 22Da and a driven roller 22Db. Further, a lower guide plate G2 and an upper guide plate G3 are arranged. The upper guide plate G3 is supported so as to be rotatable upward (in the direction of the broken line) so that a loop of the paper P can be formed on the upper side. The upper guide plate G3 may be omitted when the upper guide of the paper tip is unnecessary due to the axial distance between the registration roller 23 and the loop forming roller 22D, the stiffness of the paper P to be used, and the like. it can. Further, instead of the upper guide plate G3 being rotatably supported, an interval between predetermined positions of the lower guide plate G2 and the upper guide plate G3 may be widely provided in consideration of the loop formation of the paper P.

  The material of each roller is not particularly limited. For example, an EPDM rubber roller is preferably used for the driving roller 23a and the driving roller 22Da, and a SUS roller is preferably used for the driven roller 23b and the driven roller 22Db.

  FIG. 7 is a diagram illustrating a conveyance process of the paper P by the registration roller 23 and the loop forming roller 22D.

  As described above, the paper P accommodated in the paper feed cassette 20 is fed by the paper feed means 21 and conveyed to the loop forming roller 22D through the paper feed rollers 22A, 22B, 22C.

  The loop forming roller 22D conveys the paper P to the stopped registration roller 23 (FIG. 7A).

  The loop forming roller 22D is rotated for a predetermined time after the front end of the paper P is abutted against the registration roller 23, and is stopped after forming the loop L (FIG. 7B). As a result, the entire leading edge of the sheet is abutted against the registration roller, and the skew of the sheet P is corrected.

  After the skew correction of the sheet P, the rotation of the registration roller 23 is started in synchronization with the image formation on the photosensitive drum, and the sheet P is fed again. In this re-feeding, both the registration roller 23 and the loop forming roller 22D are rotated, and the paper P is conveyed while maintaining the loop (FIG. 7C).

  After the refeeding of the paper P is started, the driving roller 22Da and the driven roller 22Db of the loop forming roller 22D are separated at a predetermined time, and the nip is released (off). As a result, the loop L of the paper P is eliminated (FIG. 7D). The predetermined time is not limited to time, and includes, for example, the total number of rotations of the registration roller 23 after the start of refeeding.

  The nip state is referred to as nip on, and the nip release state is referred to as nip off.

  FIG. 8 is a timing chart of the conveyance process of the paper P shown in FIGS. In the figure, t1 corresponds to the time until the loop forming roller 22D shown in FIGS. 7A to 7B stops. t2 corresponds to the state in which the loop forming roller 22D in FIG. t3 corresponds to FIG. 7C, and t4 corresponds to FIG. 7D. T3 in the drawing corresponds to a predetermined time until the re-feeding of the paper P is started and the nip of the loop forming roller 22D is released. Therefore, as described above, the time is not limited, and there is, for example, the total number of rotations of the registration roller 23 after the start of refeeding.

  Here, as described above, in order to prevent the generation of the fan shape as shown in FIG. 3 and to suppress the image position deviation, the nip of the loop forming roller 22D is released as soon as possible, and only the registration roller 23 is used. It is preferable that t3 be shortened as much as possible.

  In order to shorten t3 so as not to be affected by the irregular loop and to ensure the conveyance of the paper P by the registration roller 23 to ensure the nip of the paper P, the leading edge of the paper P is registered regardless of the paper thickness. It is preferable to release the nip of the loop forming roller 22D at a constant distance after passing through the nip portion N of the roller 23. That is, it is preferable that the distance d3 between the nip portion N and the front end of the paper P in FIG.

  However, as shown in FIG. 5 described above, when the leading edge of the paper abuts against the registration roller 23, the leading edge of the thin paper is closer to the nip portion N formed by the driving roller 23a and the driven roller 23b than the leading edge of the thick paper. A difference in d2 occurs.

  For this reason, when the nip of the loop forming roller 22D is released at a fixed time after refeeding is started regardless of the paper thickness as in the past, the distance d3 between the nip portion N and the leading end of the paper P when the nip is released is In this case, there is a problem that the thin paper becomes larger than the thick paper, and it is difficult to control the distance d3 to be constant.

  In order to control the distance d3 to be constant, a method of arranging a sensor or the like for detecting the leading edge of the paper P immediately after the nip is conceivable. However, since a roller and a shaft are arranged immediately after the nip, they are arranged in a space. Is difficult.

  In the present invention, the controller AS sets the total number of rotations of the registration roller 23 from the start of the rotation of the registration roller 23 until the nip of the loop forming roller 22D is released according to the paper thickness of the paper P. The predetermined total number of revolutions is controlled.

  That is, after the rotation of the registration roller 23 is started (after re-feeding is started), the total rotation number of the registration roller 23 becomes a predetermined total rotation number set according to the paper thickness of the paper P. Sometimes control is performed to release the nip of the loop forming roller 22D. This means that the conveyance amount conveyed from the start of refeeding to the release of the nip of the loop forming roller 22D becomes a conveyance amount corresponding to the paper thickness. The loop forming roller 22D is stopped from rotating at the same time as the nip is released.

  Hereinafter, the description of simply the total number of rotations refers to the total number of rotations of the registration rollers 23 after the rotation of the registration rollers 23 is started (after refeeding is started).

  The detection of the total number of rotations described above can be performed using a rotation detection mechanism (not shown) for detecting the total number of rotations connected to the registration roller 23 or a drive source, for example, a drive motor, such as a rotary encoder. Thus, when detecting the total number of rotations of the registration roller 23 or the drive motor, it is possible to eliminate influences such as fluctuations in the rotation speed of the drive motor and fluctuations in the rotation speed at the start of rotation at the time of startup.

  Alternatively, a stepping motor may be used to rotate the registration roller 23, and the total number of revolutions may be controlled by pulse control.

  Here, as shown in FIG. 5 described above, when the leading edge of the paper abuts against the registration roller 23, the leading edge of the thin paper is closer to the nip portion N formed by the driving roller 23a and the driven roller 23b than the leading edge of the thick paper. A difference in distance d2 occurs. For this reason, in order to make the distance d3 substantially constant regardless of the paper thickness, the predetermined total number of revolutions needs to be set to be larger for thick paper than for thin paper.

  The predetermined total number of revolutions is set in advance according to the paper thickness by design specifications, experiments, and the like. The predetermined total number of revolutions may be set for each sheet thickness, or a group of sheet thicknesses may be set for each range of sheet thickness, and may be set for each group.

  This paper thickness and the predetermined total number of revolutions set accordingly are created as a data table, for example, and stored in advance in a storage unit (not shown) of the image forming apparatus main body GH.

  When printing, the operator selects a paper type (paper thickness) on the operation panel (not shown) of the image forming apparatus main body GH, or selects the paper feed cassette 20, or the like according to the paper thickness from the data table described above. Data on the total number of revolutions is selected, and the control unit performs control based on the data.

  When the nip of the loop forming roller 22D is released, the loop disappears, and the paper P is conveyed in a state where the skew is corrected. In this conveyance process, the deviation in the width direction of the paper P is corrected.

  FIG. 9 is a diagram showing this deviation correction. An end surface detection sensor S101 that detects the position of the end surface in the width direction of the paper P is provided on the upstream side of the registration roller 23. The registration roller 23 is supported so as to be movable in the width direction of the paper P, and is moved in the arrow Y11 direction by a moving mechanism (not shown).

  When the deviation of the paper P is detected by the end face detection sensor S101, the moving mechanism moves the registration roller 23 in the direction of correcting the deviation and corrects the deviation.

  For example, as shown in FIG. 9, when the end face detection sensor S101 detects that the paper P is offset toward the front side (downward in the drawing) (solid line portion), the moving mechanism moves the registration roller 23 back (upward in the drawing). , The paper P is moved (broken line portion), and the deviation is corrected.

  As described above, regardless of the paper thickness, the distance d3 can be made the minimum distance and substantially constant, thereby preventing the occurrence of a fan shape due to loop deformation (atypical loop), Transport after resist can be performed stably. Thereby, image position shift can be suppressed.

  In the above description, the control for making the distance d3 substantially constant regardless of the paper thickness is performed by detecting the total number of rotations of the registration roller 23 or the drive motor (referred to as control example 1). The rotation time or the rotation speed can be controlled. An example will be described below.

<Control example 2>
In the control example 2, the rotation speed of the registration roller 23 is constant regardless of the paper thickness, and the rotation time is controlled according to the paper thickness. Therefore, the rotation time of thick paper is set longer than that of thin paper. In the case of the control example 2, if a motor having a constant rotational speed is used for rotational driving of the registration roller 23, it can be performed only by controlling the rotation time, and a mechanism for detecting the total number of rotations of the registration roller 23 can be omitted. The mechanism can be simplified.

  The rotation time in the control example 2 is preset according to the paper thickness by design specifications, experiments, and the like. Further, it is preferable that the rotation time is set in consideration of fluctuations in the rotation speed at the start of rotation when the drive motor is started. Note that the rotation time may be set for each paper thickness, or may be set for each group by setting a group of paper thicknesses for each range of paper thickness.

<Control example 3>
In the control example 3, the rotation time of the registration roller 23 is constant regardless of the paper thickness, and the rotation speed is controlled according to the paper thickness. Therefore, the rotation speed of thick paper is set to be higher than that of thin paper. In the case of the control example 3, it is preferable to use a drive motor whose rotation speed can be easily changed for rotational driving of the registration roller 23. Further, a mechanism for detecting the total number of rotations of the registration roller 23 can be omitted, and the mechanism can be simplified.

  The rotation speed in the control example 3 is preset according to the paper thickness by design specifications, experiments, and the like. The rotation speed is preferably set in consideration of the fluctuation of the rotation speed at the start of rotation when the drive motor is started. The rotation speed may be set for each sheet thickness, or a group of sheet thicknesses may be set for each range of the sheet thickness, and may be set for each group. In the control examples 2 and 3, one of the rotation speed and the rotation time is made constant and the other is controlled according to the paper thickness, but the present invention is not limited to this. That is, the control unit may control both the rotation speed and the rotation time according to the paper thickness.

GH Image forming apparatus main body YS Image reading apparatus 8 Fixing apparatus 10Y, 10M, 10C, 10K Image forming section 22D Loop forming roller 23 Registration roller 600 Reverse discharge section 700 ADU
AS control unit L, L1, L2 Loop P Paper

Claims (7)

A registration roller having a pair of rollers for correcting and conveying the skew of the paper;
An upstream roller provided with a pair of rollers disposed upstream of the registration roller in the paper transport direction and forming a loop on the paper by abutting the leading edge of the paper against the registration roller;
A controller that controls rotation of the registration roller and the upstream roller and contact / separation of the nip of the upstream roller,
The control unit rotates the upstream roller so that the leading edge of the paper abuts against the registration roller, and then feeds the paper in which a loop is formed between the registration roller and the upstream roller in the paper conveyance direction. To rotate the registration roller and the upstream roller in order to convey in the downstream direction, and to control the nip of the upstream roller so as to release the nip of the upstream roller during the conveyance, In addition, the total number of rotations of the registration roller from the start of the conveyance of the sheet by the rotation of the registration roller and the upstream roller to the release of the nip of the upstream roller according to the sheet thickness of the sheet An image forming apparatus that controls to a set predetermined total number of revolutions.   The image forming apparatus according to claim 1, wherein the predetermined total number of revolutions is set to be larger when the paper thickness is thicker than when the paper thickness is thin.   The predetermined total number of revolutions is set so that a distance between a front end of the sheet and the center of the nip portion of the registration roller when releasing the nip of the upstream roller is constant regardless of the thickness of the sheet. The image forming apparatus according to claim 1 or 2.   4. The control unit according to claim 1, wherein the control unit controls the total number of rotations of the registration rollers by changing at least one of a rotation speed and a rotation time of the registration rollers. 5. The image forming apparatus described.   The control unit controls the total number of rotations of the registration rollers by making the rotation speed constant regardless of the paper thickness and changing the rotation time according to the paper thickness. Item 5. The image forming apparatus according to Item 4.   The control unit controls the total number of rotations of the registration rollers by making the rotation time constant regardless of the paper thickness and changing the rotation speed according to the paper thickness. Item 5. The image forming apparatus according to Item 4.   The control unit, after releasing the nip of the upstream roller, moves a registration roller in a width direction of the sheet perpendicular to a sheet conveyance direction, and corrects the deviation of the sheet. Item 7. The image forming apparatus according to any one of Items 1 to 6.

Images