JP5510203B2 - Image forming apparatus - Google Patents

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 driving roller is disposed at a position opposite to the image forming surface of the sheet, and the driven roller is disposed at a position in contact with the image forming surface.

  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.

  In double-sided printing in which image formation is performed on both front and back sides of a sheet, after image formation is performed on the front side of the sheet, the front side (hereinafter also referred to as the first side) and the back side (hereinafter also referred to as the second side) of the sheet. Is reversed, and the sheet is conveyed again to the loop forming roller, and the skew of the sheet is corrected in the same manner as described above. Even when printing on both sides, there is a case where no image is formed on the first side.

  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 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, the leading end of the sheet P is abutted against the registration roller 23 to form a loop and correct the skewing. 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 (hatching) occurs.

  When paper is transported in such a fan shape, in particular in the case of double-sided printing in which the front and back of paper P are reversed and printed on the back (second side) after printing on the front (first side), the front and back Image position accuracy decreases.

  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, as described above, the registration roller 23 and the loop forming roller 22D are each composed of a pair of rollers including a driving roller and a driven roller. The driving roller is a surface opposite to the image forming surface of the paper, and the driven roller is an image forming roller. It is arranged at a position in contact with the surface.

  Hereinafter, unless otherwise specified, the opposite surface refers to a paper surface that contacts the drive roller.

  FIG. 5 is a view showing the positional relationship between the driving roller 23a and the driven roller 23b of the registration roller 23. As shown in FIG.

  In double-sided printing that forms images on both the front and back sides of paper, after image formation on the first side of the paper, the first and second sides of the paper are reversed, and the paper is again conveyed to the loop forming roller, The skew of the paper is corrected in the same way as when the first side image is formed.

  Here, the friction coefficient of the opposite surface changes depending on whether or not an image is formed on the surface (opposite surface) of the sheet that contacts the driving roller 23a, that is, whether or not the first surface has a toner image. The coefficient of friction is lower when the toner image is present than when the toner image is absent.

  Accordingly, 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 presence or absence of the toner image. The transported amount is greater when there is no toner image on the opposite surface. The broken line shown in FIG. 5 is the tip position without the toner image on the opposite surface, and the solid line is the case with the toner image. Thus, depending on the presence / absence of the toner image, a difference d2 occurs in the distance from the center of the nip of the registration roller 23 to the front end of the sheet.

  For this reason, the fan-shaped portion is larger when there is no toner image. 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 conveyance amount becomes insufficient when there is a toner image, and the nip at the registration roller 23 becomes insufficient. May be damaged.

  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. In an image forming apparatus capable of double-sided printing in which an image is formed on the first side of the paper and then the front and back sides of the paper are reversed to form an image on the second side.
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 until the release of the nip of the upstream roller is imaged on the first surface of the sheet. An image forming apparatus that controls to a predetermined total number of rotations that is set according to whether or not the image is formed.

  2. The predetermined total number of revolutions is set to be larger when an image is formed on the first surface of the paper than when an image is not formed on the first surface of the paper. 2. The image forming apparatus as described in 1 above.

  3. The predetermined total number of rotations depends on whether the distance between the leading edge of the sheet and the center of the nip portion of the registration roller when releasing the nip of the upstream roller is an image formed on the first surface of the sheet. The image forming apparatus according to 1 or 2, wherein the image forming apparatus is set to be constant.

  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 controller controls the total number of rotations of the registration rollers so that the rotation speed is constant regardless of whether or not an image is formed on the first surface of the sheet, and image formation is performed on the first surface of the sheet. 5. The image forming apparatus as described in 4 above, wherein the rotation time is changed according to whether or not the rotation is performed.

  6). The control unit controls the total number of rotations of the registration rollers so that the rotation time is constant regardless of whether or not an image is formed on the first surface of the paper, and image formation is performed on the first surface of the paper. 5. The image forming apparatus as described in 4 above, which is performed by changing the rotation speed according to whether or not the operation is performed.

  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 positional relationship of the drive roller of a registration roller, and a driven 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, it is possible to discharge the paper P by reversing the first side and the second side.

  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 print mode using the ADU 700, the sheet P after the fixing process on which the image is formed on the first side is guided downward along the right side of the path switching member 601, and the rear end of the sheet P is set to the roller pair. The conveyance is stopped in a state of being sandwiched by 605.

  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 nip of the paper P at the registration roller 23 and to stabilize the conveyance, the paper can be used regardless of the presence or absence of the toner image on the opposite surface. It is preferable to release the nip of the loop forming roller 22D at a constant distance after the leading end of P passes through the nip portion N of the registration 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 described above, the friction coefficient of the opposite surface changes depending on the presence or absence of the toner image on the opposite surface of the paper P. The coefficient of friction is lower when the toner image is present than when the toner image is absent. For this reason, the presence of the toner image increases the slip, and the conveyance amount per rotation of the registration roller 23 decreases.

  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 presence or absence of the toner image on the opposite surface as in the prior art, the nip portion N and the paper when the nip is released The distance d3 at the tip of the P causes a problem that the absence of the toner image is larger than the presence of the toner image, and it is difficult to control the distance d3 substantially constant.

  In order to control the distance d3 to be substantially 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, space is required. Placement is difficult.

  In the present invention, the control unit AS determines 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. Control is made to a predetermined total number of revolutions set according to the presence or absence.

  That is, after the rotation of the registration roller 23 is started (after re-feeding is started), the total number of rotations of the registration roller 23 is set according to the presence or absence of the toner image on the opposite side of the paper P. When the total number of rotations is reached, control is performed to release the nip of the loop forming roller 22D.

  This is because the transport amount transported from the start of refeeding to the release of the nip of the loop forming roller 22D becomes a substantially constant transport amount regardless of the presence or absence of the toner image on the opposite side of the paper P. Means. The loop forming roller 22D is stopped from rotating at the same time as the nip is released.

  The detection of the total number of rotations described above can be performed using a rotation detection mechanism (not shown) for detecting rotation, such as a rotary encoder, connected to the registration roller 23 or a drive source such as a drive motor (not shown). it can. 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 described above, the friction coefficient of the opposite surface of the sheet P changes depending on the presence or absence of the toner image. The coefficient of friction is lower when the toner image is present than when the toner image is absent. For this reason, the presence of the toner image increases the slip, and the conveyance amount per rotation of the registration roller 23 decreases.

  For this reason, in order to make the distance d3 substantially constant regardless of the presence or absence of the toner image on the opposite surface of the paper P, the predetermined total number of revolutions is set to be more with a toner image than with no toner image on the opposite surface. There is a need.

  The predetermined total number of rotations is set in advance according to the presence or absence of the toner image on the opposite surface by design specifications, experiments, or the like.

  The presence / absence of the toner image and the predetermined total number of revolutions set according to the presence / absence of the toner image 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.

  In printing, a toner image is formed on the opposite side of the paper P based on information such as whether or not double-sided printing has been selected by the operator and whether or not a toner image has been formed on the first side by double-sided printing. Depending on whether or not there is, data of a predetermined total number of revolutions is selected from the aforementioned data table, 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 whether or not a toner image is formed on the surface of the paper P that contacts the driving roller 23a of the registration roller 23, the distance d3 can be made the minimum distance and substantially constant. Thus, fan-shaped occurrence due to loop deformation (deformed loop) can be prevented, and conveyance 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 is the control of the total number of rotations according to the presence or absence of the toner image by detecting the number of rotations of the registration roller 23 or the drive motor (referred to as control example 1). However, it can also be performed by controlling the rotation time or rotation speed of the registration roller 23. 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 presence or absence of the toner image on the opposite side of the paper P, and the rotation time is controlled according to the presence or absence of the toner image, thereby controlling the total number of rotations of the registration roller 23. It is something to control. Therefore, the rotation time is set longer when the toner image is present than when the toner image is absent. In the case of the control example 2, if a motor having a constant rotational speed is used for the rotational drive of the registration roller 23, it can be performed only by controlling the rotation time, and the mechanism for detecting the rotational speed of the registration roller 23 can be omitted. Can be simplified.

  The rotation time in the control example 2 is set in advance according to the presence / absence of a toner image 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.

<Control example 3>
In the control example 3, the rotation time of the registration roller 23 is constant regardless of the presence or absence of the toner image on the opposite surface of the paper P, and the rotation speed is controlled according to the presence or absence of the toner image, thereby controlling the total number of rotations of the registration roller 23. It is something to control. Accordingly, the rotational speed is set to be larger when the toner image is present than when the toner image is absent. 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.

  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 whether or not the toner image is formed on the opposite surface of the paper P. It is not limited to. 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)

In an image forming apparatus capable of double-sided printing in which an image is formed on the first side of the paper and then the front and back sides of the paper are reversed to form an image on the second side.
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 until the release of the nip of the upstream roller is imaged on the first surface of the sheet. An image forming apparatus that controls to a predetermined total number of rotations that is set according to whether or not the image is formed.   The predetermined total number of revolutions is set to be larger when an image is formed on the first surface of the paper than when an image is not formed on the first surface of the paper. The image forming apparatus according to claim 1.   The predetermined total number of rotations depends on whether the distance between the leading edge of the sheet and the center of the nip portion of the registration roller when releasing the nip of the upstream roller is an image formed on the first surface of the sheet. The image forming apparatus according to claim 1, wherein the image forming apparatus is set to be constant.   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 controller controls the total number of rotations of the registration rollers so that the rotation speed is constant regardless of whether or not an image is formed on the first surface of the sheet, and image formation is performed on the first surface of the sheet. The image forming apparatus according to claim 4, wherein the rotation time is changed according to whether or not the rotation is performed.   The control unit controls the total number of rotations of the registration rollers so that the rotation time is constant regardless of whether or not an image is formed on the first surface of the paper, and image formation is performed on the first surface of the paper. The image forming apparatus according to claim 4, wherein the rotation speed is changed in accordance with whether or not the operation is performed.   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.

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