JP2006349701A - Image forming apparatus and image forming method - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide an image forming apparatus for dealing with multi-color, which can be further down sized by not using an intermediate transfer body. <P>SOLUTION: The image forming apparatus 40 is provided with a photoreceptor 14 forming an electrostatic latent image on its surface, a plurality of developing machines Y, M, C and K corresponding to different color toner making a toner image by developing the electrostatic latent image, a transferring machine 42 transferring the toner image on paper, a fixing machine 52 fixing the toner image transferred to the paper and a paper transporting path 54 transporting the paper from the transferring machine 42 to the fixing machine 52. A color image is formed by passing the paper with the toner image formed on it a plurality of times through the transfer machine 42 and the fixing machine 52 and superposing the toner images for a plurality of colors on the paper. <P>COPYRIGHT: (C)2007,JPO&INPIT

Description

    The present invention relates to an electrophotographic image forming apparatus and an image forming method.

  Conventionally, a four-cycle color image forming apparatus 10 as shown in FIG. 8 is known. The image forming apparatus 10 includes a developing rack 12 that is rotationally driven in the direction of arrow A. The developing rack 12 is equipped with developing devices Y, M, C, and K corresponding to toners of four colors of yellow, magenta, cyan, and black, respectively.

  Each of the developing devices Y, M, C, and K is opposed to the photosensitive drum 14 at a predetermined position. Around the photosensitive drum 14, a charging device 16 that uniformly charges the surface of the photosensitive drum 14, and an exposure device that exposes the surface of the uniformly charged photosensitive drum 14 according to image data to form an electrostatic latent image. 18 and toner remaining on the surface of the photosensitive drum 14 after the toner image formed by developing the electrostatic latent image by the developing devices Y, M, C, and K is transferred to the intermediate transfer belt 20. A cleaning device 22 for recovery is disposed.

  The intermediate transfer belt 20 is, for example, an endless sheet made of a resin film. The intermediate transfer belt 20 is stretched from the inside by a plurality of rollers 24, 26, 28, 30 and 32. Among the plurality of rollers, the roller 24 is a driving roller, the rollers 26 and 30 are tension rollers, the roller 28 is a driven roller, and the roller 32 is a primary transfer roller (transfer roller).

  The drive roller 24 is driven to rotate in the direction of arrow B, whereby the intermediate transfer belt 20 is rotated in the direction of arrow D. The tension rollers 26 and 30 apply a predetermined tension to the intermediate transfer belt 20. The primary transfer roller 32 is disposed in the vicinity of the position where the outer surface of the intermediate transfer belt 20 comes into contact with the photosensitive drum 14, and has a primary transfer voltage having a polarity (for example, plus) opposite to the normal charging polarity (for example, minus) of the toner. It is to be applied.

  A secondary transfer roller 34 is disposed on the portion of the intermediate transfer belt 20 supported by the drive roller 24 so as to be able to contact and separate. A secondary transfer voltage having a polarity (for example, plus) opposite to the normal charging polarity (for example, minus) of the toner is applied to the secondary transfer roller 34. A sheet path 36 is formed between the intermediate transfer belt 20 and the secondary transfer roller 34 to convey the sheet from below to above.

  A cleaning blade 38 is in contact with a portion of the intermediate transfer belt 20 supported by the driven roller 28. The cleaning blade 38 contacts and separates from the intermediate transfer belt 20 by swinging in the direction of arrow E.

  When a color image is formed in the image forming apparatus 10 having the above-described configuration, first, the developing rack 12 is rotated so that the developing device Y faces the photosensitive drum 14. The surface of the photosensitive drum 14 is uniformly charged by the charging device 16, and the exposure device 18 selectively exposes the surface of the uniformly charged surface of the photosensitive drum 14 based on the yellow image data. The electrostatic latent image is developed by the developing device Y, whereby a yellow toner image is formed on the surface of the photosensitive drum 14. Thereafter, the yellow toner image formed on the surface of the photosensitive drum 14 is primarily transferred onto the outer surface of the intermediate transfer belt 20 by the electrostatic force of the primary transfer roller 32.

  Subsequently, the developing rack 12 rotates and the developing device M faces the photosensitive drum 14. Then, a magenta toner image is formed on the surface of the photosensitive drum 14 by the same process as that for the yellow toner image described above. Thereafter, the magenta toner image is primary-transferred over the yellow toner image on the surface of the intermediate transfer belt 20 that has rotated one turn while carrying the yellow toner image. When the intermediate transfer belt 20 rotates while carrying a yellow toner image (the same applies to a magenta toner image and a cyan toner image), the secondary transfer roller 34 and the cleaning blade 38 are separated from the intermediate transfer belt 20. ing. Thereafter, similarly, a cyan toner image and a black toner image are sequentially transferred onto the intermediate transfer belt 20 in a primary transfer.

  The four-color toner image primarily transferred and superimposed on the intermediate transfer belt 20 is conveyed from below to above along the paper path 36 between the intermediate transfer belt 20 and the secondary transfer roller 34 in contact therewith. Secondary transfer is collectively performed on the surface of the surface by the electrostatic force of the secondary transfer roller 34. The sheet on which the four-color toner image has been secondarily transferred is discharged from the image forming apparatus 10 after the toner image is heated and fixed when passing through a fixing device (not shown). The toner remaining on the surface of the intermediate transfer belt 20 after the secondary transfer is scraped off and collected by the cleaning blade 38 in contact with the intermediate transfer belt 20.

  Since the image forming apparatus 10 forms an image by transferring four color toner images on the intermediate transfer belt 20 in an overlapping manner, the intermediate transfer belt 20 has a surface area larger than that of the maximum print image. Of course it is necessary. Then, the use of such an intermediate transfer belt 20 itself becomes a factor that hinders further downsizing of the image forming apparatus.

Patent Document 1 below discloses an image forming apparatus in which a plurality of printers are connected to superimpose toner images of different colors on one sheet in order to create a simple color image. However, if a multicolor color is to be achieved by this method, the apparatus must be enlarged.
JP-A-7-295326

  SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus and an image forming method for multi-color that can be further miniaturized without using an intermediate transfer member.

In order to solve the above problems, an image forming apparatus according to a first aspect of the present invention includes:
A photoreceptor having an electrostatic latent image formed on the surface;
One or a plurality of developing units corresponding to a single color toner or a different color toner to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
A sheet conveyance path through which the sheet is conveyed through the transfer unit and the fixing unit,
A monochrome image or a color image is formed by passing the paper onto which the toner image has been transferred through the transfer device and the fixing device a plurality of times and forming the toner image on the paper in an overlapping manner. .

The image forming apparatus according to the second aspect of the present invention includes:
A photoreceptor having an electrostatic latent image formed on the surface;
A plurality of developing units corresponding to different color toners to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
A sheet conveyance path through which the sheet is conveyed through the transfer unit and the fixing unit,
The paper on which the toner image is transferred is passed through the transfer device and the fixing device a plurality of times to form a color image by superimposing a plurality of color toner images on the paper. .

  In the image forming apparatus according to the aspect of the invention, the sheet conveyance path extends so that the sheet that has passed through the fixing unit circulates around the photosensitive member and the developing unit and is conveyed to an upstream position of the transfer unit. May be.

  In the image forming apparatus according to the aspect of the invention, the paper conveyance path extends so that the paper that has passed through the fixing device is conveyed to the upstream position of the transfer device through the back side of the fixing device and the transfer device. You may do it.

  In the image forming apparatus of the present invention, in the paper transport path, the toner image forming portion may switch back the paper that has passed through the fixing device and return it to the upstream position of the transfer device.

  In the image forming apparatus according to the aspect of the invention, the sheet conveyance path may convey the sheet that has passed through the fixing unit to the upstream position of the transfer unit through the back side of the fixing unit and the transfer unit after switching back the sheet. And you may extend | stretch so that it may switch back once again.

  In the image forming apparatus of the present invention, after the sheet on which the toner image has not been transferred passes at least once through the fixing unit, the sheet is returned to the upstream position of the transferring unit, and then the transferring unit and the fixing unit are moved. The toner images may be overlapped and formed on a sheet by passing the sheet a plurality of times.

  In addition, the image forming apparatus of the present invention includes a detecting unit that detects a registration toner pattern, and a plurality of registration toner patterns are formed at the first toner image formation on the photoconductor. After the toner image including the alignment toner pattern is transferred and fixed on a sheet, the alignment toner pattern is detected by the detection unit, and the second and subsequent toners are detected based on the detection result of the detection unit. Corrects the second and subsequent toner images formed on the photoconductor so that the image has a print start position, a main scanning direction width, a sub-scanning direction width, and a main scanning direction inclination coincide with the first toner image. May be.

  The image forming apparatus according to the present invention further includes an end detection unit that detects an end of the sheet at an upstream position of the transfer unit, and the end position of the sheet before passing the transfer unit for the first time is the end. Detected by the edge detection means, the edge position of the paper before passing the transfer unit after the second time is detected by the edge detection means, and the edge position detection result after the second time and the previous edge position detection result. Based on the comparison with the first and second toner images on the photoconductor so that the print start position, the main scanning direction width, the sub-scanning direction width, and the main scanning direction inclination coincide with the first toner image on the sheet. You may correct | amend the toner image after the 2nd time formed.

The image forming method according to the first aspect of the present invention includes:
A photoreceptor having an electrostatic latent image formed on the surface;
One or a plurality of developing units corresponding to a single color toner or a different color toner to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
An image forming method in an image forming apparatus comprising a sheet conveyance path through which a sheet is conveyed through the transfer unit and the fixing unit,
A monochrome image or a color image is formed by passing the paper onto which the toner image has been transferred through the transfer device and the fixing device a plurality of times and forming the toner image on the paper in an overlapping manner. .

The image forming method according to the second aspect of the present invention includes:
A photoreceptor having an electrostatic latent image formed on the surface;
A plurality of developing units corresponding to different color toners to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
An image forming method in an image forming apparatus comprising a sheet conveyance path through which a sheet is conveyed through the transfer unit and the fixing unit,
The paper on which the toner image is transferred is passed through the transfer device and the fixing device a plurality of times to form a color image by superimposing a plurality of color toner images on the paper. .

  According to the image forming apparatus and the image forming method of the first aspect of the present invention, when there is one developing device, for example, a halftone image is transferred and fixed on a sheet at the first time, and then a character is printed at the second time. It is possible to create a monochrome image by superimposing two toner images with different image forming conditions, such as by transferring and fixing the image on a halftone image of paper, or by developing a plurality of developing devices. In this case, it is possible to create a multicolor image by superposing toner images of different colors on a sheet by passing one sheet through a transfer unit and a fixing unit a plurality of times. Thus, in the image forming apparatus of the present invention, since the intermediate transfer member is not provided, the apparatus can be further downsized.

  Further, according to the image forming apparatus and the image forming method of the second aspect of the present invention, different color toner images respectively formed by a plurality of developing units by passing one sheet through a transfer unit and a fixing unit a plurality of times. Are superimposed on a sheet to create a multicolor image, so that the intermediate transfer member can be eliminated, and the apparatus can be further miniaturized.

Embodiments of the present invention will be described below with reference to the accompanying drawings.
FIG. 1 is a schematic configuration diagram of a four-cycle image forming apparatus 40 according to the first embodiment of the present invention. The image forming apparatus 40 includes a developing rack 12 that is rotationally driven in an arrow A direction. On the outer periphery of the developing rack 12, developing units Y, M, C, and K corresponding to toners of four colors of yellow, magenta, cyan, and black are mounted.

  Each of the developing units Y, M, C, and K is opposed to a photosensitive drum (photosensitive member) 14 that is rotationally driven in the direction of arrow F at a predetermined position. A transfer roller (transfer device) 42 is disposed in contact with the photosensitive drum 14. Although not shown around the photosensitive drum 14, as in the image forming apparatus 10 shown in FIG. 8, a charging device that uniformly charges the surface of the photosensitive drum 14 and a uniformly charged photosensitive drum 14. An exposure device that forms an electrostatic latent image by exposing the surface according to image data, and a toner image formed by developing the electrostatic latent image by the developing units Y, M, C, and K A cleaning device for collecting the toner remaining on the surface of the photosensitive drum 14 after being transferred to the photosensitive drum 14 is provided.

  A paper feed unit 44 is provided below the image forming apparatus 40. The sheets P stacked and accommodated in the sheet feeding unit 44 are sent out one by one from the upper side by the sheet feeding roller 46. The paper P sent out from the paper supply unit 44 is composed of a nip portion of the timing roller pair 48, a transfer unit 50 between the photosensitive drum 14 and the transfer roller 40, and a fixing roller pair disposed above the transfer roller 42. The paper is conveyed along a paper conveyance path 54 formed through the nip portion of the container 52. Note that the timing roller pair 48 is located upstream of the transfer roller 42 and the transfer unit 50 in the sheet conveyance direction.

  The paper transport path 54 branches into a paper discharge path 56 and a circulation path 58 on the downstream side of the fixing device 52 in the paper transport direction. The paper discharge path 56 is a path for discharging the paper P to the outside of the apparatus via the paper discharge roller pair 60. On the other hand, the circulation path 58 circulates around the photosensitive drum 14 and extends to a position upstream of the timing roller pair 48 in the sheet conveyance direction and is connected to the sheet conveyance path 54. Above the fixing device 52, a switching claw 62 for distributing the paper P that has passed through the fixing device 52 to the paper discharge path 56 or the circulation path 58 is disposed.

Next, the operation of the image forming apparatus 40 having the above configuration will be described.
When a color image is formed, first, the developing rack 12 rotates so that the developing device Y faces the photosensitive drum 14. Then, the surface of the photosensitive drum 14 is uniformly charged by the charging device, and the exposure device selectively exposes the uniformly charged surface of the photosensitive drum 14 based on the yellow image data to form an electrostatic latent image. The electrostatic latent image is developed by the developing unit Y, whereby a yellow toner image is formed on the surface of the photosensitive drum 14. Thereafter, the yellow toner image formed on the surface of the photosensitive drum 14 moves to the transfer unit 50 according to the rotation of the photosensitive drum 14. At the same time, the paper P fed from the paper feed unit 44 by the paper feed roller 46 is introduced into the transfer unit 50 by the timing roller pair 48, and the yellow on the photosensitive drum 14 is transferred by the electrostatic force of the transfer roller 42 in the transfer unit 50. The toner image is transferred to the paper P.

  The paper P on which the yellow toner image has been transferred is transported along the paper transport path 54, and the yellow toner image is heat-fixed on the paper P when passing through the fixing device 52. The sheet P that has passed through the fixing device 52 is sent to the circulation path 58 by the guide of the switching claw 62, circulates around the photosensitive drum 14, and returns to the upstream position of the timing roller pair 48.

  After the yellow toner image is transferred to the paper P, the developing rack 12 rotates and the developing device M faces the photosensitive drum 14. Then, a magenta toner image is formed on the surface of the photosensitive drum 14 by the same process as that for the yellow toner image described above. Thereafter, the magenta toner image moves to the transfer unit 50 according to the rotation of the photosensitive drum 14, and is transferred onto the yellow toner image on the paper P conveyed by the timing roller pair 48 there. The paper P on which the magenta toner image has been transferred is heated and fixed by the fixing device 52 as described above, and then sent to the circulation path 58 and returns to the timing roller pair 48.

  Thereafter, similarly, after the cyan toner image and the black toner image are sequentially transferred and fixed on the paper P, the paper P is sent to the paper discharge path 56 by the guide of the switching claw 62 and discharged. The paper is discharged to the outside by the paper roller pair 60.

  As described above, according to the image forming apparatus 40 of the present embodiment, a single sheet P is passed through the transfer unit 50 and the fixing unit 52 a total of four times to form each of the four developing units Y, M, C, and K. The toner images of different colors are superimposed on the paper P to create a multicolor image. Therefore, the intermediate transfer belt 20 as in the conventional image forming apparatus 10 can be eliminated, and the apparatus can be further downsized.

  Next, a case where a monochrome image is formed in the image forming apparatus 40 will be described. For example, when forming a monochrome image of black and white, a black toner image is formed on the photosensitive drum 14 by the developing device K, the black toner image is transferred to the paper P by the transfer unit 50, and heated and fixed by the fixing device 52. If the black toner image is, for example, a character image, the paper P is introduced as it is into the paper discharge path 56 and discharged outside the apparatus. On the other hand, when the monochrome image is composed of, for example, a halftone portion and a character portion, the optimum image forming conditions for exposure and development differ between the halftone image and the character image. Therefore, after forming a halftone image at the first time and transferring and fixing it onto the paper P, the paper P is returned via the circulation path 58, and a character image is formed at the second time and transferred and fixed onto the paper P. Then, the paper P is discharged out of the apparatus via the paper discharge path 56. By forming the monochrome image in a plurality of times as described above, an optimal monochrome image can be formed.

  Next, an image forming apparatus 70 according to the second embodiment will be described with reference to FIG. In this image forming apparatus 70, the circulation path 58 for the paper P that has passed through the fixing device 52 to return to the upstream position of the timing roller pair 48 is the back side of the fixing device 52 and the transfer roller 42 (that is, with respect to the paper conveyance path 54). It is formed by extending through the opposite side of the developing rack 12. Since other configurations, image forming operations, and operational effects are the same as those of the image forming apparatus 40 of the first embodiment, description thereof is omitted.

  As an image forming apparatus that forms a color image by passing one sheet P through the transfer unit 50 and the fixing device 52 a plurality of times, the sheet P may be switched back. For example, as in the image forming apparatus 80 shown in FIG. 3, the paper P in which the toner image forming portion has passed through the fixing device 52 is transferred to the paper discharge roller pair 60, the fixing device 52, the transfer roller 42, the photosensitive drum 14, and the timing roller. The pair 48 may be switched back by being driven in reverse, and may be sent back to the timing roller pair 48 along the paper conveyance path 54. Further, as in the image forming apparatus 90 shown in FIG. 4, the paper P that has passed through the fixing device 52 is switched back by the reverse rotation driving of the paper discharge roller pair 60, and is sent to the circulation path 94 by the switching claw 92. May be returned to the upstream position, and then switched back and sent to the transfer unit 50 again.

  As described above, in an image forming apparatus that repeats transfer and fixing a plurality of times, alignment of each color image on the paper P is a major technical problem. When the paper P passes through the fixing device 52, the paper P tends to shrink, and even if the paper P is returned to the upstream position of the transfer unit 50, the paper P is not always in the same state as the first paper feeding. There is also a possibility of leaning towards. Therefore, if some image alignment technique is not applied, there is a risk of color misregistration occurring in the final image.

  Before the toner image is transferred for the first time, the paper P passes through the fixing device 52 at least once, and then returns to the upstream position of the transfer roller 42, and then passes through the transfer roller 42 and the fixing device 52 a plurality of times. Thus, the toner images may be formed on the paper P in an overlapping manner. By doing so, color misregistration due to thermal contraction of the paper P can be minimized.

In addition, a technique for positively correcting toner images and matching the image positions is also necessary.
When the first toner image is formed, a plurality of position detection marks (positioning toner patterns) are formed in the image. In the example shown in FIG. 5, as the position detection marks, (●) marks 100 and (“) marks 102 are formed at both corners on the leading end side of the paper P, and (”) marks 104 are formed at one corner of the rear end. After fixing the first toner image including such a position detection mark, the position of the position detection mark is detected by a plurality of optical sensors (detection means) 112 provided near the exit of the fixing device 52 as shown in FIG. . From the detected value, the control unit 114 can calculate the printing start point (x ′, y ′), the main scanning direction width X ′, the sub scanning direction width Y ′, and the main scanning direction inclination Z ′. These values do not coincide with the printing start point (x, y), the main scanning direction width X, the sub-scanning direction width Y, and the main scanning direction inclination Z before fixing the first toner image. Therefore, at the time of the second exposure, the control unit 114 makes the print start point (x ′, y ′), the main scanning direction width X ′, the sub scanning direction width Y ′, and the main scanning direction inclination Z ′ match each other. The first toner image and the second toner image are completely matched by correcting the second toner image formed on the photosensitive member 14 by correcting the image exposure data and inputting it to the exposure device 18. Can do. In the above description, the main scanning direction width X ′ and the sub scanning direction width Y ′ may be the main scanning direction magnification X ′ / X and the sub scanning direction magnification Y ′ / Y.

  Before the third and subsequent toner image formation, the position detection marks 100, 102, and 104 after fixing the previous toner image are detected, and the third and subsequent toner images are similarly corrected. By doing so, color misregistration on the paper P can be prevented.

  Two or more optical sensors 112 for detecting the position detection mark need to be installed in the main scanning direction. In order to increase the detection accuracy, it is better to increase the distance between the sensors. However, in order to cope with a small-sized paper P, the distance may have to be reduced. Accordingly, a pair of optical sensors may be installed for each paper size, or the optical sensor may be moved in the main scanning direction according to the paper size.

  Since it is necessary to make the position detection mark less noticeable, it is preferable to print with yellow toner. In the above description, the marks “()” and “()” are used. However, when the marks (<) and (>) are used, the shift amount (particularly, the magnification X ′ / X in the main scanning direction) can be accurately detected. can do. The position detection mark is not limited to the one described above, and may have any shape.

  The shift in the sub-scanning direction can be matched by adjusting the paper conveyance speed instead of the image exposure data. In addition, correcting the inclination of the paper P in the main scanning direction (so-called skew) with the image exposure data appears as a step in the horizontal line. Therefore, it is desirable to correct it with registration control of the paper P as much as possible.

  Next, another image registration technique is shown in FIG. In this example, the contour of the paper P is detected instead of the position detection mark. Specifically, the positions of the end portions 106, 108, and 110 of the paper P before passing the transfer roller 42 for the first time are detected by the optical sensor (edge detection means) 116 shown in FIG. 6, and transferred after the second time. The position of the end portions 106, 108, 110 of the paper P before passing through the roller 42 is detected by the optical sensor 116. As a result, the control unit 114 causes the second and subsequent toner images to be printed on the paper P based on the comparison between the second and subsequent edge position detection results and the previous edge position detection result, the main scanning direction width, The second and subsequent toner images formed on the photoconductor 14 are corrected by correcting the image exposure data so that the sub-scanning direction width and the main scanning direction inclination coincide with the first toner image and inputting the data to the exposure device 18. It is to correct.

  The present invention can be applied to various image forming apparatuses such as a monochrome or color copying machine, a printer, a facsimile, and a complex machine of these.

1 is a schematic configuration diagram of an image forming apparatus according to a first embodiment of the present invention. FIG. 5 is a schematic configuration diagram of an image forming apparatus according to a second embodiment of the present invention. FIG. 9 is a schematic configuration diagram of an image forming apparatus according to a third embodiment of the present invention. FIG. 10 is a schematic configuration diagram of an image forming apparatus according to a fourth embodiment of the present invention. The figure for demonstrating the image positioning method. The figure which shows arrangement | positioning of the optical sensor which detects a position detection mark etc. The figure for demonstrating another image alignment method. FIG. 6 is a main part configuration diagram of a conventional four-cycle image forming apparatus.

Explanation of symbols

12 ... Development rack 14 ... Photoconductor drum (photoconductor)
40: Image forming apparatus 42: Transfer roller (transfer device)
44 ... feeding section 46 ... feeding roller 48 ... timing roller pair 50 ... transfer section 52 ... fixing unit 54 ... paper transport path 56 ... discharge path 58 ... circulation path 62 ... switching claw Y, M, C, K ... development vessel

Claims (11)

A photoreceptor having an electrostatic latent image formed on the surface;
One or a plurality of developing units corresponding to a single color toner or a different color toner to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
A sheet conveyance path through which the sheet is conveyed through the transfer unit and the fixing unit,
An image forming apparatus for forming a monochrome image or a color image by passing a sheet onto which the toner image has been transferred through the transfer unit and the fixing unit a plurality of times and forming the toner image on the sheet in an overlapping manner. . A photoreceptor having an electrostatic latent image formed on the surface;
A plurality of developing units corresponding to different color toners to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
A sheet conveyance path through which the sheet is conveyed through the transfer unit and the fixing unit,
An image forming apparatus for forming a color image by passing a sheet onto which the toner image has been transferred through the transfer unit and the fixing unit a plurality of times and forming a plurality of color toner images on the sheet. .   The sheet conveyance path is extended so that the sheet that has passed through the fixing unit circulates around the photosensitive member and the developing unit and is conveyed to an upstream position of the transfer unit. The image forming apparatus according to claim 1.   The sheet conveyance path is extended so that the sheet that has passed through the fixing device is conveyed to the upstream position of the transfer device through the fixing device and the back side of the transfer device. The image forming apparatus according to claim 1.   3. The image forming apparatus according to claim 1, wherein the toner image forming portion switches back the paper that has passed through the fixing device to the upstream position of the transfer device in the paper conveyance path.   The paper conveyance path is configured to switch back the paper that has passed through the fixing device and then to the upstream position of the transfer device through the back side of the fixing device and the transfer device. The image forming apparatus according to claim 1, wherein the image forming apparatus is stretched.   After passing the sheet on which the toner image has not been transferred at least once through the fixing unit and then returning the sheet to the upstream position of the transferring unit, the toner image on the sheet is passed through the transferring unit and the fixing unit a plurality of times. The image forming apparatus according to claim 1, wherein the image forming apparatus is formed so as to overlap each other.   A toner having detection means for detecting an alignment toner pattern, wherein a plurality of alignment toner patterns are formed at the first toner image formation on the photoreceptor, and the toner includes the alignment toner pattern. After the image is transferred and fixed on the paper, the positioning toner pattern is detected by the detection means, and the second and subsequent toner images are printed on the paper at the print start position, main position based on the detection result by the detection means. 2. The second and subsequent toner images formed on the photosensitive member are corrected so that a scanning direction width, a sub-scanning direction width, and a main scanning direction inclination coincide with the first toner image. The image forming apparatus according to claim 2.   It has an edge detection means for detecting the edge of the paper at the upstream position of the transfer device, and the edge detection means detects the edge position of the paper before passing the transfer device for the first time, and the second time and thereafter. The edge position of the sheet before passing through the transfer device is detected by the edge detection means, and the second and subsequent edge position detection results are compared based on the comparison between the second edge position detection result and the previous edge position detection result. Second and subsequent toner images formed on the photoconductor so that the toner image has a printing start position, a main scanning direction width, a sub-scanning direction width, and a main scanning direction inclination on the sheet coincide with the first toner image. The image forming apparatus according to claim 1, wherein correction is performed. A photoreceptor having an electrostatic latent image formed on the surface;
One or a plurality of developing units corresponding to a single color toner or a different color toner to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
An image forming method in an image forming apparatus comprising a sheet conveyance path through which a sheet is conveyed through the transfer unit and the fixing unit,
An image forming method comprising: forming a monochrome image or a color image by passing a sheet onto which the toner image has been transferred through the transfer unit and the fixing unit a plurality of times and forming the toner image on the sheet in an overlapping manner. . A photoreceptor having an electrostatic latent image formed on the surface;
A plurality of developing units corresponding to different color toners to develop the electrostatic latent image into a toner image;
A transfer device for transferring the toner image to paper;
A fixing device for fixing the toner image transferred to the paper;
An image forming method in an image forming apparatus comprising a sheet conveyance path through which a sheet is conveyed through the transfer unit and the fixing unit,
An image forming method comprising: forming a color image by passing a sheet onto which the toner image is transferred through the transfer unit and the fixing unit a plurality of times to form a plurality of color toner images on the sheet. .

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