JP4627049B2 - Image forming apparatus - Google Patents

Description

  In the present invention, after the conveyance sheet is temporarily stopped by the registration roller, the leading edge of the image information visualized on the electrostatic latent image carrier and the leading edge of the sheet stopped by the registration roller are set in advance. The present invention relates to an image forming apparatus including a conveyance control unit that resumes conveyance of a sheet at a fixed timing, and more specifically, image formation corresponding to a high-speed machine in which the number of sheets conveyed to a transfer unit exceeds 100 sheets per minute. Relates to the device.

  In recent image forming apparatuses, a vertical conveyance type that does not take up space is mainly used due to the installation space. That is, a plurality of paper feed cassettes are arranged in a multi-stage at the lower part of the apparatus main body, and a transfer part and a fixing part are arranged at the upper part. In such a structure, the transport paper fed from the paper feed cassette is transported upward once, is bent approximately 90 degrees in front of the transfer unit, and is converted into transport in the horizontal direction, and then transferred to the transfer unit. It is intended to be conveyed toward

  FIG. 6A shows the configuration of the paper conveyance path on the front side of the transfer unit.

  As described above, a pair of conveyance rollers R31 and R32, pre-registration rollers R41 and R42, and registration rollers R51 and R52 are sequentially arranged along the sheet conveyance direction S in the sheet conveyance path 28 arranged in a curved shape. The registration rollers R51 and R52 are opposed to the nip portion N1 that is a contact portion between the electrostatic latent image carrier (photosensitive drum) 14 and the transfer roller 17a with a distance of about 50 mm.

  FIG. 7 is a chart showing the operation timing of the registration rollers R51 and R52 and the pre-registration rollers R41 and R42.

  That is, when the sheet P is conveyed through the sheet conveyance path 28 to the registration rollers R51 and R52 and the leading end of the sheet abuts (nips) the nip portion N5 of the registration rollers R51 and R52, the registration rollers R51 and R52 first stop at time t1. At slightly later time t2, the pre-registration rollers R41 and R42 are stopped. Due to such a stop due to the time difference, the paper P sandwiched between the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 is held in a slightly curved state as shown in FIG.

  At this time, in the conventional image forming apparatus, the leading end stop position X (see FIG. 6B) of the paper P slightly protruding from the nip portion N5 of the registration rollers R51 and R52 is always a constant stop position (appropriate stop position). The precondition is that In other words, it is assumed that the amount L1 of the sheet P protruding from the nip portion N5 of the registration rollers R51 and R52 is always a constant amount.

  Thereafter, the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 are visualized on the leading edge of the paper P stopped by the registration rollers R51 and R52 and on the electrostatic latent image carrier (photosensitive drum) 14. The conveyance of the paper P is resumed again at time t3 when the leading edge of the image information is made coincident (that is, a certain timing set in advance with reference to the appropriate stop position X). At that time, since the paper P is slightly bent, the feeding timing of the front end of the paper P from the registration rollers R51 and R52 can be matched in the width direction of the paper, and the skew of the paper P can be prevented. The distortion of the formed image in the width direction of P is eliminated. That is, the paper P once stopped by the registration rollers R51, R52 is finely adjusted by the registration rollers R51, R52 in the direction parallel to the paper transport direction S, the center of the transport paper, and transported during the stop. The skew of time is corrected.

  Here, conventional conveyance speed control of the sheet by the registration rollers R51 and R52 when conveyance of the sheet P is resumed at time t3 will be described.

  When the conveyance of the sheet is resumed at time t3, as shown in FIG. 8, the rotation speeds of the registration rollers R51 and R52 (that is, the sheet conveyance speed) are accelerated at a predetermined constant acceleration a1, and the rotation speed at time t5. Reaches the process speed Vp of the apparatus, the state of the process speed Vp is maintained thereafter. By such speed control, the sheet conveyed at time tn when the leading edge of the image information visualized on the electrostatic latent image carrier (photosensitive drum) 14 reaches the transfer portion (nip portion N1). Similarly, the leading end of the toner reaches the transfer portion (nip portion N1) so that the two leading end portions coincide with each other.

  Incidentally, recent image forming apparatuses are required to have a higher printing processing speed than conventional image forming apparatuses. For example, in the past, 60 sheets / min (in the case of A4 horizontal transport) or more was regarded as a high-speed machine, but in recent years, 80 sheets / min or more has been called a high-speed machine, and further 100 to 120 sheets. Development of an image forming apparatus of more than / min is also in progress.

  In such an image forming apparatus, it is necessary to improve the printing quality when the toner image on the electrostatic latent image carrier (photosensitive drum) is transferred onto the paper. In order to ensure this printing quality, Increases the accuracy of alignment between the leading edge of the image information visualized on the electrostatic latent image carrier (photosensitive drum) and the leading edge of the paper P stopped by the registration rollers R51 and R52. Is an important factor.

  By the way, in the conventional image forming apparatus, as described above, the leading edge stop position X of the paper P slightly protruding from the nip portion N5 of the registration rollers R51 and R52 is always a constant stop position (appropriate stop position). As a premise, the sheet conveyance speed is controlled by the registration rollers R51 and R52. However, in practice, the amount L1 of the paper P protruding from the nip N5 of the registration rollers R51 and R52 varies depending on the size and thickness of the paper, the waist of the paper, the balance of the pressure applied to the registration rollers R51 and R52, and the like. appear. For this reason, the conventional sheet conveyance speed control by the registration rollers has a problem that the alignment accuracy between the leading edge of the sheet and the leading edge of the image information cannot be said to be sufficiently high.

  Therefore, as a technique for improving the accuracy of such alignment, a paper transport device described in Patent Document 1 has been proposed.

This paper conveying device compares the time from the start of rotation of the timing roller until the sensor detects the leading edge of the paper with a preset reference time, and based on the difference between the two, the timing roller is moved upstream in the paper conveying direction. The leading edge of the sheet is made to coincide with the leading edge of the image forming area on the photosensitive drum by being moved to the side or the downstream side.
JP 09-156802 A

  However, in recent high-speed machines, the paper conveyance speed has been increased, and the variation in the stop position of the paper leading edge at the registration roller has increased, and the timing of the subsequent conveyance paper excluding the first sheet can be adjusted when printing multiple sheets continuously. It has become difficult in terms of time. That is, since there is a time margin for the first printed sheet in the initialization process of the apparatus, it is possible to secure a time margin for adjustment at the registration roller portion by increasing the paper feeding timing from the paper feeding unit. Can do. However, the sheet conveyance timing for the second and subsequent sheets depends on the printing processing speed, that is, the conveyance speed, and for the second and subsequent sheets, the electrostatic latent image carrier (photosensitive member) before the leading edge of the sheet comes into contact with the nip portion of the registration roller. Since writing of image information onto the drum) has started, there is no time margin for adjusting the paper leading edge position at the registration roller portion.

  The present invention was devised in view of such circumstances, and an object of the present invention is to variably control the sheet conveyance speed at which conveyance is always resumed at a constant timing based on the sheet leading edge stop position during the printing of a plurality of sheets. Accordingly, an object of the present invention is to provide an image forming apparatus capable of accurately aligning the leading edge of a sheet and the leading edge of image information even in a high-speed machine.

  In order to solve the above-described problems, in the image forming apparatus of the present invention, a registration roller is disposed immediately in front of the electrostatic latent image carrier, and after the conveyance paper is temporarily stopped by the registration roller, the electrostatic latent image carrier. An image forming apparatus including a conveyance control unit that resumes conveyance of the sheet at a predetermined preset timing for matching the leading edge of the image information visualized above with the leading edge of the sheet stopped by a registration roller. , And a detecting means for detecting a leading edge stop position of the conveyed paper once stopped by the registration roller, and during the continuous printing process of a plurality of sheets, the conveying control means is arranged at the paper leading edge stop position detected by the detecting means. On the basis of this, the conveyance speed of the sheet that resumes conveyance at the predetermined timing is controlled. As the detection means, a line sensor arranged in front of and behind the registration roller along the paper conveyance direction can be used. However, it is not limited to the line sensor, and any detection means may be used as long as the leading edge stop position of the paper once stopped by the registration roller can be detected.

Specifically, the control of the sheet conveyance speed by the conveyance speed control unit is performed by using the electrostatic force with respect to the appropriate stop position in which the sheet leading end stop position detected by the detection unit is set corresponding to the certain timing. In the case of the latent image carrier side, a reduction speed and a reduction maintenance time with respect to the process speed are calculated from a difference between the tip stop position and the appropriate stop position, and based on the calculated reduction speed and reduction maintenance time. If the sheet leading edge stop position detected by the detecting means is on the paper feed unit side with respect to the appropriate stopping position, the leading edge stopping position and the appropriate stopping position are controlled. From the difference, an increase speed and an increase maintaining time with respect to the process speed are calculated, and the conveyance speed is controlled to be accelerated based on the calculated increasing speed and the increase maintaining time .

In this case, the transfer control means controls to return to the process speed when the reduction maintenance time or the increase maintenance time elapses. That is, it is important to control the sheet so that it returns to the process speed of the apparatus when the sheet reaches the transfer portion of the electrostatic latent image carrier. Even if the leading edge of the image information visualized on the electrostatic latent image carrier and the leading edge of the paper that has been transported are matched in the transfer section by acceleration control or deceleration control, the process speed is restored at that time. If not, the formed image itself will be destroyed. That is, when the actual paper conveyance speed is faster than the process speed, the image is rubbed and stretched, and when it is slower than the process speed, the image is overprinted and crushed.

  According to the image forming apparatus of the present invention, at the time of printing a plurality of sheets, based on the stop position of the leading end of the sheet, the sheet transport speed at which the transport is resumed at a constant timing is variably controlled. And the image information front end can be accurately aligned.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings.

  FIG. 1 is a side view showing the overall configuration of the image forming apparatus according to the present embodiment.

  The image forming apparatus 1 of the present embodiment is a digital image forming apparatus having copy, printer, scanner, and fax modes, for example, and an operation panel 10 is provided on the front side.

  On the upper surface of the image forming apparatus 1, a manuscript table 11 made of a hard transparent glass body is arranged, an automatic manuscript feeder 12 is arranged above the manuscript table 11, and an optical unit 13 is arranged below the manuscript table 11. .

  Below the optical unit 13, a photosensitive drum 14 whose surface is made of a photoconductive material is rotatably supported. Around the photosensitive drum 14, a charger 15, a developing device 16, a transfer unit 17, and a cleaner 18 are disposed so as to face the peripheral surface of the photosensitive drum 14.

  In the image forming apparatus 1 configured as described above, when the start of the image forming process is instructed by operating the operation panel 10, the optical unit 13 scans the image surface of the document placed on the document table 11, and the inside of the optical unit 13 The light reflected from the copy lamp on the original image surface is irradiated onto the surface of the photosensitive drum 14.

  Prior to the irradiation of the reflected light from the original, the surface of the photosensitive drum 14 is uniformly charged with a single polarity charge by the charger 15, and the photoconductor is caused by the photoconductive action caused by the irradiation of the reflected light from the original. An electrostatic latent image is formed on the surface of the drum 14. Toner is supplied from the developing device 16 to the surface of the photosensitive drum 14 on which the electrostatic latent image is formed, and the electrostatic latent image is visualized as a developer image.

  A fixing unit 20 including a heating roller and a pressure roller is disposed on the downstream side of the photosensitive drum 14. A transfer belt 50 and a paper guide 19 of the transfer unit 17 are disposed between the fixing unit 20 and the photosensitive drum 14. The transfer belt 50 and the paper guide 19 allow the transfer belt 50 and the paper guide 19 to move from the photosensitive drum 14. A sheet conveyance path to the fixing unit 20 is formed.

  A sheet discharge tray 33 is provided on the side surface of the image forming apparatus 1, and a sheet discharge conveyance path 22 is formed between the fixing unit 20 and the sheet discharge tray 33. A part of the paper discharge conveyance path 22 branches through a branch gate 25 into a re-conveyance path 24 that is continuous with an automatic double-sided paper feeder 23 disposed below the photosensitive drum 14.

  Below the image forming apparatus 1, four paper feed cassettes 26 that are detachably mounted from the front side of the image forming apparatus 1 are provided. Each sheet cassette 26 stores sheets of different sizes, and the sheet from any one of the four sheet cassettes 26 is fed by a sheet feed roller 27 prior to the rotation of the photosensitive drum 14. Is fed through. The fed paper is conveyed in the direction of the photosensitive drum 14 by the conveyance rollers R31 and R32 via the common conveyance path 28, the rear end is sandwiched by the pre-registration rollers R41 and R42, and the front end is registered by the registration rollers R51 and R51. Stops in contact with R52. The configuration of this part is the same as that shown in FIG. The operation timings of the registration rollers R51 and R52 and the pre-registration rollers R41 and R42 are also the same as the operation timing shown in FIG.

  Further, the image forming apparatus 1 of the present embodiment includes a large-capacity paper feeding unit (LCC) 60. Although details of the structure of the large-capacity paper feed unit 60 are omitted, the paper fed from the large-capacity paper feed unit 60 is joined to the common transport path 28 before the transport rollers R31 and R32. , And is conveyed in the direction of the photosensitive drum 14 by the conveying rollers R31 and R32, the rear end is held between the pre-registration rollers R41 and R42, and the front end is brought into contact with the registration rollers R51 and R52 and stopped.

  The registration rollers R51 and R52 rotate in synchronization with the rotation of the photosensitive drum 14, and guide the sheet to the nip portion (image forming region) N5 between the photosensitive drum 14 and the transfer unit 17. The paper guided to the image forming area is subjected to corona discharge of the transfer unit 17, and the developer image carried on the surface of the photosensitive drum 1 is transferred to the surface of the paper.

  The sheet on which the developer image has been transferred is conveyed to the fixing unit 20 along the transfer belt 50 and the paper guide 19, and is heated and pressurized in the fixing unit 20, and the developer image is melted and fixed on the surface of the sheet. To do.

  In the single-sided printing mode in which an image is printed on one side of the paper, the paper that has passed through the fixing unit 20 is discharged from the paper discharge port 32 onto the paper discharge tray 33 by the paper discharge roller 31 via the paper discharge conveyance path 22. At this time, the paper discharge roller 31 is driven to reciprocate in the paper transport direction by the paper discharge roller driving unit 60.

  In the double-sided printing mode in which images are printed on both sides of the paper, the branch gate 25 is exposed in a part of the paper delivery conveyance path 22, and the paper that has passed through the fixing unit 20 passes through the re-conveyance path 24 including the conveyance roller 34. Then, it is conveyed to the automatic double-sided paper feeder 23. The paper transported to the automatic duplex paper feeder 23 is fed in a state where the front and back are reversed by the refeed roller 35, and again with the front and back surfaces reversed by the retransport roller 36 via the common transport path 28. It is conveyed in the direction of the photosensitive drum 14, the rear end is sandwiched between the registration rollers R41 and R42, and the front end is brought into contact with the registration rollers R51 and R52 and stopped.

  FIGS. 2A and 2B show a configuration example of the sheet leading edge detecting means 70 for detecting the leading edge stop position of the sheet that is stopped by contacting the registration rollers R51 and R52. As shown in FIG. 2B, the registration rollers R51 and R52 have four sets of registration rollers R51 and R52 arranged at a predetermined interval in the width direction. The line sensor 70a and the light receiving side line sensor 70b face each other up and down via the paper transport path and are disposed along the paper transport direction S perpendicular to the registration rollers R51 and R52. As shown in FIG. 2A, these line sensors 70a and 70b are arranged so as to extend in the front-rear direction (paper transport direction S) through the nip portion N5 of the registration rollers R51 and R52. The photosensitive drum 14 side is arranged to be slightly longer than the portion N5. In the figure, reference numeral 81 denotes a pre-registration paper guide, and reference numeral 82 denotes a post-registration paper guide.

  Next, the configuration of the control system in the image forming apparatus 1 having the above configuration will be described with reference to the block diagram shown in FIG.

  The central processing unit (control unit) 101 manages each drive mechanism unit constituting the image forming apparatus 1 such as the automatic document feeder 12, the optical unit 13, the image forming unit 102, and the sheet conveying system 103 by sequence control. A control signal is output to each unit based on the detection values of the various sensor units 106 including the paper leading edge detecting means 70 (the light emitting side line sensor 70a and the light receiving side line sensor 70b).

  An operation panel 10 is connected to the control unit 101 in a state where mutual communication is possible, and the operation of the operation panel 10 causes the image forming apparatus 1 to operate in accordance with print processing conditions set and input by the user. Yes.

  In addition, a memory 104 and an image data communication unit 105 are connected to the control unit 101. The memory 104 stores various control information necessary for controlling each drive mechanism unit constituting the image forming apparatus 1. In addition, the memory 104 stores a detected value of the paper leading edge stop position detected by the paper leading edge detection means 70. The image data communication unit 105 is a communication unit provided for enabling information communication of image information and image control signals with other digital image devices.

  The control unit 101 controls the printing process according to the printing process condition input and set by the user through the operation of the operation panel 10. At this time, during the continuous printing process for a plurality of sheets, the control unit 101 temporarily stops at the registration rollers R51 and R52. When resuming conveyance of a sheet at a predetermined timing, the sheet conveyance speed is controlled based on the detection value of the sheet leading edge detection means 70. That is, the image forming apparatus according to the present embodiment is based on the premise that the conveyance resumption timing of the paper once stopped by the registration rollers R51 and R52 is always a constant timing (time t3 in FIG. 8).

  Hereinafter, with respect to the control operation of the sheet conveyance speed when the sheet temporarily stopped by the registration rollers R51 and R52 is resumed at a predetermined timing, the flowchart shown in FIG. 4 and FIG. This will be described with reference to a graph showing the state of speed control.

  When a print request for continuous printing processing for a plurality of sheets is received by operating the operation panel 10 (step S1), the control unit 101 feeds paper from one paper feed cassette 26 (step S2), and passes through the common transport path 28. Then, the toner is conveyed in the direction of the photosensitive drum 14 by the conveying rollers R31 and R32. Thereafter, when the leading edge of the paper reaches the registration rollers R51 and R52 (when it is determined Yes in step S3), the control unit 101 sandwiches the trailing edge of the paper with the pre-registration rollers R41 and R42, and the front edge of the registration roller R51. , R52 abutting (holding), the rotation of the registration rollers R51, R52 is temporarily stopped (step S4). At this time, the leading end stop position of the sheet is detected by the sheet leading end detection means 70 (step S5), and this detected value (leading end stop position) A is stored in the memory 104.

  Thereafter, the control unit 101 compares the leading end stop position A detected by the sheet leading end detection means 70 with the appropriate stop position X stored in advance in an internal memory of the control unit 101 (not shown) (step S6, step S6). S7). As a result, when the tip stop position A coincides with the appropriate stop position X (when it is determined Yes in step S7), the same control as before, that is, the speed control indicated by the solid line in FIG. 8 is executed ( Step S8). That is, the rotation speeds of the registration rollers R51 and R52 are controlled such that the registration rollers R51 and R52 are rotated again and accelerated at the acceleration a1 so that the sheet conveyance speed becomes the process speed Vp of the apparatus.

  On the other hand, as a result of comparing the tip stop position A and the appropriate stop position X, if the two stop positions do not match (if determined No in step S7), then the tip stop position A is set to the appropriate stop position X. It is judged whether or not (step S11). As a result, when the leading end stop position A exceeds the proper stop position X (when it is determined Yes in step S11), the leading end stop position A of the sheet is closer to the transfer unit than the proper stop position X. From the difference between the tip stop position A and the appropriate stop position X, a reduction speed (−β) and a reduction maintaining time Tβ with respect to the process speed Vp are calculated (step S12), and this reduction speed (−β) and the reduction maintaining time Tβ are calculated. The speed control considering the above is executed (steps S13 to S15).

  That is, as indicated by a one-dot chain line in FIG. 5, the registration rollers R51 and R52 are re-rotated at time t3 and accelerated at the acceleration a2 so that the sheet conveyance speed becomes the deceleration speed (Vp−β). , R52 is controlled (step S13). At the same time, the control unit 101 starts measuring the reduction maintenance time Tβ from time t3 by an internal timer unit (not shown) (step S14), and when the reduction maintenance time Tβ has elapsed (when it is determined Yes in step S14), The sheet conveyance speed, that is, the rotation speed of the registration rollers R51 and R52 is returned (accelerated) from the deceleration speed (Vp-β) up to that to the process speed Vp of the apparatus (step S15). When the process speed Vp is reached, the process speed Vp is maintained thereafter. By such speed control, the sheet conveyed at time tn when the leading edge of the image information visualized on the electrostatic latent image carrier (photosensitive drum) 14 reaches the transfer portion (nip portion N1). Similarly, the leading end of the toner reaches the transfer portion (nip portion N1), and both leading end portions coincide with each other.

  On the other hand, as a result of comparing the leading edge stop position A with the appropriate stopping position X, when the leading edge stop position A is in front of the appropriate stopping position X (when it is determined Yes in step S17), the leading edge stop of the sheet is stopped. Since the position A is closer to the paper feed unit than the proper stop position X, an increase speed (+ α) and an increase maintaining time Tα with respect to the process speed Vp are calculated from the difference between the leading end stop position A and the proper stop position X (step S18), speed control considering the increase speed (+ α) and the increase maintaining time Tα is executed (steps S19 to S21).

  That is, as indicated by a broken line in FIG. 5, the registration rollers R51 and R52 are rotated again at time t3 and accelerated at an acceleration a3 so that the sheet conveyance speed becomes the acceleration speed (Vp + α). The rotation speed is controlled (step S19). At the same time, the control unit 101 starts measuring the increase maintenance time Tα from time t3 by an internal timer unit (not shown) (step S120), and when the increase maintenance time Tα elapses (when it is determined Yes in step S20), The sheet conveyance speed, that is, the rotation speed of the registration rollers R51 and R52 is returned (decelerated) from the acceleration speed (Vp + α) up to that time to the process speed Vp of the apparatus (step S21). When the process speed Vp is reached, the process speed Vp is maintained thereafter. By such speed control, the sheet conveyed at time tn when the leading edge of the image information visualized on the electrostatic latent image carrier (photosensitive drum) 14 reaches the transfer portion (nip portion N1). Similarly, the leading end of the toner reaches the transfer portion (nip portion N1), and both leading end portions coincide with each other.

  The image processing apparatus of the present invention is suitably used for a high-speed digital multi-function peripheral that has a copy mode, a printer mode, a scanner mode, and a fax mode, and that processes a large amount of printed matter at a high speed.

1 is a side view showing an overall configuration of an image forming apparatus of the present invention. (A) is a side view of the paper leading edge detecting means, and (b) is a plan view of the same. 3 is a block diagram illustrating a configuration of a control system of the image forming apparatus of the present invention. FIG. 6 is a flowchart for explaining a control operation of a sheet conveyance speed by a registration roller. 6 is a graph showing a state of sheet conveyance speed control of registration rollers by the image forming apparatus of the present invention. (A) is explanatory drawing which shows the structure of the paper conveyance path before this transfer part, (b) is explanatory drawing which expands and shows a registration roller part. It is a chart which shows the operation timing of a registration roller and a pre-registration roller. 6 is a graph showing a state of sheet conveyance speed control of registration rollers by a conventional image forming apparatus.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Image forming apparatus 10 Operation panel 11 Document stand 12 Automatic document feeder 13 Optical unit 14 Photosensitive drum 15 Charger 16 Developing device 17 Transfer unit 18 Cleaner 20 Fixing unit 22 Paper discharge conveyance path 23 Automatic duplex paper feeder 24 Re-conveyance Path 26 paper feed cassette 28 common transport path 31 (31a, 31b) paper discharge roller 32 paper discharge port 33 paper discharge tray 50 transfer belt 60 large capacity paper feed unit 61 unit side transport path 70 paper leading edge detection means 70a light emission side line sensor 70b Light-receiving-side line sensor 81 Pre-registration paper guide 82 Post-registration paper guide 101 Central processing unit (control unit)
DESCRIPTION OF SYMBOLS 102 Image formation part 103 Paper conveyance system 104 Memory 105 Image data communication unit R31, R32 Conveyance roller R41, R42 Pre-registration roller R51, R52 Registration roller

Claims (3)

A registration roller is disposed immediately in front of the electrostatic latent image carrier, and after the conveying paper is temporarily stopped by the registration roller, the leading edge of the image information visualized on the electrostatic latent image carrier and the registration roller In the image forming apparatus including a conveyance control unit that resumes conveyance of the sheet at a predetermined timing that matches the leading edge of the stopped sheet.
A detection means for detecting a leading edge stop position of the conveyance paper once stopped by the registration roller;
During the continuous printing process for a plurality of sheets, the conveyance control unit is configured to detect the leading edge stop position detected by the detection unit with respect to an appropriate stop position set corresponding to the certain timing. In this case, a reduction speed and a reduction maintenance time with respect to the process speed are calculated from the difference between the tip stop position and the appropriate stop position, and the transport speed is calculated based on the calculated reduction speed and reduction maintenance time. When the paper front end stop position detected by the detection means is on the paper feed unit side with respect to the appropriate stop position, the difference between the front end stop position and the appropriate stop position is image formation is calculated and increased speed and increased maintenance time for the process speed, and controls so as to accelerate the conveyance speed based on the calculated rate of increase was increased maintenance time and Location.   The image forming apparatus according to claim 1, wherein the detection unit is a line sensor disposed in front of and behind the registration roller along a sheet conveyance direction. It said conveyance control means, the image forming apparatus according to claim 1 or claim 2, wherein the controller controls so as to return to the process speed and the reduced maintenance time or the increase maintenance time elapses.

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