JP2002014494A - Image forming device, image forming method and storage medium - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an image forming device capable of realizing efficient double-sided image forming control. SOLUTION: This image forming device is provided with a transfer means, a 1st paper feeding means for feeding paper to the transfer means, a reversing means for reversing the paper on the first side of which an image has been already formed from a front surface to a back surface, and a double-sided path for guiding the reversed paper to the transfer means again in order to form the image on both sides of the paper, and a 2nd paper feeding means for feeding the reversed paper to the transfer means in order to form an image on the 2nd side of the paper. In the device, the image is consecutively formed on plural sheets of paper in the arrival order to the transfer means whether the paper is fed by the 1st paper feeding means or the 2nd paper feeding means. In the case of deciding that there is possibility that the papers with different size are consecutively fed, the number of times of performing other paper feeding operation is controlled to fit to maximum length in the size of the paper on both sides of which the image can be formed is performed between the 1st and the 2nd paper feeding means for the same paper in control operation for alternately feeding the paper.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an image forming apparatus having a two-sided pass.

[0002]

2. Description of the Related Art Conventionally, in order to form an image on both sides of a sheet, there has been known a control for alternately performing image formation on a first side and image formation on a second side using a two-sided pass. I have.

In the control of alternately feeding sheets, other sheet feeding operations are performed while the same sheet is fed by the first sheet feeding means for the first side and the second sheet feeding means for the second side. It is well known that the number of times that can be performed depends on the length of the sheet and the distance between the drive units that convey the sheet, and varies depending on the length of the sheet.

[0004]

However, if the size of the paper to be continuously fed is different in order to form an image on both the front and back sides, the paper on the double-sided path is used to avoid a problem such as a jam. However, in order to avoid a plurality of sheets, the sheet feeding by the first sheet feeding unit for forming an image on the first side is performed by all the sheets fed by the first sheet feeding unit during the previous operation. The image forming operation is performed after the image forming operation for inverting the image by the inverting means and then feeding by the second sheet feeding means and forming an image on the second surface is completed.

[0005] For this reason, image formation on both sides of continuously fed paper sheets of different sizes greatly reduces productivity.

The present invention has been made in view of the above-described circumstances, and if it is determined that there is a possibility that the sizes of sheets to be continuously fed are different, before reading all the originals, In the control operation for alternately feeding sheets, the number of times that other sheet feeding operations are performed between the first sheet feeding unit and the second sheet feeding unit for the same sheet is determined by the number of sheet sizes that can form images on both sides. The image forming apparatus is capable of performing the two-sided image forming control more efficiently than the conventional control that performs the two-sided image forming for each sheet by deciding to match the maximum length and starting the image forming control. It is an object to provide an apparatus, an image forming method, and a storage medium.

[0007]

The present invention can solve the above-mentioned problems by providing the following constitution.

(1) Transfer means for forming an image on both sides of a sheet of paper, and first paper feeding means for feeding the sheet to the transfer means for forming an image on the first side of the sheet Reversing means for reversing the sheet after image formation on the first side, a two-sided path for guiding the reversed sheet to the transfer means again, and a two-sided path for forming an image on the second side of the sheet. A second sheet feeding unit for feeding the inverted sheet to the transfer unit, and regardless of the sheet from the first sheet feeding unit and the second sheet feeding unit, In an image forming apparatus that performs continuous image formation on a plurality of sheets in the order of arrival at the transfer unit, if it is determined that sheets of different sizes may be continuously fed, In the control operation for feeding paper, the first paper feeding unit and the second paper feeding During the stage, the number of times of the other paper feeding operation, among the imageable sheet size on both surfaces, an image forming apparatus and controls to match that of the maximum length.

(2) Transfer means for forming an image on both sides of a sheet of paper, and first paper feed means for feeding the sheet to the transfer means for forming an image on the first side of the sheet Reversing means for reversing the sheet after image formation on the first side, a two-sided path for guiding the reversed sheet to the transfer means again, and a two-sided path for forming an image on the second side of the sheet. A second sheet feeding unit for feeding the inverted sheet to the transfer unit, and regardless of the sheet from the first sheet feeding unit and the second sheet feeding unit, An image forming method in an image forming apparatus for forming a continuous image on a plurality of sheets in the order of arrival at the transfer unit, wherein it is determined that sheets of different sizes may be fed continuously. In the case, in the control step of alternately feeding paper, the first Between the paper process and the second sheet feeding step,
An image forming method including a step of controlling the number of times of performing another sheet feeding operation so as to match the maximum length among paper sizes that can form images on both sides.

(3) A storage medium storing a program for realizing the image forming method described in (2) above.

[0011]

Embodiments of the present invention will be described below.

(Embodiment) FIG. 1 is a longitudinal sectional side view showing a main part of an image forming apparatus according to an embodiment of the present invention.

In FIG. 1, the image forming apparatus includes a main body image output section 10 which is a device for placing and outputting a document image on a sheet, a main body image input section 11 which is a device for reading image data from a document, and a main body image. The automatic document feeder 12 includes an automatic document feeder 12 mounted above the input unit 11 and a discharge tray 13 on which sheets discharged from the main body image output unit 10 are stacked.

This image forming apparatus is an example of a digital copying machine. The image data read from the document as image data by the CCD of a main body image input unit 11 is a device for reading image data from a document. After necessary image processing is performed, the image data is stored in the image memory. The image data is transferred to the main body image output unit 10, where the image is reproduced and an image is formed on a sheet.

A main body image input section 11 is provided with a light source 21 for scanning while irradiating a document stacked on a document table on the upper surface of the input section.
Is provided. The light source 21 obtains a driving force from an unillustrated optical system motor and reciprocates in the left-right direction on the plane of FIG. The light generated by the light source 21 is reflected by the stacked originals, and an optical image is obtained. The optical image is transferred to mirrors 22, 2
The light is transmitted to the CCD 26 via the lenses 3 and 24 and the lens 25. The mirrors 22, 23 and 24 are driven integrally with the light source 21. The CCD 26 is constituted by an element for converting light into an electric signal, and the transmitted optical image is converted into an electric signal and further converted into a digital signal (image data) by the operation of the element.

The image data of the read original is subjected to various correction processes and image processing by a process desired by the user, and is stored in an image memory (not shown).

The main body image output unit 10 reads out the image data stored in the image memory, reconverts the digital data into an analog signal, and amplifies it to an appropriate output value by an exposure control unit (not shown). The light is converted into an optical signal by 27. The optical signal propagates through the scanner 28, the lens 29 and the mirror 30, and is irradiated on the photosensitive drum 31.
An electrostatic latent image is formed. This latent image forms an image with toner, is transferred onto a sheet conveyed in the main body, and the toner is fixed on the sheet by a fixing roller 32, image data is recorded, and the image data is sent to the discharge tray 13. .

The paper discharge tray 13 is a device installed on the left side (on the paper surface) of the main body image output section 10 and stacks sheets output from the main body image output section 10.

The paper feed trays 33 and 34 are provided at the lower part of the main body.
It is possible to accumulate recording paper to some extent. The control section conveys the recording paper accumulated from the paper feed trays 33 and 34 and outputs an image.

The paper feed deck 35 is a device installed on the left side (on the paper surface) of the main body image output unit 10 and can store a large amount of paper. Similarly to the paper feed trays 33 and 34, the control unit transports the stored paper and outputs an image.

On the left side of the main body image output unit 10, a manual feed tray 36 is provided, which enables the operator to relatively easily feed a small number of arbitrary types of paper. The manual tray 36 is also used when special paper such as OHP sheets, thick paper, and postcard size paper is used.

Reference numeral 37 denotes a vertical path conveying roller, 38 denotes a pre-registration roller, 39 denotes an internal discharge roller, 40 denotes a reversing roller, 41 denotes a double-sided conveying roller, and 42 denotes an external discharge roller. When feeding paper, it plays a role of actually transporting the paper.

Reference numeral 43 denotes a flapper 1, which switches between a direction in which the paper is conveyed to the discharge tray 13 and a direction in which the paper is conveyed to both sides. 4
Reference numeral 4 denotes a flapper 2, which is a reversing roller 4
Switching is performed between the direction in which the sheet is conveyed to the upper side and the direction in which it is conveyed to the lower side.

FIGS. 2 (a) to 2 (d) and FIGS. 3 (a) to 3 (a)
4D and 4A to 4D are schematic explanatory diagrams showing a series of flows of a two-sided image forming operation using the two-sided pass of the present invention.

Reference numeral 45 denotes a sheet on which a double-sided image is formed.
First, in order to form an image on the first side of the paper 45,
The paper 45 starts to be conveyed from the paper feed trays 33, 34, the paper feed deck 35, or the manual feed tray 36 shown in FIG. (A)).

The paper 45 is moved by the pre-registration roller 38
After forming a loop, performing skew removal by conveyance, and performing timing adjustment with respect to the electrostatic latent image formed on the photosensitive drum 31, the sheet is conveyed to a transfer unit including the photosensitive drum 31 and the like ( FIG. 2 (b).

Next, the electrostatic latent image radiated on the photosensitive drum 31 forms an image with toner, is transferred onto the paper 45 conveyed, and the toner is fixed on the paper 45 by the fixing roller 32. Then, the image data of the first surface is recorded (FIG. 2C).

Next, in order to form a double-sided image, the flapper 1 is used to switch the transport path of the sheet 45 to a direction in which the sheet 45 is guided to a double-sided pass section. Furthermore, in order to form the image on the second side, the sheet 45 is switched to a direction in which the conveyance path of the sheet 45 is guided to the upper part of the reversing roller 40 by using the flapper 2 to reverse the sheet 45. The paper 45 is guided to the upper part of the reversing roller 40 by driving the fixing roller 32 and the inner discharge roller 39 (FIG. 2D).

The paper 45 is conveyed by the reversing roller 40, and when the rear end of the paper 45 reaches the upper part of the reversing roller 40, the conveyance is stopped (FIG. 3A).

Subsequently, the rotation direction of the reversing roller 40 is reversed, and the transport operation is started again with the trailing end of the first surface of the paper 45 as the head. That is, the conveyance operation is restarted with the leading end of the second surface of the paper 45 as the head, and the paper 45 is reversed to form an image on the second surface (FIG. 3 ( b)).

Further, the sheet 45 is conveyed to the standby position for the double-sided sheet by using the double-sided conveying roller 41 (FIG. 3).
(C)).

At this position, the control unit waits until receiving a sheet feed request for forming an image on the second surface.

When a paper feed request is received from the control unit, the paper 45
Is restarted, and the sheet is conveyed toward the pre-registration roller 38 in the same manner as in the case of the first surface (FIG. 3D).

As with the first surface, the paper 45 forms a loop with the pre-registration roller 38, skews the paper 45 by conveyance, and determines the timing with the electrostatic latent image formed on the photosensitive drum 31. After the transfer, the sheet is conveyed to a transfer unit including the photosensitive drum 31 (FIG. 4A).

Next, the electrostatic latent image radiated on the photosensitive drum 31 forms an image with toner, is transferred onto a sheet of paper 45 conveyed, and is further fixed on the sheet of paper 45 by the fixing roller 32. , The image data of the second surface is recorded. This means that the image has been formed on both sides of the sheet 45 (FIG. 4B).

Next, in order to discharge the paper 45 on which the double-sided image is formed, the conveyance path of the paper 45 is switched to the direction of leading to the discharge tray 13 by using the flapper 1, and the inner discharge roller 39 and the outer discharge The paper is conveyed by the paper roller 42 (FIG. 4).
(C)).

Finally, the paper 45 is conveyed by using the external paper discharge roller 42 until the rear end of the paper 45 comes off and passes, is discharged out of the apparatus, and is stacked on the paper discharge tray 13 (FIG. 4D). .

FIG. 5 is a schematic explanatory view showing a double-sided image forming operation when five sheets are circulated.

In FIG. 5, uppercase alphabetic characters indicate image formation on the first side of the sheet, and lowercase alphabetic letters indicate image formation on the second side of the sheet. The handling of the characters on the first side and the second side on the same sheet will follow this.

First, sheet feeding by the first sheet feeding means for forming an image on the first side is performed by the sheet feeding trays 34 and 3.
The three sheets are continuously conveyed in order of A, B, and C from 5 to the transfer section. On the transported sheet, an image is transferred and fixed to the first surface in the order of A, B, and C, respectively.

Thereafter, the sheet is reversed by the above-described sheet reversing operation using the two-sided pass, and is conveyed toward the both-side standby position in order. When the image formation on the first side of the three sheets is completed, the image formation a, b, and c by the second paper feeding unit for forming the image on the second side of each sheet and the image formation on the first side are performed. Image formation D, E, and F are performed alternately. Then, the sheets on which the image has been formed on the second side are respectively discharged to the discharge tray 13. By alternately feeding the first side and the second side, an efficient image forming operation can be performed. Processing then proceeds to d, G, e, H, f, I
Is repeated. Since image formation is performed five times, including the first side of the next sheet, from the start of alternate sheet feeding until the image is formed on the second side of the same sheet, this control is performed by circulating five sheets. I will call it. For example, by the time the image formation on both sides D → d is completed, including D, D,
That is, image formation is performed five times with b, E, c, and F.

FIG. 6 is a schematic explanatory view showing a double-sided image forming operation when three sheets are circulated.

In FIG. 6, as in FIG. 5, uppercase alphabetic characters indicate image formation on the first side of the sheet, and lowercase English letters indicate image formation on the second side of the sheet. The handling of the characters on the first side and the second side on the same sheet will follow this.

In FIG. 5, the image forming operation at the time of circulating five sheets has been described. However, depending on the distance between the respective conveying rollers in the sheet conveying path, the length of the two-sided path, and the sheet length, depending on the sheet length, The number of sheets that can be simultaneously placed on the road is determined. When the paper length is long, first, two sheets of paper are fed from the paper feed trays 34 and 35 to the transfer unit by the first paper feeding means for forming an image on the first side. , B in this order. On the conveyed sheet, the image is transferred and fixed to the first surface in the order of A and B, respectively.

Thereafter, the sheet is reversed by the sheet reversing operation using the above-described two-sided pass, and is sequentially conveyed toward the both-side standby position. When the image formation on the first side of the two sheets is completed, image formation a and b by the second paper feeding means for forming the image on the second side of each sheet and the image formation on the first side C and D are performed alternately. Then, the sheets on which the image formation on the second side is completed are respectively output to the discharge tray 1.
It is discharged to 3. That is, an efficient image forming operation can be performed by alternately feeding the first side and the second side. The processing is thereafter repeatedly performed for c, E, d, and F. After the start of the alternate sheet feeding and before the image is formed on the second side of the same sheet, three times including the first side of the next sheet.
This control is called three-sheet circulation because image formation is performed twice. For example, image formation on both sides (C,
By the time c) is completed, C, b, and D, including C, are formed three times.

FIG. 7 is a schematic explanatory view showing a conventional double-sided image forming operation when different sizes are mixed.

In FIG. 7 as well, as in FIGS. 5 and 6, uppercase letters indicate image formation on the first side of the sheet, and lowercase letters indicate image formation on the second side of the sheet. The handling of the characters on the first side and the second side on the same sheet will follow this.

As described above, the number of sheets circulated depends on the sheet length on which double-sided image formation is performed. Become.

However, in recent years, with the development of technology, an image forming apparatus capable of forming a double-sided image on paper having different paper sizes has appeared.

For example, there is an image forming apparatus having a function of automatically selecting a paper size according to the size of a document to be copied. In this case, all the documents are read in order to start the image forming operation early. Instead of waiting for
Image formation is performed sequentially from the read document. Therefore, when the image forming operation is started, all the sheet sizes cannot be determined, so that the number of circulation sheets is not determined.

Conventionally, when the size of paper to be continuously fed is different in order to form an image on both sides, in order to avoid a problem such as a jam, the number of sheets on the two-sided pass does not become plural. In this manner, the sheet feeding by the first sheet feeding unit for forming an image on the first side is reversed by the reversing unit, and all the sheets fed by the first sheet feeding unit last time are inverted.
The sheet is fed by the second sheet feeding means, an image is formed on the second side, and the process is performed after the completion of the image formation.

First, the paper is fed from the first paper feeding means for forming an image on the first side to the paper feeding trays 34 and 3.
One sheet A is conveyed from 5 or the like to the transfer section. And
The image is transferred and fixed to the first side of the conveyed sheet A.

Thereafter, the sheet A is inverted by the sheet reversing operation using the above-described two-sided pass, and is conveyed toward the both-side standby position. In order to avoid a problem such as a jam, alternate feeding cannot be performed. When image formation on the first side of the sheet A is completed, a second sheet for forming an image on the second side a of the sheet A is left as it is. The image forming of the second surface a is performed by the sheet feeding unit. Then, the sheet on which the image has been formed on the second surface a is discharged to the discharge tray 13. When the image forming operation on the front and back sides of one sheet is completed, the image forming operation on the first side of the sheet B is started. Also on the sheet B, the image forming operation B on the first side
Is completed, an image is formed on the second surface b as it is.
After forming the image on both the front and back sides of each sheet, the image formation on the next sheet starts.
It can be seen that the control is effective but very inefficient.

FIG. 8 is a schematic explanatory view showing a double-sided image forming operation according to the present invention when different sizes are mixed.

In FIG. 8, as in the case of FIG.
Uppercase letters indicate image formation on the first side of the paper,
The lowercase alphabetic characters indicate image formation on the second side of the sheet.
The handling of the characters on the first side and the second side on the same sheet will follow this.

In recent years, there is also an image forming apparatus having a function of automatically selecting a paper size according to the size of a document to be copied. In this case, all the documents are read in order to start the image forming operation early. Instead of waiting for
Image formation is performed sequentially from the read document. Therefore, at the time when the image forming operation is started, all the sheet sizes cannot be determined, and thus the number of circulating sheets is not determined.

In the prior art, in order to avoid a jam, both sides of the image are formed on one sheet and then both sides are formed on the next sheet. He explained that.

Therefore, in the double-sided image forming operation according to the present invention when different sizes are mixed, even before all the originals are read,
By performing the double-sided image formation with the double-sided circulating number as the double-sided circulating number with respect to the maximum sheet length of the double-sided sheet in the image forming apparatus, it is possible to control to increase the efficiency of the double-sided image forming operation.

More specifically, if it is determined that the size of the paper that is continuously fed may be different, the maximum size of the double-sided paper in the image forming apparatus is read before reading all the originals. In the configuration according to the present invention, the double-sided image forming operation is performed by temporarily setting the double-sided circulating number with respect to the sheet length to three sheets.

First, the paper is fed by the first paper feeding means for forming an image on the first surface by the paper feeding trays 34 and 3.
The two sheets are continuously conveyed in order of A and B from 5 and the like to the transfer section. On the conveyed sheet, the image is transferred and fixed to the first surface in the order of A and B, respectively.

For example, even if the sheet length of A is a sheet size capable of circulating five sheets, it is determined whether there is no problem even if the double-sided image forming operation for all sheets is circulated five sheets until all the originals are read. Since it is not determined, the control is performed by circulating three sheets.

Thereafter, the sheet is reversed by the sheet reversing operation using the above-described two-sided pass, and is sequentially conveyed toward the both-side standby position. When the image formation on the first side of the two sheets is completed, image formation a and b by the second paper feeding means for forming the image on the second side of each sheet and the image formation on the first side C and D are performed alternately. Then, the sheets on which the image formation on the second side is completed are respectively output to the discharge tray 1.
It is discharged to 3. That is, an efficient image forming operation can be performed by alternately feeding the first side and the second side. The processing is thereafter repeatedly performed for c, E, d, and F.

As a result, even if the paper length of all the papers is a paper length capable of circulating five sheets, the determination is not finalized until all the originals have been read. It is more efficient than the conventional control method to be formed.

A program for executing various processes (the processes described with reference to the drawings and the like) according to the present embodiment is stored in a ROM (not shown) in the control unit of the image forming apparatus. It is read out and executed by a CPU (not shown). Needless to say, the present invention can be applied to a case where the present invention is achieved by supplying a program to a system or an apparatus. In this case, by reading out a storage medium storing a program represented by software for achieving the present invention into a system or an apparatus, the system or the apparatus can enjoy the effects of the present invention. The storage medium for supplying the program is not limited to the ROM. For example, a floppy (registered trademark) disk, hard disk, optical disk, magneto-optical disk, CD-ROM, CDR, DVD, magnetic tape,
A non-volatile memory card or the like can be used.

[0065]

As described above, according to the present invention,
If it is determined that there is a possibility that the size of the paper continuously fed may be different, the first paper feeding unit for the same paper may be controlled in an alternate paper feeding control operation before reading all the originals. The number of times that other sheet feeding operations are performed between the second sheet feeding unit and the second sheet feeding unit, out of sheet sizes that can form images on both sides,
By deciding to match the maximum length and starting the image formation control, it is possible to perform the two-sided image formation control more efficiently than the conventional control which performed the two-sided image formation for each sheet. Can improve productivity.

[Brief description of the drawings]

FIG. 1 is a longitudinal sectional side view showing a main part of an image forming apparatus according to an embodiment of the present invention;

FIG. 2 is a schematic explanatory view showing a series of flows of a two-sided image forming operation using a two-sided pass of the present invention (part 1);

FIG. 3 is a schematic explanatory diagram showing a series of flows of a two-sided image forming operation using a two-sided pass of the present invention (part 2);

FIG. 4 is a schematic explanatory view showing a series of flows of a two-sided image forming operation using a two-sided pass of the present invention (part 3);

FIG. 5 is a schematic explanatory view showing a double-sided image forming operation when circulating five sheets.

FIG. 6 is a schematic explanatory view showing a double-sided image forming operation when three sheets are circulated.

FIG. 7 is a schematic explanatory view showing a conventional two-sided image forming operation when different sizes are mixed.

FIG. 8 is a schematic explanatory view showing a double-sided image forming operation when different sizes are mixed according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 10 Main-body image output part 11 Main-body image input part 12 Automatic document feeder 13 Discharge tray 21 Light source 22-24 Mirror 25 Lens (input system) 26 CCD 27 Optical irradiation part (laser) 28 Scanner 29 Lens (Output system) 30 Mirror REFERENCE SIGNS LIST 31 photosensitive drum 32 fixing roller 33, 34 paper feed tray 35 paper feed deck 36 manual feed tray 37 vertical path transport roller 38 pre-registration roller 39 inner discharge roller 40 reverse roller 41 double-sided transport roller 42 outer discharge roller 43 flapper 1 44 flapper 2 45 paper

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Keizo Isemura 3-30-2 Shimomaruko, Ota-ku, Tokyo Inside Canon Inc. (72) Inventor Hiroto Nishihara 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inside (72) Inventor Ichiro Sasaki 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. (72) Inventor Tomoyasu Yoshikawa 3-30-2 Shimomaruko, Ota-ku, Tokyo Canon Inc. F Terms (reference) 2C062 RA06 2C087 AA09 AC08 BA03 CB12 DA17 2H027 DC19 ED16 EF09 FA11 2H028 BA05 BB02 BB04 3F100 AA02 DA04 EA14

Claims (3)

[Claims] A transfer unit configured to form an image on both sides of a sheet of paper; and a first paper supply unit configured to supply the sheet to the transfer unit to form an image on a first side of the sheet. A reversing unit for reversing the sheet after image formation on the first side, a two-sided pass for guiding the reversed sheet back to the transfer unit, and a two-sided pass for forming an image on the second side of the sheet. A second sheet feeding unit for feeding the inverted sheet of paper to the transfer unit, regardless of the sheet from the first sheet feeding unit and the second sheet feeding unit, In an image forming apparatus that performs continuous image formation on a plurality of sheets in the order of arrival at the transfer unit, if it is determined that there is a possibility that sheets of different sizes are continuously supplied, the sheets are fed alternately. In the control operation for paper feeding, the first paper feeder and the second paper feeder for the same paper During, the number of times of performing the other of the paper feed operation, among the imageable sheet size on both surfaces, an image forming apparatus and controls to match that of the maximum length. 2. A transfer device for forming an image on both sides of a sheet of paper, and a first paper feeding device for feeding the sheet to the transfer device for forming an image on the first side of the sheet. A reversing unit for reversing the sheet after image formation on the first side, a two-sided pass for guiding the reversed sheet back to the transfer unit, and a two-sided pass for forming an image on the second side of the sheet. A second sheet feeding unit for feeding the inverted sheet of paper to the transfer unit, regardless of the sheet from the first sheet feeding unit and the second sheet feeding unit, An image forming method in an image forming apparatus for forming a continuous image on a plurality of sheets in the order of arrival at a transfer unit, wherein it is determined that there is a possibility that sheets of different sizes are continuously supplied. In the control process of alternately feeding sheets, the first sheet feeding for the same sheet And a step of controlling the number of other sheet feeding operations between the first sheet feeding step and the second sheet feeding step so as to match the maximum length of sheet sizes that can be image-formed on both sides. Image forming method. 3. A storage medium storing a program for realizing the image forming method according to claim 2.