JP2000214639A - Image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To optimize the sticking position of recording material to a recording material carrier and to avoid image defect on the second surface of both surfaces so as to restrain the lowering of image productivity to the minimum by repeating two sheets and one sheet as the number of carried sheets of recording material per one round of the recording material carrier in the alternate rotation of the recording material carrier in the case of forming an image on the backside of the recording material. SOLUTION: Recording paper P passes through a reversing path 21A, is reversed by a reversing roller 21B and is housed in an intermediate tray 22. After stacking the necessary number of sheets of recording paper, the recording paper P' on whose one surface the image has been already recorded is fed one by one from the tray 22 by a pickup roller 25. After feeding the paper by a paper feeding roller again, the paper is carried to a carrying part 50A. In such a case, the interval of the recording paper at the time of feeding the paper again is set to be longer than that in a state where the first surface of the recording paper is fed so as to prevent the mutual collision of the recording paper even when the slip of the recording paper occurs. Then, '2', '1', '2', '1' are repeated as the number of sticking sheets of the recording paper on a transfer drum 5.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an image forming apparatus employing an appropriate transfer type or direct type image forming process by an electrophotographic system such as a printer, a facsimile, and a copying machine.

[0002]

2. Description of the Related Art In FIG. 9, recording is performed by temporarily stopping the sheet feeding section and the leading end of a recording sheet (recording sheet) as a sheet material fed from the sheet feeding section to form a loop on the recording sheet. 1 schematically illustrates an example of an image forming apparatus having a registration unit that corrects skew of a sheet and sends a recording sheet to an image forming unit at a predetermined timing.

[0003] The image forming apparatus of this embodiment is roughly divided into an image input section 90R and an image output section 90P.

The image input unit 90R is, for example, an image information photoelectric reading mechanism, which inputs image information to be recorded, performs various processes, and sends the image signal to the image output unit 90P. The image input unit 90R may be an optical device for projecting and exposing a document image to the image carrier of the image forming unit 91 of the image output unit 90P.

The image output section 90P is an image recording mechanism of a transfer system using a transfer drum 93 which is a recording material carrier in this embodiment, and mainly produces an electrophotographic image in accordance with image information input to the image output section 90R. Photoreceptor by appropriate image forming process such as process, electrostatic recording process, magnetic recording process, etc.
An image forming unit 91 for forming a transferable recording image such as a toner image on an image carrier such as a dielectric or magnetic material, and first, second, and third sheet feeding units 92A and 92B as sheet feeding units.
92C, a pre-registration conveyance unit 97 that conveys the recording paper P fed from any of the paper feeding units to a registration roller pair 96 as a registration unit, and holds the recording paper P sent from the registration roller pair 96. And a fixing unit 94 that performs an image fixing process on the recording paper P that has undergone image transfer at the transfer unit T of the image forming unit 91, and a fixing unit 94 that conveys the image to the transfer unit T of the image forming unit 91. And a control unit 95 for controlling.

In the case of creating a double-sided image, the fixing unit 9 is used.
The recording paper P exiting from No. 4 is loaded on the duplex unit 99 via the reverse transport unit 98.

[0007] The first and second paper feed unit 92A,
Reference numeral 92B denotes an automatic single sheet feeding mechanism incorporated in the image output unit 90P in two upper and lower stages. The recording sheet P stacked and stored in the sheet cassette is supplied to a sheet feeding roller based on a sheet feeding start signal. One sheet is separated and fed in order from the top one by cooperation with the separating member. The third paper feed unit 92C is a manual paper feed device. When the recording paper P is inserted into the apparatus from a manual paper feed table, the recording paper P is drawn into the apparatus by the paper feed roller and fed. Also, from the duplex unit 99, one sheet is separated and fed from the sheet re-feeding means 92D in order from the top one in the same manner as the sheet feeding cassette.

[0008] When the operation of the apparatus is started, the first to the first are performed.
The recording paper P is fed from any of the third paper feeding units 92A to 93C, and is conveyed to the registration roller pair 96 by the pre-registration conveyance unit 97.

On the other hand, the image forming section 91 forms an image corresponding to the target image information on the image carrier,
The rotation of the registration roller pair 96 is started in synchronization with the image recording start signal, and the pre-registration conveyance unit 97 is again in the recording paper conveyance state.
The recording paper P is sent to the transfer unit T while being held on the outer surface of the transfer drum 93, so that the transfer of the image formed and carried on the image carrier of the image forming unit 91 is sequentially performed. receive.

There are several methods for holding the recording paper P on the outer surface of the transfer drum 93, such as electrostatic attraction from a thin dielectric film or the like onto a conveyance medium or air suction.

The recording paper P is sent out by the pair of registration rollers 96 when the leading end of the image formed and carried on the image carrier of the image forming section 91 reaches the transfer section T, and the leading end of the recording paper P also moves to the transfer section T. The timing is controlled so that the timing just arrives and the two coincide (registration).

The recording paper P, to which the image has been transferred in the transfer unit T, is separated from the outer surface of the transfer drum 93 and conveyed to a fixing unit 94, where it is subjected to fixing processing of the transferred image by heat and / or pressure. After that, the paper is discharged to a paper discharge tray outside the machine. In the fixing section, oil for preventing the recording paper P from winding around the fixing roller is applied to the recording paper P.

Here, the position where the recording paper P is adhered to the transfer drum 93 will be described. First, as shown in FIG. 10, the transfer drum 93 is formed by winding a recording sheet holding sheet film 93S in a cylindrical shape around a frame 93F in which annular members 93A at both ends are connected by a connecting member 93B. The film 93S is stuck so that the start end and the end thereof are on the connecting member 93B, and a seam 93C is formed there. Then, if the recording paper P is pasted on this portion, it causes image failure and paper jam, so that the recording paper P is sucked over the seam 93C.

The circumferential length of the transfer drum 93 is slightly longer than that of the large-size recording paper P such as A3, and by adjusting the registration roller pair 96 and the transfer position T, the transfer drum 93 shown in FIG.
As shown in (a), the recording paper P is adhered substantially over the circumference. In the case of small-size recording paper P such as A4, in the case of single-sheet feeding, as shown in FIG. 11B, the recording paper P is attached at the same position as the large size. In the case of continuous printing of two or more sheets, the sheet feeding operation is repeated two or more times, so that the circumference is divided into two as shown in FIG. By this operation, one recording paper P per rotation
Can increase the productivity twice as compared with the case of sticking.

[0015]

However, both sides 2
There are the following problems in continuously attaching the small-sized recording paper P of the face (back side) to the transfer drum 93.

When the recording paper P on which the image and the fixing oil are loaded on the first side is fed by the re-feeding unit 92D, the coefficient of surface friction of the recording paper P is reduced due to the influence of oil and the like. The slip of the recording paper P occurs at the time of refeeding, and FIG.
It becomes impossible to attach the recording paper P to the transfer drum 93 at the interval (c).

This state will be further described with reference to FIG. FIG. 5 is a view in which the sticking position of the transfer drum 92 is developed and shown continuously.

FIG. 5A corresponds to FIG.
A state is shown in which large-sized recording paper such as three sizes is stuck. In normal image formation, as shown in FIG. 5C, two sheets of recording paper are attracted to the transfer drum 93 per circumference, and image formation is performed one after another for monochromatic image formation. However,
In the case of the second side of both sides, since the slip occurs more than the interval d of the recording paper P shown in FIG. 11C, it is impossible to continuously attach two recording papers to the transfer drum 93 two by two. Here, if the recording sheet sticking operation of 2 sheets / 1 turn is continued forcibly, the phenomenon of running over the seam occurs as described above, which causes an image defect.

In order to prevent image defects, a method of adsorbing the recording paper P one by one in every other sheet as shown in FIG. 5B is conceivable. However, in this method, the usual 2
The productivity of the image is reduced by half compared with the case where the sheets are pasted one by one, resulting in a remarkable decrease in productivity.

Accordingly, it is an object of the present invention to optimize the position where the recording material is attached to the recording material carrier, and to minimize a decrease in image productivity while avoiding image defects on the second side of both surfaces. It is an object of the present invention to provide an image forming apparatus capable of performing the above.

[0021]

The above object is achieved by an image forming apparatus according to the present invention. In summary, the present invention provides:
The recording material conveyed from the paper supply unit is simultaneously carried on two sheets per one circumference of the recording material carrier, and after receiving the transfer of the image on the surface from the image carrier, the recording material is carried on the recording material carrier again via the intermediate tray. In the image forming apparatus for forming an image on the back surface of the recording material, when forming an image on the back surface of the recording material, the number of recording material carried per rotation of the recording material carrier is determined by the recording material carrier. An image forming apparatus is characterized in that two sheets and one sheet are repeated every other rotation.

In order to form an image on the back surface of the recording material, the interval between the recording materials when the recording material is conveyed before the recording material is adsorbed and carried on the recording material carrier is determined when the image is formed on the front surface. And the interval between the recording material carrying members on the recording material carrying member.

According to another aspect of the present invention, two sheets of recording material conveyed from the paper feeding unit are simultaneously carried per rotation of the recording material carrier, and the intermediate image is transferred after the image on the surface is transferred from the image carrier. In an image forming apparatus that forms an image on the back surface of a recording material by carrying the recording material again on the recording material carrier via a tray, the number of recording materials held per rotation of the recording material carrier during image formation on the back surface is determined. An image forming apparatus is provided in which the number of sheets is two, and one sheet is provided per rotation only when it is impossible to supply two sheets of recording material to the recording material carrier.

[0024]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, an image forming apparatus according to the present invention will be described in more detail with reference to the drawings.

Embodiment 1 A first embodiment of the present invention will be described with reference to FIGS. First, the image forming apparatus of the present embodiment will be described with reference to FIG.

In FIG. 1, the image forming apparatus according to the present embodiment is a color image forming apparatus utilizing a transfer type electrophotographic process, and has a digital color image reader unit 100 at an upper part and a printer part 200 at a lower part.

[Reader Unit] In the reader 100 unit, the original 30 is placed on an original platen glass 31 and is exposed and scanned by an exposure lamp 32. The reflected light image from the original 30 is condensed on the full color sensor 34 by the lens 33 to obtain a color separation image signal. This signal is subjected to various processing in a video processing unit (not shown) via an amplification circuit (not shown), and then sent to the printer unit 200.

[Printer Unit] The photosensitive drum 1 as an image carrier is driven to rotate at a predetermined peripheral speed (process speed) in a clockwise direction indicated by an arrow around a central axis.

A pre-exposure lamp 11, a primary charger 2, an exposure optical system 3 using a laser or the like, an electric potential sensor 12, a developing device 4, and an on-drum light amount detecting means 13 opposing the outer peripheral surface of the photosensitive drum 1 along the rotation direction. A transfer drum 5 as a recording material carrier, a cleaning device 6, and the like are arranged in this order.

The pre-exposure lamp 11 and the primary charger 2 apply a uniform charge to the surface of the photosensitive drum 1.
Next, a light beam E, such as a laser beam, modulated according to the recording image signal by the optical system 3 is applied to the polygon mirror 3.
A, photosensitive drum 1 via lens 3B, mirror 3C, etc.
Exposure on top forms an electrostatic latent image thereon.

The electrostatic latent image is developed by the developing device 4. The developing device 4 includes a yellow developing device 4Y, a cyan developing device 4C, and a magenta developing device 4M that contain yellow (cyan), magenta, and black developers (toners), respectively.
The black developing unit 4Bk is composed of four developing units, and is selectively moved closer to the photosensitive drum 1 by the operation of the eccentric cams 24Y, 24C, 24M, and 24Bk to perform a developing operation.

Then, the toner image formed and carried on the photosensitive drum 1 is sequentially transferred to the surface of the recording paper P supplied and held on the transfer drum 5 in the transfer section T as described later.

The recording paper P to which the toner image has been transferred is separated from the transfer drum 5 and introduced into the fixing unit 9 where the image is subjected to an image fixing process and is discharged to a paper output tray 10 outside the machine or transported vertically. The path is guided by a path switching guide 19 to a transport vertical path 20 of a double-sided recording paper path described later.

The fixing unit 9 is composed of a pair of a fixing roller 9A having a heat source such as a halogen heater therein and a pressure roller 9B (sometimes having a heat source) which is pressed against the roller 9A. The toner on P is melted by heat and fixed. At this time, oil is applied to the fixing roller 9A by the oil application roller to prevent the image from being transferred to the fixing roller 9A, but the oil adheres to the recording paper P on the contrary.

As other components, a conveyance belt 9C and an entrance guide 9D for guiding the recording paper P to the nip portion of the roller pair 9A, 9B, and the roller pair 9A, 9B
It comprises a discharge roller 9E and the like for further guiding the recording paper P discharged from B to the outside of the apparatus.

After the transfer of the toner image to the recording paper P, the surface of the photosensitive drum 1 is cleaned by the cleaning device 6 to remove residual deposits such as toner remaining after transfer, and is repeatedly used for image formation.

[Sheet Feeding Unit] In the apparatus of the present embodiment, the lower side of the above-mentioned image forming mechanism serves as a sheet feeding unit.
To the fourth four sheet feeding units 41A, 41B, 41C, 41D
Is provided. First to third paper feed units 41A and 41B,
41C is provided in the upper, middle and lower tiers,
This is a sheet separating and feeding mechanism. The fourth paper feeding unit 41D is a manual paper feeding mechanism. Further, the first and second sheet feeding units 4 in the upper and middle stages
Between 1A and 41B, an intermediate tray 22, a guide section 23, and a reversing path 21 for a two-sided recording paper path to be described later are provided.

The first to third paper feeding units 41A, 41B, 41
In each of the sheets C, the stacked recording paper P in the sheet cassette is separated one by one from the uppermost one by the cooperation of the pickup roller 42 and a separation member such as a separation claw.
3A and 43B, and the sheet feeding from the sheet feeding unit at the designated stage is selectively executed by driving the pickup roller 42 of the sheet feeding unit at the designated stage.

The manual paper feed mechanism, which is the fourth paper feed unit 41D, places the recording paper P on the manual paper feed table and inserts it into the apparatus, so that the recording paper P is connected to the paper feed rollers 42M (not shown). It is drawn into the apparatus by the separation pad and fed.

[Transfer / Convey Unit] The recording paper P fed from one of the first to fourth paper feed units 41A to 41D or the intermediate tray 22 is introduced into the transfer / convey unit 50.

The transfer and transport unit 50 is roughly divided into a transport section 50A and a transfer drum 5. The transfer unit 50A is shown in FIG.
A description will be given based on a partially enlarged view of FIG.

The transport section 50A is provided with guides 48A, 48 for guiding the recording paper P inherited from the paper feeding unit 40.
B, 48C, 48D, 48E, and the transport roller 46
A, 46B, 46C, 47A, 47B, a pair of registration rollers 44 for sending the recording paper P to a transfer drum 5 to be described later in synchronization with the image forming timing of the image forming section, and a suction guide 48F for guiding the recording P at that time. , 48G and the like.

Here, the transport rollers 46A, 46B, 46
C has a curling mechanism for the recording paper P.

The guide 48D is swingably supported in the direction of the arrow as shown by the solid line and the broken line in the figure. This is to secure a pre-registration loop space of the recording paper P which is generated when the leading end of the recording paper is temporarily stopped by the registration roller pair 44.

[Transfer Drum] As shown in FIG. 3, the transfer drum 5 is formed by wrapping a recording paper holding sheet film 55S in a cylindrical shape around a frame 55F in which annular members 55A at both ends are connected by a connecting member 55B, as described above. The drum 5 is formed. As a material of the film 55S, for example, PET (polyethylene terephthalate), PVdF (polyvinylidene fluoride), or the like is used.

A suction roller 5G is supported at a position where the recording paper P sent from the registration roller pair 44 protrudes into the transfer drum 5 so as to be able to be separated from and contacted with the transfer drum 5. 5G suction roller on the side
And a charging member 5C for suction to oppose the pressing force of.

In the image transfer area (transfer section) T where the photosensitive drum 1 and the transfer drum 5 are opposed, a transfer charger 5B (FIG. 1) is also arranged on the back side of the film 55S. Downstream thereof, a separating member for separating the recording paper P, specifically, a static eliminator 5H, a separation claw 8A, a film pushing roller 8B, and the like are provided, followed by the film static eliminators 5D, 5E. Are arranged with the film 55S interposed therebetween.

Further downstream, the brush roller 14 and the corona discharger or brush type static eliminator 15 for cleaning toner and paper dust adhered to the recording paper holding surface of the film 55S, and the fixing unit 9 are used. The oil to be removed may adhere to the film surface through the recording paper P during double-sided recording (described later), and the oil removing roller 16 and the backup brush 1 for cleaning it may be used.
7 are provided.

Here, the transfer drum 5 is configured to be able to be separated from and contacted with the photosensitive drum 1, and is separated except during normal operation. This is to prevent the transfer drum 5 from contacting the photosensitive drum 1 for an excessively long time, and to prevent the recording paper P or the like from contacting the photosensitive drum 1 and damaging the surface thereof during jam processing. . Further, in the present apparatus, the transfer / conveyance unit 50 includes sliders 62A, 62B.
, So that it can be pulled out from the main body to the near side. This is to make jam clearing or maintenance more reliable and easier.

[Double-sided recording paper path] This apparatus has a mechanism for automatically recording images on both sides of the recording paper P. That is, the conveyance path switching guide 19 is located immediately downstream of the fixing unit 9, and the
Automatically selects one of the paper path for discharging the single-sided recording paper P that has exited from the machine and the two-sided recording paper path.
You will be guided.

The double-sided recording paper sheet path includes a transport vertical path 20, a reversing path 21A, a reversing roller 21B, an intermediate tray 22, and the like. As a result of the reverse rotation, the sheet is retracted in a direction opposite to the direction in which the sheet is fed, with the rear end of the sheet being fed, and is stored in the intermediate tray 22. afterwards,
The paper is fed again from the intermediate tray 22, and an image is formed on the other surface of the recording paper P 'by the above-described image forming process. The guide section 23 on the upper portion of the intermediate tray 22 is configured to selectively move up and down a plurality of guides according to the size of the recording paper to be stored. Thus, paper of any size can be stored and stacked at the top of the intermediate tray 22.

[Color (Multicolor) Image Recording] When an image forming operation start signal is issued, first, the cam 71 rotates half a turn, and accordingly, the transfer drum 5 swings around the photosensitive drum 1 and moves close to the operating position. Then, the photosensitive drum 1 rotates in synchronization with the photosensitive drum 1.

With this, the recording paper P is fed from the paper feeding unit 40. Here, for convenience, it is assumed that paper feeding from the second paper feeding unit 41B, which is the middle paper feeding unit, is designated. That is, the second sheet feeding unit 41B
, The recording paper P is sent out one by one from the paper feed cassette. And the paper feed roller 43
The paper is guided between the paper feed guides by A and 43B and the transport rollers 45B and 45C and transported to the transport unit 50A.

In the transport section 50A, the recording paper P is first curled by a predetermined amount in the direction in which it is wound around the transfer drum 5 when passing through the curled rollers 46A, 46B, 46C (FIG. 2).

Thereafter, the sheet is conveyed to the pair of registration rollers 44 by the action of the conveying rollers 47A and 47B and the paper guide. At that time, the registration roller pair 44 has stopped rotating, and the leading end of the paper abuts the nip portion of the registration roller pair 44. After a certain period of time, the driving of the transport rollers 47A and 47B is stopped.

Thereafter, the registration roller pair 44 and the transport rollers 47A and 47B start rotating after a certain period of time with reference to the timing at which the image forming section starts forming an image.
The rotation timing is set such that the recording paper P and the toner image on the photosensitive drum 1 exactly coincide with each other in the image transfer area T.

When the recording paper P enters and comes into contact with the transfer drum 5, the recording paper P is held by the holding sheet film 55 by the corona discharge from the attraction charger 5C and the action of the attraction roller 5G.
It is electrostatically attracted onto S. The suction roller 5G is normally separated from the transfer drum 5 to reduce the load, and is pressed against the transfer drum 5 only when the paper enters.

The transfer drum 5 rotates in synchronization with the photosensitive drum 1, and the recording paper P is transported to the image transfer area T while being held thereon. Then, after the toner image formed on the photosensitive drum 1 by the above-described process is transferred to the surface by the transfer charger 5B, the recording paper P is conveyed to the separation unit.

Here, since one color image is transferred at one time, the recording paper P is separated from the transfer drum 5 by a later-described operation in the case of a single color image. In the case of forming an image, the recording paper P is conveyed to the transfer area T again by making one revolution while being held on the transfer drum 5 without performing the separating operation. Then, the next toner image is further transferred onto the previous image, and this process is repeated a required number of times. By this multiple transfer process, a target color (four colors in the case of full color) image is synthesized and formed on the recording paper P. The brush roller 14 and the like are supported so as to be detachable from the surface of the transfer drum, and are separated at least in the multiple transfer step.

Here, the position at which the recording paper P is pasted to the transfer drum 5 is A as described in the conventional example.
In the case of a large size such as 3, the recording paper is stuck substantially over the circumference as shown in FIG. In the case of small-size recording paper such as A4, and in the case of single-sheet feeding, FIG.
Paste at the same position as the large size as shown in (b).
Then, in the case of two or more sheets, by repeating the sheet feeding operation twice, the circumference is divided into two as shown in FIG.

In the case of full-color image formation, as described above, during the rotation of the transfer drum 4, the same recording paper rotates while being attracted to the transfer drum 5, so that the recording paper P is attached to the transfer drum 5, and the separation operation is performed. This is performed once every four rotations, and the sticking timing for continuous image formation is as shown in FIG.

When the operation is completed, the adsorbing force between the recording sheet P and the holding sheet film 55S of the transfer drum 5 is reduced by the operation of the charge removing charger 5H in the separation section, and the recording is performed by the subsequent operation of the separation claw 8A and the like. The paper P separates from the transfer drum 5.

The recording paper P separated from the transfer drum 5 is transported to the fixing unit 9 by the transport belt 9C, and is guided on the entrance guide 9D accurately to the fixing roller nip. Then, the toner image is fixed on the recording paper surface by the heat of the fixing roller 9A. Thereafter, the recording paper P is conveyed by the paper discharge rollers 9E, and is discharged outside the apparatus. Finally, the transfer drum 5 is separated from the photosensitive drum 1 to the retreat position, and the apparatus stops.

[Double-sided image recording] In the case of double-sided image recording,
The single-sided recording paper P exiting the fixing unit 9 is guided to the transport vertical path 20 by switching of the switching guide 19, and thereafter, the recording paper P is reversed by the reversing roller 21 B via the reversing path 21 A, and Is stored in. After the required number of sheets are stacked, the pickup roller 25
The single-sided recording paper P 'is sent out from the intermediate tray 22 one by one, and is fed again by the paper feed roller 26C, and then conveyed to the conveyance unit 50A.

Here, as shown in the conventional problem, the coefficient of friction of the recording paper surface decreases due to the influence of the image on the first side and the oil during re-feeding as compared with the normal paper feeding operation. Large slippage during paper feeding. Due to this slip, the interval between the recording papers is larger than a predetermined interval (d dimension in FIG. 5C) between the recording papers to be pasted on the transfer drum. Becomes impossible.

Therefore, when a single-color image is formed, the operation of attaching two sheets of recording paper on a normal transfer drum and the operation of attaching one sheet of paper are alternately performed (see FIG. 5B ').

FIG. 6 shows a time chart of the flow of the recording paper at this time. In FIG. 6, the thick line indicates the position of the recording paper, the horizontal axis indicates time, and the vertical axis indicates the position. Also, when forming the first side image,
That is, FIG. 7 shows a time chart when the slip is small as a reference.

In FIG. 6, the interval between the recording sheets at the time of re-feeding is longer than that in the first-side sheet feeding state (FIG. 7) so that the recording sheets do not collide with each other even if the recording sheet slips. Keep it. With the recording paper interval as it is, 2
Since it is impossible to affix a sheet of recording paper, a relatively long standby is performed in a state where the first recording paper is applied to the registration roller pair 44, during which the second recording paper is conveyed to reduce the interval. Further, since the third sheet is re-fed at a longer interval than usual, it is impossible to attach the third sheet to the transfer drum at a normal recording sheet interval (for example, FIG. 5C). . Therefore, the third sheet is attached to a position different from the normal position on the second rotation of the transfer drum.

For the fourth to sixth sheets, re-feeding and pasting of the recording paper are performed in the same manner as the first to third sheets. 1
, Two, one.

FIG. 5 (B ') shows a diagram in which the recording paper is pasted on the transfer drum in this case.

By the above operation, even if the image productivity is reduced due to slippage of the recording paper at the time of forming the image on both sides, the state of FIG. 5B of the conventional example is changed from the state of FIG.
It is possible to prevent the recording paper from being stuck to the joint of the transfer drum in that state.

After the recording paper P is pasted on the transfer drum 5 as described above, an image is recorded on the other side of the recording paper through the same steps as in the case of single-sided image formation.
Finally, the double-sided recording paper is discharged outside the apparatus without operating the switching guide 19.

Embodiment 2 A second embodiment of the present invention will be described mainly with reference to FIGS.

In the first embodiment, when the recording paper is carried on the transfer drum, one, two, one, and two sheets are completely and repeatedly carried, but the present invention is not limited to this construction. Not something.

Even in the case of re-feeding, the slip amount of the recording material may be small as shown in FIG. 6 depending on the state of the re-feed roller and the image density at that time. At this time, it is possible to attach two sheets on the transfer drum in the state of FIG.

Therefore, even in the case of re-feeding, the recording paper is re-fed in a sequence in which the interval between the recording papers is short as shown in FIG. 7, and two sheets are pasted on the transfer drum as shown in FIG. Are performed continuously to improve the image productivity.

A further description will be given with reference to the flowchart of FIG.

When the refeeding operation is started (S1), the uppermost sheet among the recording sheets P having an image formed on the surface stacked on the intermediate tray 22 is picked up, and refeeding is performed (S2). ). At this time, the slip amount in FIGS. 6 and 7 and the like is detected by the recording paper detection sensor 45C ′ disposed immediately before the recording paper P is transported by the transport roller 45C. This is performed for each recording sheet (S3).

Then, based on the result, it is determined whether or not the slip amount is smaller than the amount of sticking on the transfer drum 5 two / one round (S4). When the slip amount is small (that is, the sequence of FIG. 7 is possible). ) Is 2 sheets / 1
The re-feeding of the circumference and the sticking to the transfer drum 5 are performed, and as a result, the sticking is performed as shown in FIG. 5C (S5).

When the detection result of the recording paper detection sensor 45C 'indicates that the slip amount is large (when the sequence of FIG. 7 is not possible), the re-feed interval is extended to be equivalent to that of FIG. The sticking of the recording paper per rotation is one sheet / perimeter.

By repeating this, the image is stuck to the transfer drum 5 as shown in FIG.
The mode of the circumference is mixed.

[0082]

As is apparent from the above description, according to the image forming apparatus of the present invention, when forming an image on the back surface of a recording material, the number of recording material carried per rotation of the recording material carrier is reduced. By repeating two and one sheets every other rotation of the recording material carrier, or when forming an image on the back surface of the recording material, the number of recording materials held per rotation of the recording material carrier can be reduced. The number of recording materials to the recording material carrier is set to one per revolution only when it is impossible to supply two recording materials to the recording material carrier per revolution. By optimizing the sticking position, it is possible to avoid the image defect on the second side of both sides and minimize the decrease in image productivity.

[Brief description of the drawings]

FIG. 1 is an overall configuration diagram illustrating an image forming apparatus according to a first embodiment of the present invention.

FIG. 2 is a configuration diagram illustrating a transport unit of the image forming apparatus of FIG. 1;

FIG. 3 is a perspective view showing a transfer drum of the first embodiment.

FIG. 4 is a schematic diagram showing a state in which a recording material is carried on a transfer drum. FIG. 4A shows a state in which one sheet of A3 size paper is carried, FIG. 4B shows a state in which one sheet of A4 size paper is carried, and FIG. The state (c) in which the sheets are carried is shown.

FIG. 5 is an explanatory view showing a state in which the recording material is carried when the transfer drum in the first and second embodiments and the conventional example rotates.

FIG. 6 is a time chart for carrying a recording material in the first embodiment.

FIG. 7 is a time chart for supporting the recording material when the slip of the recording material is small.

FIG. 8 is a flowchart of control in a second embodiment.

FIG. 9 is a schematic configuration diagram of a conventional image forming apparatus.

FIG. 10 is a perspective view showing a transfer drum in the image forming apparatus of FIG. 9;

11 is a schematic diagram showing a state in which a recording material is carried on the transfer drum of FIG. 10; FIG. 11 (a) shows a state in which one A3-size sheet is carried; FIG.
The state (c) in which two sheets of size paper are carried is shown.

[Explanation of symbols]

 Reference Signs List 1 photosensitive drum (image carrier) 5 transfer drum (recording material carrier) 22 intermediate tray 50A transport unit

Claims (3)

[Claims] 1. A recording material conveyed from a paper supply unit is simultaneously carried by two per one circumference of a recording material carrier, and after receiving a transfer of an image on the surface from the image carrier, the recording material passes through an intermediate tray and is again recorded. An image forming apparatus that forms an image on the back surface of a recording material by supporting the recording material on a back surface of the recording material. The recording material carrier 1
An image forming apparatus wherein two sheets and one sheet are repeated every rotation. 2. An image is formed on the back surface of a recording material. Therefore, the distance between recording materials during conveyance of the recording material before the recording material is adsorbed and carried on the recording material carrier is determined when the image is formed on the front surface. 2. The image forming apparatus according to claim 1, wherein the distance between the recording materials and the distance between the recording materials on the recording material carrier are larger than the intervals between the recording materials. 3. The recording material conveyed from a paper feeding unit is simultaneously carried by two per one circumference of the recording material carrier, and after receiving the transfer of the image on the surface from the image carrier, the recording material is again passed through the intermediate tray. In an image forming apparatus which forms an image on the back surface of a recording material by supporting the recording material on a back surface of the recording material, the number of recording materials held per rotation of the recording material carrier is 2 when forming an image on the back surface of the recording material. An image forming apparatus, wherein the number of recording sheets supplied to the recording material carrier is one per rotation only when two sheets cannot be supplied per rotation.