JPH11212308A - Both-side image forming device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a both-side image forming device constituted so that productivity one recording sheet is identical to that at a one-side recording time and images can be simultaneously recorded on both surfaces of the recording material without causing position slippage between both surfaces if the respective intermediate transfer belts of a first and a second image forming parts are asymmetrically laid. SOLUTION: The both-side image forming device is constituted so that the first and the second image forming parts are respectively provided with image carries 1 and 2 and the intermediate transfer belts 2 and 4 and the images T1 and T2 formed on the belt 2 and 4 are simultaneously transferred on both surfaces of the recording material 5. Even when a secondary intermediate transfer means 8 is asymmetrically constituted and the belts 2 and 4 must be asymmetrically laid, the carrying distances of the first image T1 and the second image T2 leading to the transfer means 8 are made to be equal by adjusting the forming start timing of the respective images in consideration of the carrying distances.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to an electrophotographic copying machine,
The present invention relates to an image forming apparatus such as a printer, and more particularly to an improvement in a double-sided image forming apparatus capable of recording an image on both sides of a recording material.

[0002]

2. Description of the Related Art Conventionally, as a double-sided image forming apparatus capable of forming a double-sided image, a first image formed on a latent image carrier such as a photoreceptor is transferred and fixed to a first surface of a sheet, and then the sheet is printed. In general, the sheet is reversed and re-fed, and subsequently, the second image formed on the latent image carrier is transferred and fixed to the second surface of the sheet. However, in this method, since the image forming unit is passed twice, the productivity per sheet becomes slower than 1/2 that of single-sided recording, and the sheet curls during the first fixing. There are various problems such as poor transfer at the second transfer and fixing, paper wrinkling, the occurrence of jams in the conveyance path, and noise when the paper is reversed and re-fed.

In order to solve such a technical problem,
The present applicant firstly primary-transfers a first image on a first photoconductor to a first intermediate transfer belt via a primary transfer device,
On the other hand, the second image on the second photosensitive member is primarily transferred to the second intermediate transfer belt via the primary transfer device, and two transfer rolls between which the intermediate transfer belts are sandwiched are used as the secondary transfer devices. There has already been proposed a technique in which the images on the respective intermediate transfer belts are simultaneously transferred to both sides of the sheet by using the secondary transfer roll (see Japanese Patent Application No. 8-108449).
According to this type, by using an intermediate transfer member such as an intermediate transfer belt, it is possible to increase the degree of freedom in the layout of the apparatus, for example, by arranging the final transfer position on paper close to the fixing device. In addition, the transfer roll has a certain degree of elasticity to absorb the sheet thickness. For this reason, it is possible to obtain a double-sided image with less image disturbance without lowering the recording speed per sheet than when recording on one side.

[0004]

In this type of double-sided image forming apparatus, a first image forming section (first photosensitive member and first intermediate transfer belt) and a second image forming section (second photosensitive member) are used. , And the second intermediate transfer belt) are symmetrically arranged. Therefore, if the image forming timing is simultaneously started for each of the first image and the second image, each image is formed on both sides of the sheet by the secondary transfer unit. Simultaneous transcription could have been easily realized. However, the first image forming unit and the second image forming unit are not always symmetrically arranged, and for example, the secondary transfer unit is configured to be asymmetric, and as a result,
When the first intermediate transfer belt and the second intermediate transfer belt are stretched asymmetrically on different tracks, the first image forming unit and the second image forming unit are necessarily arranged asymmetrically. It has to be in the form. In such a mode in which the first image forming unit and the second image forming unit are asymmetrically arranged, in particular, since the distance from the primary transfer unit to the secondary transfer unit in each intermediate transfer belt is different, If image formation is simultaneously started for each of the first image and the second image, the timings at which the first and second images conveyed on the respective intermediate transfer belts enter the secondary transfer unit do not match, and the secondary transfer is performed. A technical problem has been found in that when images are simultaneously transferred to both sides of a sheet at each section, the respective images on both sides of the sheet are misaligned.

SUMMARY OF THE INVENTION The present invention has been made to solve the above-mentioned technical problem. In an aspect in which each of the intermediate transfer belts of the first and second image forming sections is asymmetrically stretched, a recording material is provided for each recording material. The object of the present invention is to provide a double-sided image forming apparatus which has the same productivity as that of single-sided printing, and is capable of simultaneous printing on both sides of a recording material without displacement on both sides.

[0006]

That is, a first embodiment of the double-sided image forming apparatus according to the present invention is as shown in FIG.
A first image carrier 1 on which a first image T1 is formed and carried; a first intermediate transfer belt 2 disposed to face the first image carrier 1; and a first image T1 on the first image carrier 1 First primary transfer means 6 for primary transfer of the first image onto the first intermediate transfer belt 2, a second image carrier 3 on which a second image T2 is formed and carried, and an opposing arrangement on the second image carrier 3. Second intermediate transfer belt 4
A second intermediate primary transfer means 7 for primary-transferring the second image T2 on the second image carrier 3 to the second intermediate transfer belt 4;
In a region where the intermediate transfer belt 2 and the second intermediate transfer belt 4 are in contact with or close to each other, the two intermediate transfer belts 2 and 4
Secondary transfer means 8 having transfer members 8a and 8b opposed to each other across the intermediate transfer belt, and secondary transferring the primary transfer images T1 and T2 on the intermediate transfer belts 2 and 4 to both surfaces of the recording material 5. The first intermediate transfer belt 2 and the second intermediate transfer belt 4 are respectively stretched asymmetrically, and the first image T1 and the second image T1 are formed on both surfaces of the recording material 5 by the intermediate secondary transfer means 8. From the formation start position of the first image T1 and the second image T2 on each of the image carriers 1 and 3 to the secondary transfer position of the intermediate secondary transfer means 8 so that the second image T2 is simultaneously secondary-transferred. Each image carrier 1, depending on the total transport distance of each image T1, T2 up to
3 is characterized in that the formation start timing of each image T1, T2 is adjusted.

In a second embodiment of the double-sided image forming apparatus according to the present invention, as shown in FIG.
A first image carrier 1 to be carried, a first intermediate transfer belt 2 disposed opposite to the first image carrier 1, and a first image carrier 1
A first intermediate primary transfer means 6 for primarily transferring the upper first image T1 to the first intermediate transfer belt 2, a second image carrier 3 on which a second image T2 is formed and carried, and the second image carrier 3; A second intermediate transfer belt 4 opposed to the carrier 3, a second intermediate primary transfer unit 7 for primary-transferring the second image T2 on the second image carrier 3 to the second intermediate transfer belt 4, In a region where the first intermediate transfer belt 2 and the second intermediate transfer belt 4 are in contact with or close to each other, there are transfer members 8a and 8b that are arranged to face each other with the two intermediate transfer belts 2 and 4 therebetween. And an intermediate secondary transfer means 8 for secondary-transferring the primary transfer images T1, T2 on the intermediate transfer belts 2, 4, respectively, on the premise that the first intermediate transfer belt 2 and the second intermediate transfer The belts 4 are asymmetrically stretched respectively, and the first and second belts 4 are firstly applied to both surfaces of the recording material 5 by the intermediate secondary transfer means 8. Secondary transfer of the intermediate secondary transfer means 8 from the formation start position of the first image T1 and the second image T2 on each of the image carriers 1 and 3 so that the image T1 and the second image T2 are simultaneously secondary transferred. It is characterized in that the entire transport distance of each of the images T1 and T2 up to the position is made equal.

In such technical means, the first image carrier 1, the second image carrier 1 on which the first image T1 and the second image T2 are formed are formed.
The image carrier 3 may be of any form (drum, belt, or combination thereof) as long as it can carry a monochrome image or a color image. Various image forming methods such as an electrophotographic method, an electrostatic transfer method, and an ink jet method can be adopted as the image forming method for the two-image carrier 3.
As an embodiment in which the transfer of the recording material 5 to both sides is performed simultaneously, for example, in a type in which the first image T1 and the second image T2 are formed by an electrophotographic method, the first intermediate transfer belt 2 is used.
And the first and second members carried on the second intermediate transfer belt 4.
The images T1 and T2 need to have opposite polarities in the secondary transfer area. At this time, the first and second images T1 and T2 may be originally made of a material having opposite polarities, or the same image may be used to provide a polarity inversion means at an appropriate place to invert one of the images. You may. Further, the intermediate secondary transfer means 8 includes a pair of transfer members 8.
Any method may be used as long as a transfer bias is applied so that a predetermined transfer electric field is formed between a and 8b.

In the invention shown in FIG. 1 and FIG. 2, examples of the mode in which the first intermediate transfer belt 2 and the second intermediate transfer belt 4 are asymmetrically stretched are as follows. That is, a belt tension member 11 for extending the intermediate transfer belt 4 is provided on one of the transfer members of the intermediate secondary transfer means 8, for example, on both sides of the transfer member 8 b, and the intermediate tension member stretched on these belt tension members 11 is provided. This is a mode in which the surface of the transfer member 8b is positioned substantially on the track of the transfer belt 4 (see, for example, Japanese Patent Application No. 9-169615). According to this, in the secondary transfer step, the belt tension member 11 maintains the tensioned state of the intermediate transfer belt 4, so that the belt tension is not loosened due to deformation of the transfer member 8b and the like, and the intermediate transfer belt 4 and the recording material 5 can pass through the secondary transfer area at a constant speed. If the transfer member 8b has a low hardness, a wide transfer nip area can be secured while maintaining the tensioned state of the belt, and the pressure in the transfer nip area can be suppressed to a small value.

As a method for stabilizing the posture of the recording material 5 entering the fixing means and avoiding the occurrence of image disturbance and wrinkles, a bending force is applied to the recording material 5 in the secondary transfer area and before and after the secondary transfer area. (See Japanese Patent Application No. 9-36907). According to this, it is possible to bend the tip of the recording material 5 on which images are simultaneously transferred to both sides toward the fixing unit side, and to effectively use the waist (surface rigidity) of the tip of the recording material 5. Thus, the conveyance direction of the recording material 5 after the transfer can be stabilized. Therefore, it is possible to avoid the disturbance of the unfixed images on both sides of the recording material 5 due to the variation in the attitude of the recording material 5 entering the fixing unit.

Further, in the invention shown in FIG. 1, as a specific means for adjusting the timing of starting the formation of each image T1, T2 on each image carrier 1, 3, the intermediate transfer belts 2, 4 are asymmetrically stretched. The first image carrier from the first image formation start position by the first image pattern forming means 9 on the first image carrier 1 to the first intermediate primary transfer means 6 for the double-sided image forming apparatus. And the total distance Lt1 of the first image transport distance Ld1 and the first image transport distance Lb1 from the first intermediate primary transfer means 6 to the intermediate secondary transfer means 8, and the second image on the second image carrier 3. The second from the second image formation start position by the pattern forming means 10 to the second intermediate primary transfer means 7
The drive signal control unit 12 is controlled in accordance with the total distance Lt2 of the second image transport distance Ld2 on the image carrier 3 and the second image transport distance Lb2 from the second intermediate primary transfer unit 7 to the intermediate secondary transfer unit 8. , The formation start timings of the first image T1 and the second image T2 are separately set (in other words, the image formation timing is shifted by a predetermined time δ), and the images T1 and T2 are simultaneously positioned on both sides of the recording material 5. Recording without deviation.

On the other hand, according to the invention shown in FIG. 2, the first image pattern forming means 9 on the first image carrier 1 applies to the double-sided image forming apparatus in which the intermediate transfer belts 2 and 4 are asymmetrically stretched. First image transport distance L on first image carrier 1 from one image formation start position to first intermediate primary transfer means 6
d1 and the first intermediate primary transfer means 6 to the intermediate secondary transfer means 8
And the total distance Lt1 from the first image transport distance Lb1 to the first image transfer distance Lb1, and the second distance from the second image formation start position by the second image pattern forming means 10 on the second image carrier 3 to the second intermediate primary transfer means 7. Second image transport distance Ld on two image carrier 3
The total distance Lt2 between the second image transfer distance Lb2 and the second image transfer distance Lb2 from the second intermediate primary transfer means 7 to the intermediate secondary transfer means 8 is set to be equal, and the drive signal control unit 12 forms the first image pattern at the same timing. The first and second images T1 and T2, for which the forming operation by the means 9 and the second image pattern forming means 10 has been started, are simultaneously recorded on both surfaces of the recording material 5 without displacement.

In the invention shown in FIG. 1 or FIG. 2, from the viewpoint of facilitating the adjustment and the dimension setting, the first image carrier 1 and the second image carrier 3, and the first intermediate Setting the image transport speeds on the transfer belt 2 and the second intermediate transfer belt 4 equal, or
It is preferable that the circumferences of the first intermediate transfer belt 2 and the second intermediate transfer belt 4 are set equal, or that the first image carrier 1 and the second image carrier 3 are set equal.

Further, in the invention shown in FIG. 2, the distance from the first intermediate primary transfer means 6 to the intermediate secondary transfer means 8 on the conveyance path of the first intermediate transfer belt 2 and the distance of the second intermediate transfer belt 4 Second intermediate primary transfer means 7 on the transport trajectory
Or the distance from the image forming start position on the conveyance trajectory of the first image carrier 1 to the first intermediate primary transfer means 6 and the second intermediate transfer means 8 are set equal. It is preferable to set the distance from the image formation start position on the conveyance path of the image carrier 2 to the second intermediate primary transfer means 7 to be equal.

Next, the operation of the above technical means will be described. First, in the first mode shown in FIG. 1, since the transport distances of the first image T1 and the second image T2 are different, the image forming start timings are separately set according to the transport distances of the respective images T1 and T2. ing. Therefore, the first image T1 conveyed on the first intermediate transfer belt 2 and the second image T2 conveyed on the second intermediate transfer belt 4 correspond to an image transfer area on both surfaces of the recording material 5 in the intermediate secondary transfer section. And the two-sided simultaneous image transfer without the two-sided position shift is performed.

In the second embodiment shown in FIG.
Since the transport distances of the first image T1 and the second image T2 are equal, the sequence of the forming process of the first image T1 and the second image T2 can be performed substantially in the same manner. The respective image transfer areas on both sides of the recording material 5 exactly match. Therefore, it is possible to obtain a double-sided transfer image without a double-sided position shift without having to design an image forming sequence having completely different timings for the first image and the second image, or to adjust and control the mutual timing.

[0017]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described below in detail based on an embodiment shown in the accompanying drawings. First Embodiment FIG. 3 shows a schematic configuration of a first embodiment of a double-sided image forming apparatus to which the present invention is applied. In FIG. 1, the two-sided image forming apparatus includes a first image forming unit 20a that forms a first image on a first surface of a sheet P, and a second image forming unit that forms a second image on a second surface of the sheet P. 20b, and both image forming units 20a, 20
and a fixing device 50 for fixing the image on the sheet P that has passed through b. In the present embodiment, each image forming unit 20
Reference numerals a and 20b denote photosensitive drums 21a and 21b and a charging roll 22 for charging the surfaces of the photosensitive drums 21a and 21b.
a, 22b and an exposure device 23 for writing electrostatic latent images for the first and second images on the charged photosensitive drums 21a, 21b.
a, 23b and developing units 24a, 2 for visualizing the electrostatic latent images written on the photosensitive drums 21a, 21b with toner.
4b, intermediate transfer belts 25a and 25b arranged in contact with photosensitive drums 21a and 21b, and photosensitive drums 21a and 2b.
1 for primary transfer of the toner images T1 and T2 (for example, normal images in the present embodiment) on the intermediate transfer belts 25a and 25b.
Next transfer rolls 26a, 26b and photosensitive drums 21a, 2
Cleaners 27a and 27b for removing the residual toner on
The distance from the latent image writing position by the exposure device 23a in the rotation direction on the photosensitive drum 21a to the transfer position to the intermediate transfer belt 25a by the primary transfer roll 26a, and the exposure in the rotation direction on the photosensitive drum 21b The distance from the latent image writing position by the device 23b to the transfer position to the intermediate transfer belt 25b by the primary transfer roll 26b is equal. A polarity reversal corotron 28 is disposed downstream of the photosensitive drum 21b of the second image forming unit 20b at the developing position. In FIG. 3, reference numeral 100 denotes an exposure control device that sends an exposure drive signal to the exposure devices 23a and 23b at a predetermined timing, and details thereof will be described later.

In the present embodiment, the intermediate transfer belts 25a and 25b have the same circumference and an appropriate number of holding rolls including the secondary transfer rolls 40a and 40b (one is a driving roll and the other is a driven roll. ) And rotate in synchronization with the photosensitive drums 21a and 21b. In particular, in FIG. 3, the holding rolls 52a,
52b and 53a, 53b, secondary transfer rolls 40a, 4
0b and the primary transfer rolls 26a and 26b are arranged symmetrically (L1a = L1b), while the second transfer rolls 26a and 26b are arranged symmetrically.
On both sides of the secondary transfer roll 40b of the intermediate transfer belt 25b of the image forming unit 20b, tension rolls 51a and 51b as holding rolls for holding the intermediate transfer belt 25b in a state of being bridged (tension state) are substantially along the transport direction. These tension rolls 51a, 51b
Since the surface of the secondary transfer roll 40b is arranged on a substantially orbit of the intermediate transfer belt 25b stretched between the intermediate transfer belts 25a and 25b, the intermediate transfer belts 25a and 25b are stretched asymmetrically. The image transfer distance on the intermediate transfer belt from the next transfer section to the secondary transfer section is longer on the intermediate transfer belt 25b than on the intermediate transfer belt 25a. Reference numerals 30a and 30b denote intermediate transfer belt 2
It is a belt cleaner for removing residual toner on 5a and 25b.

The intermediate transfer belts 25a and 25b
Is a resin such as polyimide, acrylic, vinyl chloride, polyester, polycarbonate, polyethylene terephthalate (PET) or various rubbers containing an appropriate amount of an antistatic agent such as carbon black, and having a volume resistivity of 1
0 ⁹ is formed so as to be ~10 ¹⁴ Ω · cm, the thickness is set to 0.08mm for example.

Although the secondary transfer rolls 40a and 40b can be transferred by using both conductive ones, the first and second intermediate transfer belts 2 can be transferred to a small-sized paper.
When 5a and 25b come into direct contact, the secondary transfer roll 40
Since an excessive current flows between the transfer belts a and 40b, a sufficient transfer electric field cannot be formed and transfer failure occurs, and the intermediate transfer belts 25a and 25b are easily damaged. Coated with a semiconductive or insulating material is preferably used.

In this embodiment, the secondary transfer roll 40
For both a and 40b, a metal shaft coated with EPDM rubber dispersed with carbon black and having a volume resistivity of 10 ⁶ Ω · cm is used.
A transfer bias 41 was applied to the shaft a, and the shaft of the secondary transfer roll 40b was grounded. Secondary transfer roll 40
a is a rubber hardness of 70 degrees (Asuka C), a secondary transfer roll 40
b was set to a rubber hardness of 30 degrees (Asuka C). Here, in order to use the secondary transfer roll 40a also as a tension roll and prevent slippage of the intermediate transfer belt 25a, the rubber hardness (Asuka C) of the secondary transfer roll 40a is 50 degrees or more, preferably 70 degrees or more. It is desirable to make. It is desirable that the secondary transfer roll 40a has a rubber hardness (Asuka C) of 50 degrees or less, preferably 40 degrees or less in order to secure a transfer nip and prevent an increase in nip pressure. In addition, as the coating material, a material obtained by dispersing conductive particles (such as carbon black or aluminum) or an ion conductive material (such as LiClO ₄ ) in polyurethane rubber or silicon rubber can be used. It is preferably set to 10 ⁵ to 10 ⁹ Ω · cm. Further, the tension rolls 51a, 5
A metal roll was used for 1b.

In this embodiment, as shown in FIG. 4A, a transfer bias 41 is applied to one of the secondary transfer rolls 40a, and the other is applied to the secondary transfer portion. Although a configuration in which the transfer roll 40b is grounded is adopted, the invention is not limited to this. For example, as shown in FIG. 4B, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are sandwiched. Alternatively, transfer biases 41 (specifically, 41a and 41b) having different polarities may be applied to the pair of secondary transfer rolls 40a and 40b disposed to face each other. At this time, as shown in FIGS. 4A and 4B, the bias application method to the secondary transfer portion is 2
The transfer bias 41 may be directly applied to the next transfer roll 40a (or 40b). However, as shown in FIG. 4C, in order to apply a uniform transfer bias 41 to the entire area of the secondary transfer roll 40a. Alternatively, an intermediate member such as a contact roll 48 having a uniform surface may be disposed in contact with the secondary transfer roll 40a, and the transfer bias 41 may be applied via the intermediate member.

Further, in the present embodiment, the toner T
1, T2 uses a negatively charged toner, and primary transfer rolls 26a,
DC current +10 μA and DC voltage +300 μA and a grid voltage 5
00V and a DC voltage of -2 kV to the secondary transfer roll 40a.
Was applied.

The distance from the secondary transfer position to the fixing device 50 is shorter than the minimum sheet length, and the rotation speed of the fixing roll is set to be equal to or slightly lower than the speed of the intermediate transfer belts 25a and 25b. Further, the fixing device 50 is provided on a substantially extended line in the sheet discharging direction at the exit of the secondary transfer nip. Further, the fixing device 50
The upper and lower fixing rolls had the same shape so that the fixing nip was linear, and heaters were disposed inside the fixing rolls.
In FIG. 3, reference numeral 31 denotes a paper tray, 32 denotes a transport roll for transporting the paper P, and 33 denotes a paper guide plate.

Next, an image forming process of the double-sided image forming apparatus according to the present embodiment will be described. First photosensitive drum 2
The first toner image (first image) T1 formed on 1a is transferred by a primary transfer roll 26a onto a first intermediate transfer belt 25a moving at the same speed as the first photosensitive drum 21a. Similarly, the second photosensitive drum 21b is formed on the second photosensitive drum 21b.
The polarity of the toner image (second image) T2 is inverted by applying a voltage to the polarity inversion corotron 28, and then transferred onto the second intermediate transfer belt 25b by the primary transfer roll 26b.
Then, the sheet P is conveyed from the sheet tray 31 to the secondary transfer rolls 40a and 40b at the same time, and the toner images T1 and T2 on the intermediate transfer belts 25a and 25b are transferred to the sheet P.
Are simultaneously transferred to both sides of the sheet, and then fixed on both sides simultaneously by the fixing device 50.

FIG. 5 shows the image forming timing of the first image T1 and the second image T2 by the image forming control system used in the present embodiment. That is, in the present embodiment, the image forming control system includes the exposure control device 100 and the bias power control device 10.
And the exposure control device 100 includes a first image forming unit 20
a, the writing start timing of the electrostatic latent image of the first image T1 on the first photosensitive drum 21a whose surface is charged by the charging roll 22a by the exposure device 23a, and the surface of the first image T1 by the charging roll 22b in the second image forming unit 20b. The writing start timing of the electrostatic latent image of the second image T2 on the charged second photosensitive drum 21b by the exposure device 23b is set in accordance with each image transport distance, and the bias power control device 101 includes a developing device. 24a, 2
4b, the timing at which the first toner image T1 and the second toner image T2 visualized respectively by the primary transfer are primary-transferred to the first intermediate transfer belt 25a and the second intermediate transfer belt 25b, respectively, is set according to each image transport distance. Then, both toner images are conveyed to the intermediate secondary transfer portion at the same time.

More specifically, in the double-sided image forming apparatus shown in FIG. 3, an exposure device 23a for forming an electrostatic latent image on a first photosensitive drum 21a is moved from an intermediate transfer belt 25a.
The image transfer distance LD1 along the peripheral surface of the first photosensitive drum 21a to the nip between the primary transfer portion for transferring the first toner image T1 to the nip portion between the primary transfer roll 26a and the first photosensitive drum 21a is the second. From the exposure device 23b for forming an electrostatic latent image on the photosensitive drum 21b to the primary transfer portion for transferring the second toner image T2 to the intermediate transfer belt 25b, that is, the nip portion between the primary transfer roll 26b and the second photosensitive drum 21b. Second
The image transport distance LD2 along the peripheral surface of the photosensitive drum 21b is equal to the image transport distance LD2. However, the image transfer distance LB1 on the intermediate transfer belt 25a from the primary transfer roll 26a to the secondary transfer unit 40 (specifically, the secondary transfer rolls 40a and 40b), and from the primary transfer roll to the secondary transfer unit 40 Transfer belt 25 up to
Comparing with the image transfer distance LB2 on b, the intermediate transfer belts 25a and 25b have the same total circumference, but the tension rolls 51a and 51b are disposed on both sides of the secondary transfer roll 40b. The first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched, and as a result, LB2 is longer than LB1.

Therefore, the first and second photosensitive drums 21a, 21a,
21b, and the first and second intermediate transfer belts 2
Assuming that the transport speed of the photosensitive drums 5a and 25b is PS, the timing of forming the second electrostatic latent image (the electrostatic latent image of the second image T2) on the second photosensitive drum 21b is changed to the first electrostatic latent image on the first photosensitive drum 21a. It is necessary to start earlier by | (LB1-LB2) / PS | than that of the latent image (the electrostatic latent image of the first image T1). In addition, regarding the bias application timing by the bias power supply control device 101, the application timing to the second image T2 is set to be | (LB1-LB) more than that of the first image T2.
2) It is necessary to start earlier by / PS |.

As described above, since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are stretched asymmetrically. In the double-sided image forming apparatus having the above-described configuration, the start of the formation of the electrostatic latent image of the first image T1 on the first photosensitive drum 21a and the start of the formation of the electrostatic latent image of the second image T2 on the second photosensitive drum 21b are started. By setting the timing in consideration of the difference between the image transport distances of the first image T1 and the second image T2 caused by the asymmetric layout, the two-sided position is simultaneously shifted in the image transfer area on both sides of the sheet P in the secondary transfer unit. The first image T1 and the second image T2 can be transferred accurately without any trouble, and are conveyed to the fixing device 50 as they are and are simultaneously fixed on both sides, so that a double-sided image without misalignment on both sides is the same as one side per sheet. Record It could be obtained in degrees.

In the case of single-sided recording, for example, by applying a bias having a polarity opposite to that of a normal transfer bias to the primary transfer roll 26a or 26b on which no recording is performed, it is possible to prevent back surface contamination. In addition, a method of switching the developing bias or a method of separating the photosensitive drums 21a and 21b from the intermediate transfer belts 25a and 25b may be used.

In the present embodiment, the secondary transfer roll 40b
Tension rolls 5 on the tension members arranged on both sides of
Although 1a and 51b are used, a metal plate or the like may be appropriately used as long as it can hold the tension of the intermediate transfer belt 25b and does not hinder the rotation. Further, in this embodiment, the polarity of the toner image T2 is inverted on the second intermediate transfer belt 25b by using the toner having the same charging polarity, but may be inverted on the photosensitive drum 21b. Different toners may be used. Further, in the present embodiment, in order to control the posture of the sheet P on which the unfixed double-sided toner image is transferred when the sheet P is transported from the secondary transfer nip to the fixing device 50, for example, an auxiliary device such as a star wheel or an air flow is used. Means may be provided.

Further, in the present embodiment, the holding roll 52
a, 52b and 53a, 53b, secondary transfer roll 40
Although the configuration in which the arrangement of the primary transfer rolls 26a and 26b is symmetric has been described, the present invention is not limited to the above case, and the arrangement of the above-described roll groups is asymmetric. The first and second intermediate transfer belts 25a and 25b can be applied asymmetrically. Similarly, in the present embodiment, a primary transfer unit that transfers the first toner image T1 from the exposure device 23a that forms an electrostatic latent image on the first photosensitive drum 21a to the intermediate transfer belt 25a, that is, a primary transfer Image transport distance L along the peripheral surface of first photosensitive drum 21a up to the nip between roll 26a and first photosensitive drum 21a
D1 and a primary transfer unit or primary transfer roll 26 for transferring the second toner image T2 from the exposure device 23b for forming an electrostatic latent image on the second photosensitive drum 21b to the intermediate transfer belt 25b.
b and the image transport distance LD2 along the peripheral surface of the second photosensitive drum 21b up to the nip portion of the second photosensitive drum 21b has been described.
However, the present invention is not limited to this, and in a configuration where LD1 ≠ LD2, the image forming start time in the image forming section having a short image carrying distance is | (LD1−LD2) / PD | (where PD: photosensitive drums 21a and 21b). The peripheral speed),
A two-sided image without a two-sided position shift can be obtained, and the present invention is also applicable to the following embodiments.

Second Embodiment FIG. 6 shows a schematic configuration of a two-sided image forming apparatus according to a second embodiment of the present invention. In the figure, the configuration of the double-sided image forming apparatus is similar to that of the first embodiment, since tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b. Transfer belt 2
5a and the second intermediate transfer belt 25b are asymmetrically stretched. However, unlike the first embodiment, the entire transport distance of the first image T1 to the secondary transfer portion 40 and the second image T2
The total transport distance to the next transfer section 40 is set to be equal. It should be noted that, among the components of the present embodiment, components similar to those of the first embodiment are substantially the same as those of the first embodiment, and therefore are denoted by the same reference numerals, and detailed description thereof is omitted here. I do. This is the same for the following embodiments.

More specifically, in the present embodiment, the first primary transfer section (primary transfer roll) 26a
The image transfer distance LB1 on the intermediate transfer belt 25a to the next transfer unit 40 (secondary transfer rolls 40a, 40b) and the intermediate transfer from the second primary transfer unit (primary transfer roll 26b) to the secondary transfer unit 40 The holding rolls 52a, 52b and 53a, 5b are set so that the image transfer distance LB2 on the belt 25b is equalized.
3b, secondary transfer rolls 40a and 40b, and primary transfer rolls 26a and 26b are provided. At the same time, the first
Exposure device 23 for forming an electrostatic latent image on photosensitive drum 21a
a from the image transfer distance LD1 along the peripheral surface of the first photosensitive drum 21a to the primary transfer portion 26a for transferring the first toner image T1 to the intermediate transfer belt 25a, and the second photosensitive drum 21b.
A primary transfer section 2 for transferring the second toner image T2 from the exposure device 23b for forming an electrostatic latent image thereon to the intermediate transfer belt 25b
The image transport distance LD2 along the peripheral surface of the second photosensitive drum 21b up to 6b is equal. Therefore, in the present embodiment, LD1 = LD2, LB1 = LB2, and 2 of the first image T1
The total transport distance (LD1 + LB1) to the next transfer unit 40 and the second
The total transport distance of the image T2 to the secondary transfer section 40 (LD2 + LB
2) are equidistant.

That is, in the first embodiment shown in FIG. 3, the holding rolls 52a, 52b and 53, 53b, the secondary transfer rolls 40a, 40b, and the primary transfer roll 26a,
26b are symmetrically disposed (L1a = L1b), however, in the present embodiment, the position of the primary transfer section 26b for transferring the second image T2 to the second intermediate transfer belt 25b. Closer to the holding roll 52b side (L1a
> L1b), the first primary transfer portion 26a
The image transfer distance LB1 on the intermediate transfer belt 25a from the first transfer unit 40 to the secondary transfer unit 40 is equal to the image transfer distance LB2 on the intermediate transfer belt 25b from the second primary transfer unit 26b to the secondary transfer unit 40. Is set.

Next, the image forming process of the two-sided image forming apparatus according to the present embodiment will be described. The image forming process is performed in substantially the same manner as in the first embodiment, but the image forming timing of the first image T1 and the second image T2 is changed. Different from the first embodiment. FIG. 7 shows the image forming timing of the first image T1 and the second image T2 by the image forming control system used in the present embodiment. In the figure, an exposure control device 100 controls a first photosensitive drum 21 whose surface is charged by a charging roll 22a in a first image forming unit 20a.
Exposure device 23a for electrostatic latent image of first image T1 on a
And the writing start timing of the electrostatic latent image of the second image T2 on the second photosensitive drum 21b, the surface of which is charged by the charging roll 22b in the second image forming unit 20b, by the exposure device 23b. And the bias power control device 10
Reference numeral 1 denotes a timing at which the first image T1 and the second image T2 visualized by the developing units 24a and 24b, respectively, are first-transferred to the first intermediate transfer belt 25a and the second intermediate transfer belt 25b, respectively. The distance is set according to the distance, and both images are conveyed to the intermediate secondary transfer portion at the same time.

More specifically, in the present embodiment, a primary transfer section for transferring the first image T1 from the exposure device 23a for forming an electrostatic latent image on the first photosensitive drum 21a to the intermediate transfer belt 25a. That is, an image transport distance LD1 along the peripheral surface of the first photosensitive drum 21a to a nip portion between the primary transfer roll 26a and the first photosensitive drum 21a, and an exposure for forming an electrostatic latent image on the second photosensitive drum 21b. A primary transfer unit that transfers the second image T2 from the device 23b to the intermediate transfer belt 25b, that is, a primary transfer roll 26b and the second photosensitive drum 2
The image transport distance LD2 along the peripheral surface of the second photosensitive drum 21b up to the nip portion with the nip portion 1b is equal. Further, the intermediate transfer belt 25a from the primary transfer section 26a to the secondary transfer section 40
And the image transfer distance LB2 on the intermediate transfer belt 25b from the primary transfer section 26b to the secondary transfer section 40.
Also, tension rolls 51a and 51b are disposed on both sides of the secondary transfer roll 40b, and the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are stretched asymmetrically, but the holding rolls 52b and The position of the primary transfer from the second photosensitive drum 21b to the second intermediate transfer belt 25b, that is, the position of the primary transfer roll 26b, between the second photosensitive drum 21b and the second intermediate transfer belt 25b, is brought closer to the holding roll 52b.
Set to B2. Accordingly, the formation of the electrostatic latent image of the first image T1 on the first photosensitive drum 21a and the formation of the second photosensitive drum 21
b, the formation of the electrostatic latent image of the second image T2, the primary transfer from the first photosensitive drum 21a to the intermediate transfer belt 25a, and the primary transfer from the second photosensitive drum 21b to the intermediate transfer belt 25b are the same. Executed at the timing.

As described above, since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are stretched asymmetrically. Although the double-sided image forming apparatus having the above-described configuration is used, the total transport distance (LD1 + LB1) from the latent image writing start position of the first image T1 on the first photosensitive drum 21a to the secondary transfer unit 40, and the second image T2 of the second image T2 Photosensitive drum 2
By setting the total transport distance (LD2 + LB2) from the latent image writing start position on the first transfer drum 1b to the secondary transfer unit 40 to be equal, the start of the formation of the electrostatic latent image of the first image T1 on the first photosensitive drum 21a and the If the timing of starting the formation of the electrostatic latent image of the second image T2 on the second photosensitive drum 21b is made simultaneous, the first image T1 can be accurately and simultaneously transferred to the image transfer area on both sides of the sheet P in the secondary transfer unit 40 without any displacement. And the second image T2 can be transferred, and can be transferred to the fixing device 50 as it is and simultaneously fixed on both sides, so that a double-sided image with no misalignment on both sides can be obtained at the same recording speed as one side per sheet. Was.

In the present embodiment, the first image T 1 and the second image T 2 are transferred from the start of forming the electrostatic latent image of each image to the first and second surfaces of the sheet P simultaneously. Since the operation sequence up to this point can be made substantially the same and the execution of each operation can be controlled by the same signal,
Since there is no need to prepare an independent control sequence for each of the two image forming units 20a and 20b, it is possible to make the configuration of the entire image forming control system substantially the same as that of the single-sided image forming mode.

Third Embodiment FIG. 8 shows a schematic configuration of a double-sided image forming apparatus according to a third embodiment of the present invention. In the present embodiment, the first image forming unit 20a and the second image forming unit 2 of the duplex image forming apparatus
0b has substantially the same configuration as that of the first embodiment, so that an exposure device 23a for writing a latent image on the first photosensitive drum 21a and an exposure device 23b for writing a latent image on the second photosensitive drum 21b
The devices around the photosensitive drums 21a and 21b except for the above are omitted. In FIG. 8, the basic configuration of this double-sided image forming apparatus is substantially the same as that of the first embodiment, but is different from the first embodiment in that the intermediate transfer belts 25a and 25b are arranged in an area where the intermediate transfer belts 25a and 25b come into contact with or approach each other. Next transfer roll 40a, 4
0b are also configured to also function as tension rolls, and apply a bending force to the sheet P before and after the secondary transfer area to bend and deform the leading end toward the fixing device 50, thereby forming a surface of the sheet itself. The transport direction can be stabilized using the rigidity.

In particular, in the present embodiment, the secondary transfer rolls 40a and 40b are arranged such that a straight line L connecting the central axes thereof is non-orthogonal to the sheet P intrusion direction as shown in FIG. The first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched. In FIG. 8, the transfer of the belts from the primary transfer section to the secondary transfer section of the intermediate transfer belts 25a and 25b to which the first image T1 and the second image T2 are respectively conveyed is performed by holding rolls 52a and 52b, 53a and 53b are symmetrical, and the positions of the primary transfer rolls 26a and 26b are also symmetrical.
Secondary transfer rolls 40a, 40 also serving as tension rolls
b are arranged to be shifted back and forth with respect to the sheet traveling direction. Accordingly, as shown by the arrow m in FIG. 9, the distance at which the intermediate transfer belt 25b contacts the peripheral surface of the secondary transfer roll 40b near the entrance side of the secondary transfer portion is set to be equal to the intermediate transfer belt 25.
As a result, the transport distance of the first image T1 and the second image T2 on the intermediate transfer belts 25a and 25b is slightly longer for the second image.

In the present embodiment, the secondary transfer roll 40
Both a and 40b disperse carbon black in EPDM rubber with respect to the metal shaft and have a volume resistivity of 10 ⁶ Ω.
-Using a roll coated with a cm, applying a transfer bias 41 to the shaft of the secondary transfer roll 40a,
The shaft of the secondary transfer roll 40b was grounded. In addition, as the coating material, a material obtained by dispersing conductive particles (such as carbon black or aluminum) or an ion conductive material (such as LiClO ₄ ) in polyurethane rubber or silicon rubber can be used. It is preferably set to 10 ⁵ to 10 ⁹ Ω · cm.

Next, an image forming process of the double-sided image forming apparatus according to the present embodiment will be described. First photosensitive drum 2
The first toner image T1 formed on 1a is transferred by a primary transfer roll 26a onto a first intermediate transfer belt 25a moving at substantially the same speed as the first photosensitive drum 21a. Similarly,
Second toner image T formed on second photosensitive drum 21b
After applying the voltage to the polarity reversing corotron 28 to reverse the polarity of the second polarity, the second polarity is transferred onto the second intermediate transfer belt 25b by the primary transfer roll 26b. Then, the paper P is transferred from the paper tray 31 to the secondary transfer roll 40 at the same timing.
a, 40b, and transferred to the intermediate transfer belts 25a, 25b.
After the upper toner images T1 and T2 are simultaneously transferred to both sides of the sheet P, the sheet P is bent by the bending force applied at the transfer nip.
Are fixed at the same time on both sides by the fixing device 50 by stabilizing the front end posture of the fixing device.

At this time, the image forming control system (the exposure control device 10
0, the bias power supply control device 101) operates in substantially the same manner as in the first embodiment, and the first image T1 and the second image T2 are simultaneously transported to the secondary transfer unit 40.
1. The drive timing of the exposure devices 23a and 23b and the primary transfer timing by the primary transfer units 26a and 26b are controlled in accordance with the difference in the transport distance of the second image T2 to the secondary transfer unit 40, respectively. Therefore, each intermediate transfer belt 25a,
The first images T1 and T2 on 25b are simultaneously carried into the transfer nip of the secondary transfer unit 40, and are simultaneously transferred to both sides of the sheet P.

As described above, the secondary transfer rolls 40a, 40a
0b with respect to the traveling direction of the sheet P, the images on the intermediate transfer belts 25a and 25b are simultaneously transferred to both sides of the sheet P while applying a bending force to the sheet P. Although the intermediate transfer belts 25a and 25b are stretched asymmetrically in substantially the same manner as in the first embodiment, the start of the formation of the electrostatic latent image of the first image T1 on the first photosensitive drum 21a and the second Second image T on photosensitive drum 21b
2 is set in consideration of the difference between the image transport distances of the first image T1 and the second image T2 caused by the asymmetric layout, so that the secondary transfer unit 4
0, the first image T1 and the second image T2 can be accurately and simultaneously transferred to the image transfer areas on both sides of the sheet P without any misalignment. A two-sided image without any deviation could be obtained at the same recording speed as that for one side per sheet.

Fourth Embodiment FIG. 10 shows a schematic configuration of a four-sided image forming apparatus according to a fourth embodiment of the present invention. In FIG. 10, the basic configuration and the image forming process of this double-sided image forming apparatus are substantially the same as those of the third embodiment, and a secondary transfer roll 40a is disposed in an area where the intermediate transfer belts 25a and 25b are in contact with or close to each other. , 40b also serve as tension rolls, and apply a bending force to the sheet P before and after the secondary transfer area to bend the tip of the sheet P toward the fixing device 50 to deform the sheet itself. Although the transport direction can be stabilized by using the surface rigidity, the secondary transfer rolls 40a, 40b
Since the straight line connecting the central axes is arranged non-orthogonally with respect to the direction in which the sheet P enters, the first intermediate transfer belt 25a
The second intermediate transfer belt 25b is asymmetrically stretched. Note that components similar to those of the third embodiment are denoted by the same reference numerals as those of the third embodiment, and a detailed description thereof will be omitted.

However, in this embodiment,
Unlike the third embodiment, the first on the intermediate transfer belt 25a
L from the primary transfer section 26a to the holding roll 52a
1a and the second primary transfer section 26 on the intermediate transfer belt 25b
The primary transfer position is changed so as to cancel the difference in transport distance in the third embodiment with respect to the distance L1b from b to the holding roll 52b.
a to the secondary transfer unit 40 and the second primary transfer unit 2
The image transfer distance is set to be equal to the image transfer distance from 6b to the secondary transfer unit 40.

According to the present embodiment, the secondary transfer roll 4
Since the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are arranged asymmetrically, the double-sided image has a configuration in which the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are arranged so as to be shifted back and forth with respect to the traveling direction of the sheet P. Although it is a forming apparatus, the image transfer distance LB1 on the first intermediate transfer belt 25a from the first primary transfer section 26a to the secondary transfer section 40, and the secondary transfer from the second primary transfer section 26b. An exposure device 23a for forming an electrostatic latent image on the first photosensitive drum 21a with the same image transport distance LB2 on the second intermediate transfer belt 25b up to the unit 40;
From the image transfer distance LD1 along the peripheral surface of the first photosensitive drum 21a to the primary transfer portion 26a for transferring the first toner image T1 to the first intermediate transfer belt 25a;
b. Image transport distance along the peripheral surface of the second photosensitive drum 21b from the exposure device 23b for forming an electrostatic latent image on the second intermediate transfer belt 25b to the primary transfer portion 26b for transferring the second toner image T2 to the second intermediate transfer belt 25b. With the same configuration for LD2, the entire transport distance (LD1 + LB1) of the first image T1 to the secondary transfer unit 40 is obtained.
And the total transport distance (LD) of the second image T2 to the secondary transfer section 40.
2 + LB2), the first photosensitive drum 21
If the formation of the electrostatic latent image of the first image T1 and the formation of the electrostatic latent image of the second image T2 on the second photosensitive drum 21b are simultaneously started, the images on both sides of the sheet P in the secondary transfer unit 40 The first image T1 and the second image T2 can be simultaneously and accurately transferred to the transfer area without displacement of both surfaces.
, And fixed simultaneously on both sides, it was possible to obtain a double-sided image without misalignment on both sides at the same recording speed as on one side per sheet.

In the present embodiment, the first image T 1 and the second image T 2 are transferred from the start of forming the electrostatic latent image of each image to the first and second surfaces of the sheet P simultaneously. Since the operation sequence up to this point can be made substantially the same and the execution of each operation can be controlled by the same signal,
Since it is not necessary to prepare an independent control sequence for each of the two image forming units 20a and 20b, the configuration of the entire control device can be made substantially the same as that of the single-sided image forming device.

Fifth Embodiment FIG. 11 shows a schematic configuration of a double-sided image forming apparatus according to a fifth embodiment of the present invention. In the figure, the basic configuration of the double-sided image forming apparatus is substantially the same as that of the first embodiment, and tension rolls 51a and 51 are provided on both sides of a secondary transfer roll 40b.
b, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched.
As a result, the first primary transfer section 26a to the secondary transfer section 40
(Image transfer distance L to (secondary transfer rolls 40a, 40b))
The image transport distance LB2 from the second primary transfer section 26b to the secondary transfer section 40 is longer than B1 (LB2> LB1). However, the double-sided image forming apparatus according to the present embodiment
Unlike the first embodiment, the rotary developing devices 34a and 34b for full color (in this embodiment, yellow (Y), magenta (M), cyan (C), and black (Bk))
In the secondary transfer section 40, a contact roll 48 is brought into contact with the secondary transfer roll 40b.
To apply the transfer bias 41 via
A mechanism is provided between the secondary transfer rolls 40a and 40b so as to be able to contact and separate from the secondary transfer rolls 40a and 40b, so that a full-color image can be formed by sequentially transferring each color image onto the intermediate transfer belts 25a and 25b.

In this embodiment, the secondary transfer roll 40b
Has a metal shaft coated with an insulating EPDM rubber, and the surface thereof coated with a thin layer of a conductive EPDM rubber.
0 ⁹ Ω / cm ² and the contact roll 48 was a metal shaft. On the other hand, the secondary transfer roll 40a used was a metal shaft coated with EPDM rubber dispersed with carbon black and having a volume resistivity of 10 ⁵ Ω · cm. Various rubbers and resins having a volume resistivity of 10 ¹¹ Ω · cm or more can be used for the insulating layer.
A material in which conductive particles such as carbon black are dispersed in VdF, polyester, acrylic, or the like can be used, and the surface resistance is preferably set to ^{10 8} to 10 ¹⁰ Ω / cm ² .
The rubber hardness (Asuka C) for coating the secondary transfer rolls 40a and 40b was 30 degrees and 70 degrees, respectively, and metal rolls were used for the tension rolls 51a and 51b.

The mechanism 60 for moving the secondary transfer rolls 40a and 40b toward and away from each other will be described with reference to FIG. In the present embodiment, a method in which the secondary transfer roll 40b is fixed and the secondary transfer roll 40a moves is used as the contact / separation mechanism 60. Specifically, the contact / separation mechanism 60 holds the secondary transfer roll 40 a with the lever 62 around the fulcrum 61 and pressurizes with the spring 63, while pressing the lever 65 connected to the lever 62 with the fulcrum 64. By moving the secondary transfer roll 40a, the secondary transfer roll 40a is moved toward and away from the secondary transfer rolls 40a and 40b.

Further, in the present embodiment, the toner T
1, T2 uses a negatively charged toner, and primary transfer rolls 26a,
26b, DC current +10 μm for each transfer of YMCK
A, −10 μA, a DC current of +300 μA, a grid voltage of +500 V to the polarity inversion corotron 28, and a DC voltage of +3 kV to the contact roll 48 in contact with the secondary transfer roll 40 b. The secondary transfer roll 40a was grounded. Further, the circumferences of the intermediate transfer belts 25a and 25b are equal and twice the circumference of the photosensitive drums 21a and 21b, but may be an integral multiple of the circumference of the photosensitive drums 21a and 21b due to color misregistration. desirable.

Next, an image forming process of the double-sided image forming apparatus according to the present embodiment will be described. Secondary transfer roll 4
0a, 40b, the first photosensitive drum 21
a first toner image T1 formed in the order of YMCK
Is transferred to the first intermediate transfer belt 25 by the primary transfer roll 26a.
are sequentially transferred one by one for one rotation. Similarly, the polarity of the toner image T2 formed on the second photosensitive drum 21b in the order of YMCK is inverted by applying a voltage to the polarity inversion corotron 28, and then the primary transfer roll 26b , The image is sequentially transferred onto the second intermediate transfer belt 25b. After the cyan of the third color transferred onto the intermediate transfer belts 25a and 25b passes through the secondary transfer unit 40, the secondary transfer rolls 40a and 40b are brought into contact with each other, and the paper P is conveyed at a proper timing to perform the intermediate transfer. Belts 25a, 25b
After the upper toner images T1 and T2 are simultaneously transferred onto the paper P,
The fixing device 50 fixes both surfaces simultaneously.

FIG. 13 shows the image forming timing of the first image T1 and the second image T2 of the double-sided image forming apparatus according to the present embodiment by the image forming control system. In the present embodiment, the first
The image forming process of each color of the image T1 and the second image T2 is substantially the same as in the first embodiment, and the exposure control device 100
The image forming unit 20a starts writing the electrostatic latent image of each color of the first image T1 on the first photosensitive drum 21a by the exposure device 23a, and the second image forming unit 20b writes the second image T2 on the second photosensitive drum 21b. The timing of the start of writing of the electrostatic latent image of each color by the exposure device 23b is set in accordance with the transport distance until each color component image of the first image T1 and the second image T2 reaches the secondary transfer unit 40, , The bias power supply controller 101 converts the first color image of the first image T1 and the color image of the second image T2 visualized by the rotary developing devices 34a and 34b that are sequentially switched to the first intermediate transfer belt 25a and the second intermediate transfer belt 25b. 1 each
The timing of the next transfer is set according to the transport distance until each color component image of the first image T1 and the second image T2 reaches the secondary transfer unit 40, and the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are set. After a full-color image is formed thereon (a color image is completed), the two images T1 and T2 are transferred to the secondary transfer unit 40.
Are simultaneously carried into the transfer nip, and are transferred to the image forming areas on both sides of the sheet P without displacement. Note that FIG.
Medium, the formation timing of each color component image of the second image T2 and the bias application timing are a predetermined time | (LB1−LB2) / PS | in comparison with those of each color component image of the first image T1.
(LB1: the image transport distance on the intermediate transfer belt 25a from the primary transfer section 26a to the secondary transfer section 40, LB2: 1
Image transport distance on intermediate transfer belt 25b from next transfer section 26b to secondary transfer section 40, PS: photosensitive drum 2
1a, 21b, peripheral speed of intermediate transfer belts 25a, 25b)
Only set early.

As described above, since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are stretched asymmetrically. Is a YMCK color double-sided image forming apparatus, but the first photosensitive drum 21a
The start of the formation of the electrostatic latent images of the respective colors of the first image T1 above and the second
The timing of the start of the formation of the electrostatic latent image of each color of the second image T2 on the photosensitive drum 21b is determined in consideration of the difference between the image transport distances of the first image T1 and the second image T2 caused by the asymmetric belt layout. By setting each of them, the first image T1 and the second image T2 of the YMCK color can be accurately and simultaneously placed on the image transfer areas on both sides of the sheet P in the secondary transfer unit 40 without any displacement.
Was transferred to the fixing device 50 and fixed on both sides simultaneously, whereby a double-sided image with no misalignment on both sides could be obtained at the same recording speed as one side per sheet.

Sixth Embodiment FIG. 14 shows a schematic structure of a double-sided image forming apparatus according to a sixth embodiment of the present invention. In the figure, the basic configuration and the image forming process of the double-sided image forming apparatus are substantially the same as those of the fifth embodiment, and since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, the first intermediate Although the transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched, unlike the fifth embodiment, the intermediate transfer belt 25a from the first primary transfer portion 26a to the secondary transfer portion 40 is different from that of the fifth embodiment. The holding rolls 52a, 52b and 53a are arranged such that the upper image transport distance LB1 and the image transport distance LB2 on the intermediate transfer belt 25b from the second primary transfer section 26b to the secondary transfer section 40 are equal. , 53b, and secondary transfer rolls 40a, 40b and primary transfer rolls 26a, 26b. In addition, rotary developing devices 34a and 34b for full color (yellow (Y), magenta (M), cyan (C), and black (Bk) in this embodiment).
Are also set equal to each other on the photosensitive drums 21a and 21b where the first image T1 and the second image T2 formed in full color are transported to the secondary transfer unit 40. Are equal.

FIG. 15 shows the image forming timing of the first image T1 and the second image T2 by the image forming control system of the double-sided image forming apparatus according to the present embodiment. At this time, the first image forming unit 20
The process of forming each color of the first image T1 and the second image T2 in the second image forming unit 20b is substantially the same as that in the fifth embodiment, but in the present embodiment, the secondary transfer unit 40
Since the transport distances of the respective color component images of the first image T1 and the second image T2 which reach the image forming apparatus are equal, the image forming control system (the exposure control apparatus 100,
The bias power supply control device 101) includes the first image forming unit 20.
a, a first image T on the first photosensitive drum 21a
The writing of the electrostatic latent image of each color by the exposure device 23a and the second photosensitive drum 2 in the second image forming unit 20b.
At the same time as the start of writing of the electrostatic latent image of each color of the second image T2 on the exposure image 23b by the exposure device 23b, the rotary developing devices 34a,
The timing at which the first image T1 and the second image T2, which are visualized by 34b, respectively, are primarily transferred onto the first intermediate transfer belt 25a and the second intermediate transfer belt 25b, respectively, is simultaneously executed. Then, after a full-color image is formed on the first intermediate transfer belt 25a and the second intermediate transfer belt 25b (color image is completed), the two images are simultaneously transported to the intermediate secondary transfer unit.

As described above, since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, as a result, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched. Y of the installed configuration
Although the MCK full-color double-sided image forming apparatus is used, the first photosensitive drum 21
a, the start of the formation of the electrostatic latent images of the respective colors of the first image T1 on the second photosensitive drum 21b and the start of the formation of the electrostatic latent images of the respective colors of the second image T2 on the second photosensitive drum 21b are not set separately. By the substantially same control sequence, the first image T1 and the second image T2 of the YMCK color can be transferred to the image transfer areas on both sides of the sheet P simultaneously and accurately in the secondary transfer unit 40, and the fixing unit By transporting the sheet to the sheet 50 and performing simultaneous fixing on both sides, a double-sided image with no misalignment on both sides could be obtained per sheet at the same recording speed as that on one side.

Seventh Embodiment FIG. 16 shows a schematic configuration of a double-sided image forming apparatus according to a seventh embodiment of the present invention. In the figure, the basic configuration of the double-sided image forming apparatus is substantially the same as that of the fifth embodiment. YMCK
A first photosensitive drum group 21a (specifically, 21aY, 21aM, 21aC, 21aK) for forming a toner image and a second photosensitive drum group 21b (specifically, 21bY, 21b)
M, 21bC, 21bK) and each photosensitive drum group 21a,
Exposure devices 23a, 23 for writing an electrostatic latent image on 21b
3b (specifically, 23aY, 23aM, 23aC, 23
aK, 23bY, 23bM, 23bC, 23bK);
Primary transfer rolls 26a and 26b (specifically, 26aY, 26aM, 2A) corresponding to the respective photosensitive drum groups 21a and 21b.
6aC, 26aK, 26bY, 26bM, 26bC, 2
6bK), and one of the secondary transfer rolls 4 as the secondary transfer unit 40 (secondary transfer rolls 40a and 40b).
The tension rolls 51a and 51b are disposed on both sides of the intermediate transfer belt 0b, and the intermediate transfer belts 25a and 25b are asymmetrically disposed.

In this embodiment, the toners T1 and T2 use negatively charged toner, and use the primary transfer rolls 26aY to 26a.
DC current +10 μA and −10 μA for each transfer of YMCK to K, 26bY to 26bk, DC current +300 μA, grid voltage +500 V to polarity inversion corotron 28 (specifically, 28Y, 28M, 28C, 28K),
A DC voltage of -4 kV was applied to a contact roll 48 in contact with the secondary transfer roll 40a. The secondary transfer roll 40b was grounded.

Next, an image forming process of the double-sided image forming apparatus according to the present embodiment will be described. The first toner images T1 are successively transferred from the first photosensitive drum groups 21aY, 21aM, 21aC, 21aK to the first intermediate transfer belt 25a in the order of YMCK by the primary transfer rolls 26aY, 26aM, 26aC, 26aK. . Similarly, for the second toner image T2 (YMCK) formed on the second photosensitive drum group 21bY, 21bM, 21bC, 21bK, the second toner image T2 (YMCK) is applied by applying a voltage to the polarity inversion corotron 28 (28Y to 28K). After the polarity of the toner image T2 is inverted, the primary transfer rolls 26bY, 26bM, 26bC, 2
6bK, the second toner image T2 is sequentially transferred onto the second intermediate transfer belt 25b. The sheet P is conveyed at the same timing, and the toner images T1 and T2 on the intermediate transfer belts 25a and 25b are simultaneously transferred onto the sheet P.
At 0, both sides are fixed simultaneously.

FIG. 17 shows the image forming timing of the first image T1 and the second image T2 by the image forming control system of the duplex image forming apparatus according to the present embodiment. In the figure, an exposure control device 1
Reference numeral 00 denotes an exposure device 23a (23aY-2) for the electrostatic latent image of each color of the first image T1 on the first photosensitive drum group 21a (21aY-21aK) in the first image forming unit 20a.
3aK) and the second image forming unit 20b
In the second photosensitive drum group 21b (21bY to 21b
K) The timing of the start of writing the electrostatic latent image of each color of the second image T2 on the exposure device 23b (23bY to 23bK) on the upper side is determined by the secondary transfer unit 40 using the first image T1 and each color component image of the second image T2. The bias power supply control device 101 sets each of the photosensitive drums 21
a and 21b, the respective color component images of the first image T1 and the second image T2 visualized by developing units (not shown) are provided on the first intermediate transfer belt 25a and the second intermediate transfer belt 25b.
The timing of the primary transfer to the first image T1 and the second transfer
Each color component image of the image T2 is set in accordance with the transport distance until the color component image reaches the secondary transfer unit 40, and the respective color component images (first Y color) are placed on the first intermediate transfer belt 25a and the second intermediate transfer belt 25b.
After the multiple transfer of the image to the first K image and the second Y image to the second K image), the two images T1 and T2 are simultaneously carried into the transfer nip of the secondary transfer unit 40 and located in the image forming areas on both sides of the sheet P. It is transferred without displacement.

In such an operation process, for example, the first
In the image forming unit 20a, the image transfer distance along the peripheral surface of the photosensitive drum 21aY between the image forming start position of the photosensitive drum 21aY and the primary transfer unit 26aY is denoted by LD1, and the primary transfer unit 26aY and the secondary transfer unit Assuming that the image transport distance along the intermediate transfer belt 25a between the first and second color component images is LB1, the photosensitive drums 21aM to 21aK are used for the other color component images.
Is an image transport distance on the peripheral surface of LD1, but the image transport distance along the intermediate transfer belt 25a is shortened by the span s (fixed in this example) between the photosensitive drums 21a. Specifically, the image transfer distance along the intermediate transfer belt 25a for the photosensitive drum 21aM is LB1-s, and the image transfer distance along the intermediate transfer belt 25a for the photosensitive drums 21aC and 21aK is LB1-2s, LB1-3s.
It is. On the other hand, in the second image forming section 20b, the image forming start position of the photosensitive drum 21bY and the primary transfer section 26bY
The image transport distance along the peripheral surface of the photosensitive drum 21bY between the primary transfer unit 26bY and the secondary transfer unit 40 along the intermediate transfer belt 25b is L2.
In the case of B2, for other color component images, the image transport distance on the peripheral surface of the photosensitive drums 21bM to 21bK is LD2, but the image transport distance along the intermediate transfer belt 25b is between the photosensitive drums 21b. It is shortened by the span s (it is assumed to be constant in this example). Specifically, the photosensitive drum 21
The image transport distance along the intermediate transfer belt 25b for the bM is LB2-s, and the image transport distance along the intermediate transfer belt 25b for the photosensitive drums 21bC and 21bK is LB2.
-2 s and LB2-3 s.

Here, assuming that LD1 and LD2 of each of the photosensitive drum groups 21a and 21b are equal, for example, the difference in image transport distance up to the secondary transfer unit 40 of the same color component image is | LB1
−LB2 |. Therefore, in the present embodiment, the timing of starting the writing of the electrostatic latent image by each of the exposure devices 23a and 23b of the corresponding color components is determined by the photosensitive drums 21a and 21b.
If the peripheral speed of the intermediate transfer belts 25a and 25b is PS, | (LB1-LB2) / PS |
It is necessary to advance b side. Further, each image forming unit 2
Exposure devices 23a, 23 for each color component at 0a, 20b
The latent image writing timing of b may be set to be earlier in order from the side remote from the secondary transfer unit 40, and to be delayed by (s / PS) sequentially for the exposing devices 23a and 23b of the adjacent photosensitive drums 21a. Incidentally, each photosensitive drum 21a, 2
The timing of bias application to the primary transfer units 26a and 26b corresponding to 1b is also set corresponding to the drive timing of the exposure devices 23a and 23b.

As described above, the photosensitive drum 21a (21a
YMCK toner images T on the intermediate transfer belts 25a and 25b from the groups Y to 21aK) and 21b (21bY to 21bK).
1 and T2 are superimposed and transferred, respectively, to the intermediate transfer belts 25a and 25a.
5b, and the intermediate transfer belts 25a and 25
b is simultaneously transferred onto both sides of the sheet P by the electric field between the secondary transfer rolls 40a and 40b. At this time, although a YMCK color double-sided image forming apparatus having a configuration in which the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched, the first image T of the first photosensitive drum group 21a is formed.
The timing of the start of the formation of the electrostatic latent image of each color and the start of the formation of the electrostatic latent image of each color of the second image T2 on the second photosensitive drum group 21b are determined by the first asymmetrical belt layout. By setting the image transfer distance between the image T1 and the second image T2 in consideration of the difference between the image transfer distances, the first image of the YMCK color can be accurately transferred to the image transfer areas on both sides of the sheet P at the same time in the secondary transfer unit 40. The image T1 and the second image T2 can be transferred, and the image is conveyed horizontally to the fixing device 50 and fixed on both sides at the same time, so that there is no image disturbance, fine line dropout, paper wrinkling, and no misalignment on both sides. A two-sided color image could be obtained per sheet at the same recording speed as when one side was used.

Embodiment 8 FIG. 18 shows a schematic structure of Embodiment 8 of a double-sided image forming apparatus to which the present invention is applied. In the figure, the basic configuration and the image forming process of the double-sided image forming apparatus are substantially the same as those of the seventh embodiment, and since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, the first intermediate The transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched, but different from the seventh embodiment, each photosensitive drum group 21a (21aY to 21a) of the first image forming unit 20a.
K) corresponding to the primary transfer section 26a (26aY to 26a).
K) the intermediate transfer belt 2 from the secondary transfer unit 40
Image transport distances LB1, LB1-s, LB1-2s, LB1 on 5a
-3s and each photosensitive drum group 21 of the second image forming unit 20b
b (21bY to 21bK) corresponding to the primary transfer portion 26b
Transfer belt 25b from the first transfer to the secondary transfer unit 40
Upper image transport distances LB2, LB2-s, LB2-2s, LB2-3
s is equal to the holding rolls 52a, 52b and 53a, 53b, the secondary transfer rolls 40a, 40b, 1
Next transfer rolls 26a and 26b are provided. Since the transport distance of each color component image on each of the photosensitive drum groups 21a and 21b is also set to be equal, the total transport distance of each color component image of the first image T1 and the second image T2 to the secondary transfer unit 40 is equal. Has become.

FIG. 19 shows the image forming timing of the first image T1 and the second image T2 by the image forming control system of the duplex image forming apparatus according to the present embodiment. At this time, the first image forming unit 20
The process of forming each color of the first image T1 and the second image T2 in the second image forming unit 20b is substantially the same as that of the seventh embodiment, but in the present embodiment, the secondary transfer unit 40
Since the transport distances of the respective color component images of the first image T1 and the second image T2 which reach the image forming apparatus are equal, the image forming control system (the exposure control apparatus 100,
The bias power supply control device 101) includes the first image forming unit 20.
a, the start of writing of the electrostatic latent image of each color of the first image T1 on the first photosensitive drum group 21a by the exposure device 23a, and the second image T2 on the second photosensitive drum 21b in the second image forming unit 20b. At the same time as the start of writing of the electrostatic latent image of each color by the exposure device 23b, and further, the first image T1 and the second image T1, which are visualized by developing units (not shown) of the photosensitive drums 21a and 21b, respectively.
The timing at which each color component of the image T2 is primarily transferred to the first intermediate transfer belt 25a and the second intermediate transfer belt 25b is simultaneously executed. However, each image forming unit 20a,
The drive timing of the exposure devices 23a and 23b of the photosensitive drums 21a and 21b adjacent to the photosensitive drum 20b and the drive timing of the primary transfer units 26a and 26b are the same as in the seventh embodiment.
(S: span between photosensitive drum groups 21a, 21b, PS:
The photosensitive drum groups 21a and 21b, the intermediate transfer belt 25a,
(A peripheral speed of 25b). Then, after the respective color component images (first Y image to first K image, second Y image to second K image) are multiplex-transferred onto the first intermediate transfer belt 25a and the second intermediate transfer belt 25b, both images T1 and T2 are The sheet P is simultaneously conveyed into the transfer nip of the secondary transfer unit 40 and is transferred to the image forming areas on both sides of the sheet P without displacement.

As described above, since the tension rolls 51a and 51b are provided on both sides of the secondary transfer roll 40b, as a result, the first intermediate transfer belt 25a and the second intermediate transfer belt 25b are asymmetrically stretched. Y of the installed configuration
The MCK full-color double-sided image forming apparatus is used.
The start of forming the electrostatic latent images of the respective colors of the first image T1 on the photosensitive drum group 21a and the start of forming the electrostatic latent images of the respective colors of the second image T2 on the second photosensitive drum group 21b. The first image T1 and the second image T2 of the YMCK color can be accurately and simultaneously transferred to the image transfer areas on both sides of the sheet P in the secondary transfer unit 40 at substantially the same control sequence without setting them separately. Then, the sheet was directly conveyed to the fixing device 50 and simultaneously fixed on both sides, so that a double-sided image with no misalignment on both sides could be obtained at the same recording speed as one side per sheet.

[0070]

As described above, according to the present invention,
In a double-sided image forming apparatus, the first image forming unit and the second image forming unit each have an image carrier and an intermediate transfer belt, and simultaneously transfer images on each intermediate transfer belt to both surfaces of a recording material.
For example, assume that the intermediate secondary transfer unit has an asymmetric configuration in order to improve the image quality and stabilize the recording material posture, and a situation has arisen in which the first intermediate transfer belt and the second intermediate transfer belt must be stretched asymmetrically, respectively. Also, taking into consideration the transport distance of the first image and the second image to the intermediate secondary transfer section, the start timing of formation of each image is adjusted, and the transport of the first image and the second image to the intermediate secondary transfer section is performed. Since the distances are made equal, the first image and the second image can be simultaneously and accurately transferred to the image transfer areas on both surfaces of the recording material at the intermediate secondary transfer portion without displacement of both surfaces. A double-sided image without any deviation can be obtained at the same recording speed as that for one side per recording material.

In particular, according to the second aspect of the present invention, there is no concern that a new problem will be caused in order to make the intermediate secondary transfer portion transport distance between the first image and the second image equal. It can be easily realized by taking advantage of the high degree of freedom in the layout of the apparatus, which is an advantage of the system using a belt.

[Brief description of the drawings]

FIG. 1 is an explanatory diagram illustrating a first embodiment of a two-sided image forming apparatus according to the present invention.

FIG. 2 is an explanatory view showing a second embodiment of the duplex image forming apparatus according to the present invention.

FIG. 3 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to the first embodiment;

FIG. 4A is an explanatory view showing a method of applying a bias to a secondary transfer device used in the first embodiment, and FIGS.
FIG. 7 is an explanatory diagram showing a modification of the bias application method to the secondary transfer device.

FIG. 5 is an explanatory diagram showing image forming timing by an image forming control system of the double-sided image forming apparatus according to the first embodiment.

FIG. 6 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to a second embodiment.

FIG. 7 is an explanatory diagram showing image forming timing by an image forming control system of the duplex image forming apparatus according to the second embodiment.

FIG. 8 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to a third embodiment.

FIG. 9 is an explanatory diagram showing details of a secondary transfer unit of the double-sided image forming apparatus according to the third embodiment.

FIG. 10 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to a fourth embodiment.

FIG. 11 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to a fifth embodiment.

FIG. 12 illustrates a two-sided image forming apparatus according to a fifth embodiment.
FIG. 9 is an explanatory diagram illustrating a separation mechanism of a next transfer unit.

FIG. 13 is an explanatory diagram showing image forming timing by an image forming control system of the duplex image forming apparatus according to the fifth embodiment.

FIG. 14 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to a sixth embodiment.

FIG. 15 is an explanatory diagram showing image forming timing by an image forming control system of the duplex image forming apparatus according to the sixth embodiment.

FIG. 16 is a schematic configuration diagram showing a double-sided image forming apparatus according to a seventh embodiment.

FIG. 17 is an explanatory diagram showing image forming timing by an image forming control system of the duplex image forming apparatus according to the seventh embodiment.

FIG. 18 is a schematic configuration diagram illustrating a double-sided image forming apparatus according to an eighth embodiment.

FIG. 19 is an explanatory diagram showing image forming timing by the image forming control system of the duplex image forming apparatus according to the eighth embodiment.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... 1st image carrier, 2 ... 1st intermediate transfer belt, 3 ... 2nd
Image carrier 4, second intermediate transfer belt 5, recording material 6,
First intermediate primary transfer means, 7... Second intermediate primary transfer means, 8
... intermediate secondary transfer means, 8a, 8b ... transfer member, 9 ... first
Image pattern forming means, 10 second image pattern forming means,
11: belt tension member, 12: drive signal control unit,
20a: first image forming unit, 20b: second image forming unit, 2
1a: First photosensitive drum, 21b: Second photosensitive drum, 2
2a: first charging roll, 22b: second charging roll,
23a: First exposure apparatus, 23b: Second exposure apparatus, 2
4a: first developing unit, 24b: second developing unit, 25a ...
First intermediate transfer belt, 25b... Second intermediate transfer belt, 2
6a: first primary transfer roll, 26b: second primary transfer roll, 27a: first drum cleaner, 27b: second
Drum cleaner, 28 ... polarity reversal corotron, 30a
... first belt cleaner, 30b second belt cleaner, 33 paper guide plate, 34a, 34b rotary developing device, 40a, 40b secondary transfer roll, 41 transfer bias, 48 contact roll, 50 Fixing device, 51
a, 51b: tension rolls, 52a, 52b, 53
a, 53b: holding roll, P: paper

Claims (2)

[Claims] 1. A first image carrier on which a first image is formed and carried, a first intermediate transfer belt disposed to face the first image carrier, and a first image on the first image carrier. A first intermediate primary transfer unit that performs primary transfer on the first intermediate transfer belt, a second image carrier on which a second image is formed and carried, and a second intermediate transfer that is arranged to face the second image carrier Belt and a second image for primary transfer of the second image on the second image carrier to the second intermediate transfer belt
An intermediate primary transfer unit, and a transfer member that is disposed opposite to the first intermediate transfer belt and the second intermediate transfer belt in a region where the first intermediate transfer belt and the second intermediate transfer belt are in contact with or in proximity to each other with the two intermediate transfer belts interposed therebetween;
An intermediate secondary transfer unit for secondary-transferring the primary transfer image on each intermediate transfer belt to both sides of the recording material, wherein the first intermediate transfer belt and the second intermediate transfer belt are respectively asymmetrically stretched, From the start position of the first image and the second image on each image carrier, the intermediate secondary image is transferred so that the first image and the second image are simultaneously secondary-transferred to both sides of the recording material by the intermediate secondary transfer means. A double-sided image forming apparatus, wherein the start timing of forming each image on each image carrier is adjusted in accordance with the total transport distance of each image up to a secondary transfer position of a transfer unit. 2. A first image carrier on which a first image is formed and carried, a first intermediate transfer belt disposed opposite to the first image carrier, and a first image on the first image carrier. A first intermediate primary transfer unit that performs primary transfer on the first intermediate transfer belt, a second image carrier on which a second image is formed and carried, and a second intermediate transfer that is arranged to face the second image carrier Belt and a second image for primary transfer of the second image on the second image carrier to the second intermediate transfer belt
An intermediate primary transfer unit, and a transfer member that is disposed opposite to the first intermediate transfer belt and the second intermediate transfer belt in a region where the first intermediate transfer belt and the second intermediate transfer belt are in contact with or in proximity to each other with the two intermediate transfer belts interposed therebetween;
An intermediate secondary transfer unit for secondary-transferring a primary transfer image on each intermediate transfer belt to both surfaces of the recording material, wherein the first intermediate transfer belt and the second intermediate transfer belt are asymmetrically stretched, respectively; Intermediate secondary transfer from the formation start position of the first image and the second image on each image carrier so that the first image and the second image are simultaneously secondary-transferred to both sides of the recording material by the intermediate secondary transfer means. A two-sided image forming apparatus, wherein the total transport distance of each image up to the secondary transfer position of the means is made equal.