JP3852170B2 - Image forming apparatus - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to an image forming apparatus such as a copying machine or a printer using an electrophotographic method, and more specifically, by transferring each toner image onto both sides of a recording sheet conveyed without inverting the front and back surfaces of the sheet. The present invention relates to an image forming apparatus capable of forming a double-sided image.
[0002]
[Prior art]
Conventional image forming apparatuses capable of forming double-sided images generally transfer and fix a first toner image formed on a photoreceptor on one side of a recording sheet, and then reverse the recording sheet. A method is adopted in which a double-sided image is formed by re-feeding to the transfer portion of the photosensitive member and then transferring and fixing the second toner image formed on the photosensitive member to the other surface of the recording sheet. .
[0003]
However, in this method, since the toner image must be transferred and fixed twice, the productivity (image formation efficiency) per recording sheet is almost halved compared to the case of image formation on one side. Heat curling occurs on the recording sheet during the first fixing, and this curling causes transfer failure and recording sheet conveyance failure during the second transfer / fixing, and the recording sheet is reversed and re-supplied. There are various problems, such as the fact that a transport path for feeding must be provided in the apparatus, and that operational noise is likely to occur during reversal and refeeding.
[0004]
In order to solve such problems, Japanese Patent Application Laid-Open No. 3-253881 proposes an image forming apparatus adopting a new double-sided image forming method. As shown in FIG. 4, the image forming apparatus includes a single photosensitive member 101 and an intermediate transfer member 102, and a first toner image T <b> 1 formed on the photosensitive member 101 is transferred to the intermediate transfer member 102 by an electrostatic transfer device. After the toner image T1 is transferred by the action of 103, the toner image T1 is transferred to the transfer portion of the photosensitive member 101 facing the transfer unit 103 again by the rotation of the intermediate transfer member 102, and the timing on the photosensitive member 101 is adjusted. The second toner image T2 is formed at the same time, and the polarity of the second toner image T2 is switched to the opposite polarity to the first toner image T1 by the reverse polarity charging means 104, and then the photosensitive member 101 and the intermediate transfer member 102 The first toner image T1 on the intermediate transfer member 102 and the second toner image T2 on the photosensitive member 101 are collectively transferred to the front and back surfaces of the recording sheet 105 inserted between After copy, in which so as to simultaneously fixed by the fixing device 106.
[0005]
That is, in this apparatus, the polarity switching process is performed so that the polarities of the first toner image T1 and the second toner image T2 having the same toner polarity are opposite to each other before transfer. The first and second toner images are collectively transferred to both the front and back surfaces of the recording sheet 105 and then fixed simultaneously, thereby forming a double-sided image.
[0006]
[Problems to be solved by the invention]
However, the proposed image forming apparatus can obtain a double-sided image by one transfer and fixing, but has the following problems.
[0007]
That is, since the polarity is switched by the reverse polarity charging means 104 such as a corona discharger for the second toner image T2 formed by developing on the photosensitive member 101, the uppermost layer side and the lowermost layer side of the toner image T2 are used. As a result, the transfer efficiency of the second toner image T2 is different from that of the first toner image T1, so that the images on both sides of the obtained recording sheet differ in image quality. It may end up.
[0008]
In addition, since the photoconductor 101 is charged to the reverse polarity by the reverse polarity charging unit 104, it is difficult to set the characteristics of the photoconductor, and the carrier of the reverse polarity is trapped inside the photoconductor to thereby detect the sensitivity of the photoconductor. There is a risk of lowering the sensitivity, and a latent image ghost or the like may easily occur due to this sensitivity reduction.
[0009]
Incidentally, in the image forming apparatus described above, the first and second toner images are charged to the developing unit for forming the first toner image and the developing unit for forming the second toner image as means for charging the first and second toner images with opposite polarities. However, in this case, since two types of toner must be used in the same image forming apparatus, handling becomes troublesome. Further, since development is performed by different development methods corresponding to the toners having different polarities, it is difficult to make the image quality of the first and second toner images the same level.
[0010]
SUMMARY OF THE INVENTION An object of the present invention is to provide an image forming apparatus capable of forming a double-sided image by transferring each toner image on both sides of a recording sheet conveyed without inverting the front and back surfaces of the recording sheet and fixing the image on a photoreceptor. An object of the present invention is to provide an image forming apparatus capable of easily, efficiently and stably forming a double-sided image having no image quality difference between both sides of a recording sheet without switching the toner polarity.
[0011]
[Means for Solving the Problems]
The image forming apparatus according to the present invention includes a first photosensitive member on which a first toner image is formed, and a first image that electrostatically transfers the toner image on the first photosensitive member to one surface of a recording sheet. Transfer means, a belt-type sheet conveying means for conveying the recording sheet after the transfer by the first transfer means is completed in a state where the transferred first toner image is in contact with the belt surface, A second photoconductor that is disposed in contact with the belt surface of the sheet conveying unit and forms a second toner image using toner having the same polarity as the toner used for forming the first toner image. A second transfer means for electrostatically transferring the toner image on the second photosensitive member to the other surface of the recording sheet conveyed by the sheet conveying means, and transfer by the second transfer means. after completion, the first retrieved from the second toner image transfer surface side of the recording sheet Giving charges of opposite polarity to the polarity of over image, and a transfer correction means first toner image in between the recording sheet and the belt of the sheet conveyor means to form a transfer electric field are attracted to the recording sheet side Is.
[0012]
In the apparatus having such a configuration, the recording sheet passes through the transfer portion by the first transfer means and the transfer portion by the second transfer means in this order, so that the first toner image and the second image are formed on both surfaces of the recording sheet. Each toner image is transferred. When the transfer of the second toner image is completed, the first toner image is electrostatically attracted to the belt side of the sheet conveying unit under the influence of the transfer action by the second transfer unit. become. However, the first toner image is attracted to the recording sheet side by the transfer electric field formed by the transfer correction means when the transfer by the second transfer means is completed. That is, the transfer correction means gives a charge having a polarity opposite to the polarity of the first toner image from the second toner image transfer surface side of the recording sheet, whereby the first toner image is transferred from the belt side of the sheet conveying means to the recording sheet. The electrostatic force in the direction toward the side acts, whereby the first toner image is transferred again to the recording sheet side. As a result, the first toner image and the second toner image are electrostatically attached to both surfaces of the recording sheet. Note that the first and second toner images after passing through the transfer correcting unit are fixed simultaneously by the fixing unit, for example.
[0013]
In the image forming apparatus, the transfer correction unit includes a charge eliminating unit that neutralizes the belt of the sheet conveying unit with the same polarity as the polarity of the first toner image, and a second toner on the recording sheet after the charge removal by the charge eliminating unit. from the image transfer surface side giving opposite polarity charge to the polarity of the first toner image can be the toner image is composed of a field forming means for forming a transfer electric field that is attracted to the recording sheet side.
[0014]
In the apparatus having such a configuration, since the belt of the sheet conveying unit is neutralized by the neutralizing unit, the electrostatic adhesion force with the belt of the first toner image that is electrostatically attracted to the belt side is weakened. As a result, the effect of the transfer electric field formed by the subsequent electric field forming unit is more accurately applied to the first toner image, and as a result, the toner image is reliably attracted to the recording sheet side.
[0015]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the accompanying drawings.
[0016]
FIG. 1 shows an image forming apparatus according to an embodiment of the present invention. The image forming apparatus includes a first photosensitive drum 1 rotating in the direction of an arrow, a second photosensitive drum 2 rotating in the direction of the arrow at the same speed as the photosensitive drum 1, and a plurality of support rolls (or drums) 3a and 3b. A transfer belt 3 that has a sheet conveying function and an intermediate transfer function, and is in contact with the first photosensitive drum 1 and the transfer belt 3. The first primary transfer corotron 4 disposed on the inner side of the adjacent belt, and the first primary transfer corotron 4 disposed on the inner side of the transfer belt 3 which is substantially in the middle between the first photosensitive drum 1 and the second photosensitive drum 3. A secondary transfer corotron 5, a second transfer corotron 6 disposed inside the belt in the vicinity of the contact portion between the second photosensitive drum 2 and the transfer belt 3, and a belt downstream of the second transfer corotron 6. Of the transfer correction means 7 disposed inside. The static eliminating corotron 8 constituting the recording section, the retransfer corotron 9 constituting a part of the transfer correcting means 7 disposed outside the belt on the downstream side of the static eliminating corotron 8, and the recording sheet P peel from the transfer belt 3. The peeling corotron 10 for neutralizing the electric charges so as to be easy and the roll-type heat and pressure fixing device 11 constitute the main part. Of the support rolls 3a and 3b, 3a is a drive roll and 3b is a tension roll.
[0017]
In FIG. 1, 12 is a developing device disposed around the first photosensitive drum 1, 13 is a developing device disposed around the second photosensitive drum 2, and 14 accommodates a recording sheet P. A sheet storage tray 15 is a suction type sheet conveying belt. In addition, around the first and second photosensitive drums 1 and 2, a charger, an image exposure device, and the like for toner image formation (not shown) are similarly arranged. In addition, when the image forming apparatus is used as a copying machine, an image exposure apparatus that operates based on image information from the document reading apparatus is used, and when used as a printer, an external device, a recording medium, or the like is used. That operate on the basis of image information input from is used.
[0018]
As the transfer belt 3 having both the sheet conveying function and the intermediate transfer function, a synthetic resin such as polyethylene terephthalate (PET), polyvinylidene fluoride (PVdF), polyimide, acrylic, vinyl chloride, and polycarbonate, or carbon black on various rubbers. An appropriate amount of an antistatic agent such as the one described above is used, and the volume resistivity is 10 ⁹ to 10 ¹⁶ Ω · cm and the thickness is adjusted to, for example, 75 μm, and the endless belt is used. The
[0019]
In this image forming apparatus, a predetermined direct current (for example, +550 μA) is applied to the first primary transfer corotron 4, and a predetermined direct current (for example, +500 μA) is applied to the first secondary transfer corotron 5, A predetermined direct current (for example, +550 μA) is applied to the second transfer corotron 6 from the power supply device 6a (see FIG. 2), and a direct current (for example, −20 μA) is applied to the peeling corotron 10 with a predetermined alternating voltage (for example, 8 kV _PP / 600 Hz). ) Are superimposed and applied. The numerical items in parentheses are specific examples when negatively charged toner is used for both the developing devices 12 and 13 (the same applies to the parentheses thereafter).
[0020]
In addition, in this image forming apparatus, an alternating current value (for example, 500 Hz, 550 μA) having a predetermined frequency and a predetermined DC voltage (for example, −1.5 kV) are superimposed and applied to the static eliminating corotron 8 from the power supply device 8a. A predetermined direct current (for example, +550 μA) is applied to the retransfer corotron 9 from the power supply device 9a (see FIG. 2).
[0021]
Next, the operation when a double-sided image is formed by this image forming apparatus will be described.
[0022]
First, after an electrostatic latent image is formed on the first photosensitive drum 1 by a known electrophotographic process, the latent image is developed with negatively charged toner supplied from the developing device 12, and the first toner image T1 is developed. Is formed. The first toner image T1 is electrostatically transferred from the photosensitive drum 1 to the transfer belt 3 by the transfer electric field formed by the first primary transfer corotron 4, and then the second second image is rotated by the rotation of the transfer belt 3. It is conveyed to a secondary transfer position facing the next transfer corotron 4.
[0023]
Subsequently, the toner image T1 on the transfer belt 3 is transferred to the first surface of the recording sheet P fed at a predetermined timing from the sheet storage tray 14 via the sheet conveying belt 15 at the secondary transfer position. The secondary transfer corotron 5 is electrostatically transferred by a transfer electric field. The recording sheet P after the secondary transfer is adsorbed as it is on the conveying belt 3 so that the toner image T1 is in contact with the belt surface of the transfer belt 3, and the second photosensitive drum 2 and the second transfer corotron 6. Are conveyed to the second transfer position facing each other. In FIG. 1, the alternate long and short dash line indicates the conveyance path of the recording sheet P.
[0024]
On the other hand, while the first toner image T1 is being formed and transferred, an electrostatic latent image is formed on the second photosensitive drum 2 by a known electrophotographic process. The second toner image T <b> 2 is formed by developing with the negatively charged toner supplied from 13. The second toner image T2 is electrostatically transferred from the photosensitive drum 2 to the other surface of the recording sheet P conveyed by the transfer belt 3 by a transfer electric field formed by the second transfer corotron 6.
[0025]
After this transfer, the inside of the transfer belt 3 is neutralized by the static elimination corotron 8, and immediately thereafter, the first toner image T1 is transferred to the recording sheet 9 side by the transfer electric field from the toner image T2 side formed by the retransfer corotron 9. Is attracted electrostatically. As a result, the first toner image T1 is transferred to one side of the recording sheet P, and the second toner image T2 is transferred to the opposite side.
[0026]
The recording sheet P on which the toner images T1 and T2 are formed on both sides is conveyed to the position of the peeling corotron 10 by the transfer belt 3 and then peeled off from the transfer belt 3 by the charge removing action of the peeling corotron 10 and then the fixing device. 11 and the fixing device 11 fixes the toner images T1 and T2 on both sides of the sheet at the same time. A double-sided image is formed as described above.
[0027]
In particular, in this image forming apparatus, after the transfer by the second transfer corotron 6 is performed, the recording sheet P held on the transfer belt 3 (before fixing processing) By passing through the transfer corotron 9, it is possible to obtain a double-sided image having no difference in image quality between both sides of the recording sheet.
[0028]
FIG. 2 schematically shows the time-lapse state during and after the second transfer operation in the image forming apparatus.
[0029]
As shown in the right half of FIG. 2, the first toner image T <b> 1 is sandwiched between the transfer belt 3 that rotates and moves in the direction of arrow A and the recording sheet P adsorbed by the belt 3. It is conveyed to the transfer area. The first toner image T1 is electrostatically attached to the recording sheet P side until the second transfer is executed.
[0030]
When the second transfer is executed, a positive charge is applied from the second transfer corotron 6 to the inside of the transfer belt 3, so that the second toner image T2 formed on the photosensitive drum 2 by the negatively charged toner. The electrostatic force in the direction of arrow B acts on the second toner image T2, and as a result, the second toner image T2 is transferred to the recording sheet P side. However, at this time, since the first toner image T1 that has already been transferred to the recording sheet P is also formed by the negatively charged toner, the electrostatic force in the direction of arrow B acts on the first toner image T1. As a result, the first toner image T1 is electrostatically attached to the transfer belt 3 side. T1a in the drawing indicates a first toner image portion attached to the transfer belt 3 side.
[0031]
When the second transfer is completed, as shown in the left half of FIG. 2, negative charge is first applied from the static elimination corotron 8 to the inside of the transfer belt 3, so that the surface charge inside the transfer belt 3 is eliminated. Therefore, the electrostatic adhesion between the first toner image T1a adhered to the transfer belt 3 and the transfer belt 3 is weakened. Further, as a result of the subsequent retransfer corotron 9 being charged from the second toner image transfer surface side of the recording sheet P with a charge having a polarity opposite to that of the first toner image T1 (in this case, positive polarity), the first toner image is obtained. Since the electrostatic force in the direction of arrow C acts on T1 (a), the first toner image T1 is transferred again to the recording sheet P side. The second toner image on the recording sheet P changes to the opposite polarity when passing through the retransfer corotron 9 as shown in the left half of FIG .
[0032]
As a result, the first toner image T1 is partially attached to the transfer belt 3 side at the time of the second transfer, but passes through the static elimination corotron 8 and the retransfer corotron 9 as the transfer correction unit 7. Return to the recording sheet P again. As a result, both the first toner image T2 and the second toner image T2 are reliably transferred to the recording sheet P.
[0033]
Thus, as a result of the first toner image T1 and the second toner image T2 being reliably transferred to the front and back surfaces of the recording sheet P, it is possible to obtain a double-sided image with no difference in image quality between both sides of the recording sheet. .
[0034]
FIG. 3 shows an image forming apparatus according to another embodiment of the present invention. The image forming apparatus is configured to directly transfer the first toner image T1 from the photosensitive drum 1 to one surface of the recording sheet P, and then convey the recording sheet P after the transfer to the second transfer position by a belt. Except for the above, the apparatus has the same configuration as the apparatus according to the above-described embodiment. In FIG. 3, 20 is a peeling corotron for peeling the recording sheet P from the photosensitive drum 1, and 21 is a sheet for transporting the recording sheet P peeled from the photosensitive drum 1 to the transfer belt 3 to be sent to the second transfer region. A conveyance belt 22 is an adsorption corotron for electrostatically adsorbing the recording sheet P to the transfer belt 3. A tension support roll 3b for supporting the transfer belt 3 is formed of a conductive material and is grounded.
[0035]
In this image forming apparatus, when negatively charged toner is used for both the developing devices 12 and 13, for example, a direct current of +550 μA is applied to the first primary transfer corotron 4 and an AC voltage is applied to the peeling corotron 20. A DC current of −15 μA is applied to an AC voltage of 4.0 kV _PMS and a frequency of 600 Hz, a DC current of −250 μA is applied to the adsorption corotron 22, a DC current of +550 μA is applied to the second transfer corotron 6, and a peeling corotron is applied. A DC current of −20 μA is superimposed on an AC voltage of 10 kV _PP / 600 Hz and applied. In addition, in this case, an AC current value of 550 μA having a frequency of 500 Hz and a DC voltage of −1.5 kV are superimposed and applied to the static elimination corotron 8, and a DC current of +550 μA is applied to the retransfer corotron 9. It has become.
[0036]
Next, the operation when a double-sided image is formed by this image forming apparatus will be described.
[0037]
First, a first toner image T1 made of negatively charged toner is formed on the first photosensitive drum 1, and the toner image T1 is fed to a transfer position where the photosensitive drum 1 and the first primary transfer corotron 4 face each other. The recording sheet P is electrostatically transferred. After the transfer, the recording sheet P is peeled off from the photosensitive drum 1 by the peeling action of the peeling coatroton 20, and then conveyed to the transfer belt 3 by the sheet conveying belt 21 in a state where the toner image T1 touches the belt surface. Thereafter, the toner is conveyed to the second transfer position by the transfer belt 3 while being adsorbed on the belt surface by the adsorption action of the adsorption corotron 22.
[0038]
While the first toner image T1 is being formed, transferred, and conveyed, a second toner image T2 made of negatively charged toner is formed on the second photosensitive drum 2. The second toner image T2 is electrostatically transferred to the other surface of the recording sheet P conveyed by the transfer belt 3 by a transfer electric field formed by the second transfer corotron 6.
[0039]
After this transfer, the inside of the transfer belt 3 is neutralized by the static elimination corotron 8 and immediately after that, the transfer electric field from the toner image T2 side formed by the retransfer corotron 9 is removed. Thus, the first toner image T1 is electrostatically attracted to the recording sheet 9 side. As a result, the first toner image T1 is transferred to one side of the recording sheet P, and the second toner image T2 is transferred to the opposite side. Thereafter, the recording sheet P is peeled off from the transfer belt 3 by the discharging action of the peeling corotron 10 and then sent to the fixing device 11 to fix the toner images T1 and T2 simultaneously. A double-sided image is formed as described above.
[0040]
Also in this image forming apparatus, after the transfer by the second transfer corotron 6 is performed, the recording sheet P held on the transfer belt 3 (before fixing processing) By passing through the retransfer corotron 9, a double-sided image having no difference in image quality between both sides of the recording sheet is obtained.
[0041]
In each of the above-described embodiments, the transfer correction means 7 includes the static elimination corotron 8 and the retransfer corotron 9, but the present invention is replaced with a conductive roll, for example, a metal roll, instead of the static elimination corotron 8. Alternatively, a conductive brush may be provided in contact with the back surface side of the transfer belt 3, or the transfer correction means 7 may be constituted only by the retransfer corotron 9. In these cases, substantially the same effect as in the case of the above embodiment can be obtained.
[0042]
In the above-described exemplary embodiments, the case where negatively chargeable toner is used has been described. However, it is also possible to use positively chargeable toner. In this case, the same effect can be obtained by reversing the bias polarity applied to each corotron. In the present invention, a color image may be formed on each of the photosensitive drums 1 and 2 and the color image may be transferred to both sides of the recording sheet.
[0043]
Furthermore, a roll type transfer means may be used instead of the corona discharge type transfer means (Corotron 4, 5, 6) in the above-described embodiment. In this case, examples of the transfer roll, for example, a metal sheet Yafuto applying a transfer current (bias), EPDM, polyurethane, carbon black and silicone rubber, aluminum, conductive particles and LiCL ion conductivity, such as _4, such as tin A semiconductive roll formed by forming a coating layer having a volume resistivity of about 10 ⁵ to 10 ⁹ by dispersing an agent can be used.
[0044]
【The invention's effect】
As described above, according to the present invention, when a double-sided image is formed by simultaneously transferring and fixing each toner image on both sides of a recording sheet conveyed without inverting the front and back sides of the sheet, A double-sided image having no image quality difference between both sides of the recording sheet can be formed easily, efficiently and stably. In addition, when the double-sided image is formed, the toner polarity on the photoconductor is not switched as in the prior art. Therefore, the sensitivity of the photoconductor is detected by corona charging performed on the photoconductor for the polarity switching. And the latent image ghost and toner scattering do not occur.
[0045]
In the case where the transfer correcting means is constituted by a charge removing means for discharging the belt of the sheet conveying means and an electric field forming means for forming a transfer electric field that draws the first toner image toward the recording sheet after the charge removal by the charge removing means. Since the first toner image can be more reliably attracted to the recording sheet, a double-sided image having no difference in image quality between both sides of the recording sheet can be obtained more stably.
[Brief description of the drawings]
FIG. 1 is a schematic diagram illustrating a configuration of a main part of an image forming apparatus according to an exemplary embodiment of the present invention.
FIG. 2 is an explanatory view schematically showing a time-dependent state during and after the second transfer operation.
FIG. 3 is a schematic diagram illustrating a configuration of a main part of an image forming apparatus according to another exemplary embodiment of the present invention.
FIG. 4 is a schematic diagram illustrating a main configuration of a conventional image forming apparatus capable of forming a double-sided image.
[Explanation of symbols]
DESCRIPTION OF SYMBOLS 1 ... 1st photosensitive drum, 2 ... 2nd photosensitive drum, 4, 5 ... 1st transfer corotron, 3 ... Transfer belt, 6 ... 2nd transfer corotron, 7 ... Transfer correction means, 8 ... Static elimination corotron ( Neutralizing means) 9 Retransfer corotron (electric field forming means) 11 Fixing device P Recording sheet T1 First toner image T2 Second toner

Claims (1)

A first photoreceptor on which a first toner image is formed;
First transfer means for electrostatically transferring the toner image on the first photosensitive member to one surface of the recording sheet;
A belt-type sheet conveying means for conveying the recording sheet after the transfer by the first transfer means is completed in a state where the transferred first toner image is in contact with the belt surface;
A second photosensitive member is formed in contact with the belt surface of the sheet conveying means, and a second toner image is formed using toner having the same polarity as the toner used for forming the first toner image. Body,
Second transfer means for electrostatically transferring the toner image on the second photoreceptor to the other surface of the recording sheet conveyed by the sheet conveyance means;
After the transfer by the second transfer means is completed, a charge removing means for discharging the belt of the sheet conveying means with the same polarity as that of the first toner image, and a second toner image on the recording sheet after the charge removal by the charge removing means. And a transfer correction unit configured to include an electric field forming unit that applies a charge having a polarity opposite to the polarity of the first toner image from the transfer surface side and forms a transfer electric field that attracts the toner image to the recording sheet side. An image forming apparatus.

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