Electrophotography – Image formation – Transfer
Reexamination Certificate
2001-06-11
2003-06-03
Ngo, Hoang (Department: 2852)
Electrophotography
Image formation
Transfer
C399S308000, C399S328000
Reexamination Certificate
active
06574448
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an apparatus and a method of forming images on both sides of a recording medium.
2. Discussion of the Background
Image forming apparatuses, such as copying machines, printers and facsimile machines, are known to include devices that are configured to form images on both sides of a recording medium (hereinafter sometime referred to as a sheet). Such image forming apparatuses capable of recording images on both sides of a sheet generally transfer an image of one side of an original, which has been formed and visualized on an image bearing member, onto one side of a sheet, and then fix the image onto the sheet by a fixing device. The sheet is then reversed by a reversing path etc., and is conveyed again so that an image of the other side of the original, which has been also visualized on the image bearing member, is transferred and fixed onto the other side of the sheet.
In the above image forming apparatuses, because a sheet conveying direction has to be reversed, and a sheet tends to be curled when an image is fixed onto one side of the sheet, reliability of sheet conveyance is hard to obtain. Further, a curled sheet causes inferior transfer of a toner image, resulting in deteriorating the image quality. Japanese Patent Laid-open Publications No. 1-209470, No. 3-253881 and 10-142869 respectively disclose an image forming apparatus in which toner images, which have been transferred onto both sides of a sheet from a first image bearing member and a second image bearing member, are fixed at one time.
Japanese Patent Laid-opened Publication No. 1-209470 discloses an image forming apparatus that transfers a first image formed on a photoconductor onto a transfer belt by a first transfer device, and then transfers a second image formed on the photoconductor onto one side of a sheet by a second transfer. The image forming apparatus then transfers the first image on the transfer belt onto the other side of the sheet by a second transfer device, thus transferring the images on both sides of the sheet. The sheet is then conveyed to a fixing device, where the images are fixed onto both sides of the sheet.
Japanese Patent Laid-open Publication No. 3-253881 discloses another image forming apparatus, which is similar to the one disclosed in the above-described JP Publication No. 1-209470. In the image forming apparatus, polarity of a second image formed on a photoconductor, which has been visualized as a toner image, is reversed on the photoconductor before a transfer process so that transfer of toner images onto both sides of a sheet is enabled without requiring a second transfer device. The sheet is then conveyed to a fixing device where the toner images on both sides of the sheet are fixed thereupon respectively.
Japanese Patent Laid-open Publication No. 10-142869 discloses another image forming apparatus that includes two transfer devices. After transferring color images onto both sides of a sheet, the sheet is conveyed to a fixing device, and the images are fixed onto both sides of the sheet at one time at the fixing device. The image forming apparatus includes a spur having a plurality of protrusions on its circumferential surface so as to guide conveyance of a sheet carrying unfixed toner images on both sides thereof.
Each of the above-described image forming apparatuses separates a sheet, carrying unfixed toner images on both sides thereof, from a transfer belt to convey the sheet to a fixing device. Therefore, the toner images carried on both sides of the sheet tend to be disturbed when the sheet is being conveyed separated from the transfer belt or when the sheet abuts on the fixing device.
Further, each of the image forming apparatuses disclosed in JP Publications No. 1-209470 and No. 3-253881 does not include a guide device to guide a sheet so as to be conveyed to a fixing device. Therefore, the sheet is not smoothly conveyed to the fixing device and an image carried on the sheet is disturbed or the sheet is jammed thereby reducing reliability for the image forming apparatus. Furthermore, it is relatively difficult to make the speeds of a transfer belt and a fixing device substantially the same. Therefore, when a sheet being conveyed by the transfer belt reaches a fixing device which has a conveying force generally larger than that of the transfer belt, the sheet starts to be conveyed by the fixing device at the speed of the fixing device. Thereby, images carried on the sheet are easily disturbed, causing image blurring as a result.
In the image forming apparatus of JP Publication No. 10-142869, an unfixed image tends to touch the spur, thus easily causing deterioration of an image quality. Further, as described above, because it is relatively difficult to make the speeds of a transfer belt and a fixing device substantially the same, when a sheet being conveyed by the transfer belt reaches a fixing device which has a conveying force generally larger than that of the transfer belt, the sheet is conveyed by the fixing device at the speed of the fixing device thereby causing images carried on the sheet to be easily disturbed and, as a result, image blurring.
FIGS.
1
(
a
)-
1
(
d
) are schematic drawings for explaining background processes of transferring toner images onto both sides of a sheet. In FIG.
1
(
a
) illustrating a state of a developing and a first transferring processes, a negatively charged toner image of a first side of an original formed on a photoconductor drum
1
serving as a first image bearing member is transferred onto an intermediate transfer belt
10
serving as a second image bearing member
10
by a first transfer device
21
applying a positive voltage. In FIG.
1
(
b
) illustrating a state of a second developing process, another negatively charged toner image of the second side of the original is formed on the photoconductor drum
1
, and at the same time the first side toner image carried on the intermediate transfer belt
10
reaches a position where the first side toner image has been transferred onto the intermediate transfer belt
10
from the photoconductor drum
10
after making one round. Further, a sheet is conveyed so as to be correctly positioned relative to the first and second side toner images.
In FIG.
1
(
c
) illustrating a state of a second transferring process, the negatively charged second side toner image on the photoconductor drum
1
is transferred onto the second side of the sheet by the first transfer device
21
applying a positive voltage. At this time, the first side toner image on the intermediate transfer belt
10
is overlapped with the sheet. In FIG.
1
(
d
) illustrating a third transferring process, a second transfer device
22
is turned on to apply a positive voltage, so that the negatively charged first side toner image on the intermediate transfer belt
10
is transferred onto the first side of the sheet. The sheet is then conveyed to a fixing device (not shown) and a cleaning operation is performed for the intermediate transfer belt
10
.
Thus, the transfer process is performed three times. In particular, in the third transferring process, the first side toner image on the intermediate transfer belt
10
is transferred onto the sheet by applying the voltage with the second transfer device
22
from the side of the second side of the sheet onto which the second side toner image has been transferred from the photoconductor drum
1
. Therefore, because of an effect of charging, the second side toner image transferred on the second side of the sheet tends to be disturbed. Further, increase of the charge of the sheet may cause electrostatic offsetting of the toner image in the fixing operation.
Japanese Patent Laid-open Publication No. 3-253881 discloses an image forming apparatus in which transfer of images onto both sides of a sheet is realized by a transfer device arranged at the side of the first side of the sheet. The first image formed on a photoconductor is transferred onto a transfer belt by a first transfer belt, and the polarit
Higaya Toshiaki
Matsuda Masanori
Mochimaru Hideaki
Omata Yasukuni
Seto Mitsuru
Ngo Hoang
Ricoh & Company, Ltd.
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