U.S. patent application number 12/510702 was filed with the patent office on 2010-02-11 for sheet discharging device and image forming apparatus including the sheet discharging device.
This patent application is currently assigned to CANON KABUSHIKI KAISHA. Invention is credited to Hiroshi Sahara, Takeshi Setoriyama.
Application Number | 20100032895 12/510702 |
Document ID | / |
Family ID | 41652178 |
Filed Date | 2010-02-11 |
United States Patent
Application |
20100032895 |
Kind Code |
A1 |
Sahara; Hiroshi ; et
al. |
February 11, 2010 |
SHEET DISCHARGING DEVICE AND IMAGE FORMING APPARATUS INCLUDING THE
SHEET DISCHARGING DEVICE
Abstract
In a sheet discharging unit including a pair of sheet discharge
rollers which discharges a sheet P, the outer diameter of a sheet
discharge drive roller included in the pair of sheet discharge
rollers is set so that the outer diameter of a straight portion
which is a central portion in the width direction perpendicular to
the discharging direction of the sheet P is larger than the outer
diameters of both end portions in the width direction. Further, in
the sheet discharging unit, the pair of sheet discharge rollers
respectively include metal shafts extending in the width direction
and elastic layers respectively wound around the metal shafts and
continuously formed to be longer in the width direction than the
width of a sheet P with a passable maximum size. Further, at least
one elastic layer of the pair of sheet discharge rollers is formed
of a foamed material.
Inventors: |
Sahara; Hiroshi;
(Susono-shi, JP) ; Setoriyama; Takeshi;
(Suntou-gun, JP) |
Correspondence
Address: |
FITZPATRICK CELLA HARPER & SCINTO
1290 Avenue of the Americas
NEW YORK
NY
10104-3800
US
|
Assignee: |
CANON KABUSHIKI KAISHA
Tokyo
JP
|
Family ID: |
41652178 |
Appl. No.: |
12/510702 |
Filed: |
July 28, 2009 |
Current U.S.
Class: |
271/314 |
Current CPC
Class: |
B65H 2301/5122 20130101;
B65H 2404/1314 20130101; B65H 2404/143 20130101; B65H 2301/44318
20130101; B65H 2301/51214 20130101; G03G 15/6573 20130101; B65H
29/14 20130101; B65H 2401/242 20130101; B65H 2401/111 20130101;
B65H 2404/1312 20130101; G03G 15/6552 20130101; B65H 29/70
20130101; B65H 2801/06 20130101 |
Class at
Publication: |
271/314 |
International
Class: |
B65H 29/22 20060101
B65H029/22 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2008 |
JP |
2008-201811 |
Jul 9, 2009 |
JP |
2009-162516 |
Claims
1. A sheet discharging device comprising: a sheet stacking portion
on which sheets are staked; and a pair of sheet discharge rollers
which discharges a sheet, on which a toner image is fixed by heat,
onto the sheet stacking portion in a U-shaped curve in a width
direction perpendicular to the discharging direction of the pair of
sheet discharge rollers, wherein each of the pair of sheet
discharge rollers includes a rotating shaft extending in the width
direction and an elastic layer is provided on the rotating shaft
and continuously formed to be longer in the width direction than
the width of a sheet with a passable maximum size, and at least one
elastic layer of the pair of sheet discharge rollers is formed of a
foamed material.
2. The sheet discharging device as claimed in claim 1, wherein the
outer diameter of the central portion in the width direction, of
one sheet discharge roller of the pair of sheet discharge rollers,
is larger than the outer diameters of the both end portions in the
width direction.
3. The sheet discharging device as claimed in claim 2, wherein the
one sheet discharge roller is disposed above the other sheet
discharge roller of the pair of sheet discharge rollers.
4. The sheet discharging device as claimed in claim 2, wherein the
elastic layer of the one sheet discharge roller has, in the central
portion in the width direction, a cylindrical portion without a
change of the outer diameter, a length L of the cylindrical portion
is 0 L 1/3 L1 with respect to a length L1 in the width direction of
the sheet with a maximum size that allows the sheet to pass between
the pair of sheet discharge rollers, and the elastic layer has
taper shape from the both ends in the width direction of the
cylindrical portion to the both ends in the width direction of the
one sheet discharge roller.
5. The sheet discharging device as claimed in claim 2, wherein the
elastic layer of the one sheet discharge roller has a crown shape
with a maximum outer diameter in the central portion in the width
direction of the sheet.
6. The sheet discharging device as claimed in claim 2, wherein the
elastic layer of the one sheet discharge roller has a hardness
higher than that of the elastic layer of the other sheet discharge
roller of the pair of sheet discharge rollers.
7. The sheet discharging device as claimed in claim 1, wherein a
tube-like resin film is wound around the outer circumference of at
least one of the elastic layers of the pair of sheet discharge
rollers disposed on the printed surface side of the sheet.
8. An image forming apparatus comprising: an image forming portion
which forms a toner image; a fixing portion which fixes the toner
image on a sheet by heat; and a sheet discharging device which
discharges the sheet, having passed through the fixing portion,
wherein the sheet discharging device includes: a sheet stacking
portion on which sheets are staked; and a pair of sheet discharge
rollers which discharges a sheet, on which a toner image is fixed
by heat, onto the sheet stacking portion in a U-shaped curve in a
width direction perpendicular to the discharging direction of the
pair of sheet discharge rollers, wherein each of the pair of sheet
discharge rollers includes a rotating shaft extending in the width
direction and an elastic layer is provided on the rotating shaft
and continuously formed to be longer in the width direction than
the width of a sheet with a passable maximum size, and at least one
elastic layer of the pair of sheet discharge rollers is formed of a
foamed material.
9. The image forming apparatus as claimed in claim 8, wherein the
outer diameter of the central portion in the width direction, of
one sheet discharge roller of the pair of sheet discharge rollers,
is larger than the outer diameters of the both end portions in the
width direction.
10. The image forming apparatus as claimed in claim 9, wherein the
one sheet discharge roller is disposed above the other sheet
discharge roller of the pair of sheet discharge rollers.
11. The image forming apparatus as claimed in claim 9, wherein the
elastic layer of the one sheet discharge roller has, in the central
portion in the width direction, a cylindrical portion without a
change of the outer diameter, a length L of the cylindrical portion
is 0 L 1/3 L1 with respect to a length L1 in the width direction of
the sheet with a maximum size that allows the sheet to pass between
the pair of sheet discharge rollers, and the elastic layer has
taper shape from the both ends in the width direction of the
cylindrical portion to the both ends in the width direction of the
one sheet discharge roller.
12. The image forming apparatus as claimed in claim 9, wherein the
elastic layer of the one sheet discharge roller has a crown shape
with a maximum outer diameter in the central portion in the width
direction of the sheet.
13. The image forming apparatus as claimed in claim 9, wherein the
elastic layer of the one sheet discharge roller has a hardness
higher than that of the elastic layer of the other sheet discharge
roller of the pair of sheet discharge rollers.
14. The image forming apparatus as claimed in claim 8, wherein a
tube-like resin film is wound around the outer circumference of at
least one of the elastic layers of the pair of sheet discharge
rollers disposed on the printed surface side of the sheet.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a sheet discharging device
of an image forming apparatus, which uses an electrostatic
recording system and an electrophotographic recording system, and
an image forming apparatus including the sheet discharging
device.
[0003] 2. Description of the Related Art
[0004] Recently, an image forming apparatus has reduced in size and
realized high performance, and an intermediate transfer type
full-color image forming apparatus having the constitution
disclosed in U.S. Pat. No. 6,970,665 has been developed. The
invention disclosed in U.S. Pat. No. 6,970,665 is described with
reference to FIG. 5 of the present application.
[0005] As illustrated in FIG. 5, an image forming apparatus 200
includes a plurality of image forming portions 10. The image
forming portions 10 form a latent image on photosensitive drums 11,
which are image bearing members, with the use of light, magnetism,
and charge to elicit the latent image with a developer such as a
toner, and, thus, to obtain a visible image. The image forming
apparatus 200 further includes an intermediate transfer unit 30
provided above the image forming portions 10 and a secondary
transfer roller 36. The visible images from the image forming
portions 10 are sequentially transferred to the intermediate
transfer unit 30, and the intermediate transfer unit 30 forms an
image of a plurality of colors. The secondary transfer roller 36
transfers the image of a plurality of colors on the intermediate
transfer unit 30 onto a sheet P. The image forming apparatus 200
further includes a fixing unit 40 for fixing the image of a
plurality of colors, transferred onto the sheet P, onto the sheet P
and a sheet discharging portion 250 for discharging the sheet P
fixed with the image to the outside of the image forming apparatus
200. Furthermore, a sheet conveyance unit 20 for conveying the
sheet P to a secondary transfer portion Te, and a manual paper feed
tray 71 and sheet cassette 21 for feeding the sheet P to the sheet
conveyance unit 20 are disposed below the secondary transfer roller
36 of the image forming apparatus 200.
[0006] The intermediate transfer unit 30 includes a rotating
endless belt-like intermediate transfer belt 31 stretched between a
plurality of rollers, and the intermediate transfer belt 31 is
disposed to approach the image forming portions 10. In the
intermediate transfer unit 30, a primary transfer charging devices
35 are arranged at a position facing the image forming portions 10
so that the intermediate transfer unit 30 is held between the
primary transfer charging devices 35 and the image forming portions
10. The primary transfer charging devices 35 primarily transfers
the visible images, formed on the photosensitive drums 11 by an
exposure device 6, onto the intermediate transfer-unit 30. The
visible images primarily transferred from the photosensitive drums
11 are superposed on the intermediate transfer unit 30. The
intermediate transfer unit 30 rotates to thereby convey the visible
image to the position of the secondary transfer portion Te where
the visible image is secondarily transferred onto the sheet P.
[0007] When the visible image on the intermediate transfer unit 30
arrives at the position of the secondary transfer portion Te, the
visible image is secondarily transferred onto the sheet P, selected
and conveyed from the manual paper feed tray 71 or a sheet cassette
21, by the secondary transfer roller 36 and is further fixed by the
fixing unit 40. The sheet P fixed with the image is discharged
outside the image forming apparatus 200 by the sheet discharging
portion 250, whereby a full-color image can be obtained. A sheet
discharge tray 60 is disposed on the downstream side in the sheet
conveyance direction of the sheet discharging portion 250. The
sheet discharge tray 60 can receive therein the sheet P discharged
by the sheet discharging portion 250.
[0008] For the sheet discharging portion 250, as disclosed in
Japanese Patent Application Laid-Open No. 2006-117365, a plurality
of rollers 502 with a width smaller than the width of a plurality
of concavo-convex rollers 501 are arranged in a row in the width
direction perpendicular to the discharging direction of the sheet
P. The invention disclosed in Japanese Patent Application Laid-Open
No. 2006-117365 is described with reference to FIG. 6.
[0009] As illustrated in FIG. 6, the sheet discharging portion 250
includes the plurality of concavo-convex rollers 501. According to
the sheet discharging portion 250, the sheet P is stiffened,
whereby while curl of the sheet P is removed, the sheet P can be
discharged while always stabilizing the posture of the sheet P at
the time of discharge. The lead end of the sheet P discharged does
not push out the sheets P previously stacked in the sheet discharge
tray 60, but the sheet P is discharged in a state that the sheets P
are neatly arranged in the sheet discharge tray 60.
[0010] Meanwhile, for the sheet discharging portion 250, instead of
the concavo-convex rollers 501 of Japanese Patent Application
Laid-Open No. 2006-117365 and FIG. 6, USPub2007/0201892 proposes to
use a pair of sheet discharge rollers in which a rubber layer wound
around a roller shaft has a length larger than the width of the
sheet P. The invention disclosed in the USPub2007/0201892 is
described with reference to FIG. 7.
[0011] As illustrated in FIG. 7, according to a sheet discharging
portion 350, the total width of elastic layers of a sheet discharge
drive roller 503 and a sheet discharge driven roller 504 is evenly
in contact with the sheet P, and therefore, heat of the sheet P is
uniformly conducted away, whereby uneven brightness of an image can
be prevented. Further, since the edge portions of the rollers are
not in contact with the sheet P, it is possible to prevent the
traces of the rollers from being left on the sheet P.
[0012] However, when the concavo-convex rollers 501 illustrated in
FIG. 6 are used, the surfaces of the rollers are nonuniformly in
contact with the sheet P. In this case, for the sheet P having
passed through the fixing unit 40, since the portions of the sheet
P which are in contact with the roller surfaces and the noncontact
portions are different in the amount of heat of the sheet P
conducted by the roller surfaces, the cooling conditions of a toner
image fixed onto the sheet P results in difference. Therefore, the
portions which are in contact with the roller surfaces and the
noncontact portions cause the uneven brightness of the image on the
sheet P. In particular, when the sheet P with high stiffness, like
a heavy paper, is used, the contact surface between the sheet P and
the roller surface and the noncontact surface clearly appear, and
therefore, the-uneven brightness more notably occurs.
[0013] The plurality of rollers 502 are aligned in the width
direction, and the sheet P is stiffened by each edge portion of the
rollers 502. Therefore, the traces of the rollers are left on the
sheet P, softened by the heat when fixed, by the corner portions of
the rollers 502.
[0014] In addition, the sheet discharging portion 350 of U.S. Pat.
No. 6,970,665 and FIG. 7 cause the following problem.
[0015] As illustrated in FIG. 7, the sheet discharging portion 350
is constituted of the pair of sheet discharge rollers including the
sheet discharge drive roller 503 and the sheet discharge driven
roller 504 which have a uniform diameter and are through rollers
continued in the width direction of the sheet P. The sheet
discharge tray 60 is disposed on the downstream side of the sheet
discharging unit 50.
[0016] As illustrated in FIG. 7, a pair of sheet discharge roller
505 is a straight pair of roller having a uniform outer diameter
over the whole length in the width direction and does not have a
function for stiffening the sheet P. Thus, a sheet P1 is discharged
on the sheet discharge tray 60 so that a lead end portion Pa is
discharged substantially linearly with respect to the pair of sheet
discharge roller 505 and, at the same time, so that the sheet P1 is
discharged in a curled fashion in the sheet discharging direction.
At this time, the sheet P1 is discharged in the direction of the
arrow A with respect to the sheet P0, which has been discharged and
stacked on the sheet discharge tray 60, so that the entire region
of the lead end portion Pa (a dotted line region in FIG. 7) is
pressed against the sheet P0. Therefore, the sliding resistance
between the sheet P1 and the sheet P0 increases, and the sheet P0
may be pushed out in the direction of the arrow B. In addition, the
lead end portion Pa of the sheet P1 may be caught on the sheet P0
to be curled on the sheet discharge tray 60 due to a surface
property of the sheet P or a minimal burr, which is generated at
the end of the sheet P when the sheet P is cut. Thus, the sheet to
be discharged subsequently cannot be normally discharged, and the
sheet P1 is highly likely to be pushed outside the sheet discharge
tray 60, or sheet jamming easily occurs, resulting in extreme
deterioration of sheet dischargeability.
[0017] In addition, the amount of heat of the sheet P1 conducted by
the pair of sheet discharge roller 505 is different between the
front surface and the rear surface of the sheet P1 due to the
difference in material of the pair of sheet discharge roller 505,
whereby shrinkage of the sheet P1 in the cooling of the sheet P1 is
different between the front surface and the rear surface, and thus,
the sheet may be likely to be curled.
[0018] The present invention has been made in view of the above
problems, and the present invention provides a sheet discharging
device, which can increase the rigidity of a sheet in a sheet
discharging direction, stabilize the posture of a discharged sheet,
and realize the improvement of loading performance of the
discharged sheet.
SUMMARY OF THE INVENTION
[0019] The present invention provides a sheet discharging device
comprising a sheet stacking portion on which sheets are staked, and
a pair of sheet discharge rollers which nips and discharges a
sheet, on which a toner image is fixed by heat, onto the sheet
stacking portion in a convex shape in a width direction
perpendicular to the discharging direction of the pair of sheet
discharge rollers.
[0020] Each of the pair of sheet discharge rollers includes a
rotating shaft extending in the width direction and an elastic
layer is provided on the rotating shaft and continuously formed to
be longer in the width direction than the width of a sheet with a
passable maximum size, and at least one elastic layer of the pair
of sheet discharge rollers is formed of a foamed material.
[0021] According to the present invention, a pair of sheet
discharge rollers has an elastic layer. The sheet is nipped and
discharged by the pair of sheet discharge rollers in a convex shape
(U-shaped curve) in a width direction, whereby the rigidity of the
sheet in the sheet discharging direction increases, the posture of
the discharged sheet is stabilized, and the loading performance of
the discharged sheet is improved. The sheet and the roller surface
are evenly in contact with each other by the elastic layer, at
least one elastic layer is formed of a foamed material, of the pair
of sheet discharge rollers, and thus, the uneven brightness can be
prevented when an image is fixed onto the sheet. Further, since the
corner portions of the roller are not in contact with the sheet,
the traces of the rollers are prevented from being left on the
sheet.
[0022] Further features of the present invention will become
apparent from the following description of exemplary embodiments
(with reference to the attached drawings).
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 is a cross-sectional view illustrating a
configuration of an image forming apparatus according to a first
embodiment of the present invention;
[0024] FIG. 2 is an enlarged side elevational view illustrating a
configuration of a sheet discharging unit as viewed from the
downstream side in a conveyance direction of a sheet;
[0025] FIG. 3 is a schematic perspective view illustrating
configurations of the sheet discharging unit and a sheet discharge
tray;
[0026] FIG. 4 is an enlarged side elevational view illustrating a
configuration of a sheet discharging unit, used in an image forming
apparatus according to a second embodiment of the present
invention, as viewed from the downstream side in a conveyance
direction of a sheet;
[0027] FIG. 5 is a cross-sectional view illustrating a
configuration of the prior image forming apparatus;
[0028] FIG. 6 is an enlarged side elevational view illustrating a
configuration of the prior sheet discharging unit; and
[0029] FIG. 7 is a schematic perspective view illustrating
configurations of the prior sheet discharging unit and the prior
sheet discharge tray.
DESCRIPTION OF THE EMBODIMENTS
First Embodiment
[0030] FIG. 1 is a cross-sectional view illustrating a
configuration of an image forming apparatus 100 according to the
first embodiment of the present invention. The image forming
apparatus 100 is a color image forming apparatus using an
electrophotographic image forming process.
[0031] As illustrated in FIG. 1, the image forming apparatus 100
includes an image forming portion 1a for forming a yellow image and
an image forming portion 1b for forming a magenta image. The image
forming apparatus 100 further includes an image forming portion 1c
for forming a cyan image and an image forming portion 1d for
forming a black image. The image forming apparatus 100 includes
those four image forming portions (image forming unit). Those four
image forming portions 1a, 1b, 1c, and 1d are arranged in a row at
a constant interval. The image forming portions 1a, 1b, 1c, and 1d
respectively include drum-type electrophotographic photoreceptor
(hereinafter referred to as photosensitive drums) 2a, 2b, 2c, and
2d serving as image bearing members forming a visible image. Around
the respective photosensitive drums 2a, 2b, 2c, and 2d, there are
provided chargers 3a, 3b, 3c, and 3d, developing devices 4a, 4b,
4c, and 4d, drum cleaning devices 5a, 5b, 5c, and 5d. An exposure
device 6 is provided below the image forming portions 1a, 1b, 1c,
and 1d. The developing devices 4a, 4b, 4c, and 4d respectively
contain yellow, magenta, cyan, and black toners.
[0032] Each of the photosensitive drums 2a, 2b, 2c, and 2d is a
negatively charged OPC photoreceptor having a photoconductive layer
on a drum base body made of aluminum, and is driven and rotated by
a drive (not illustrated) in the direction of the arrow (the
clockwise direction) at a predetermined process speed. The chargers
3a, 3b, 3c, and 3d serving as charging portions uniformly charge
the surface of the photosensitive drums 2a, 2b, 2c, and 2d to a
predetermined negative electrode potential by a charging bias
applied from a charging bias supply (not illustrated). The
developing devices 4a, 4b, 4c, and 4d cause the toners of the
respective colors to adhere to the electrostatic latent images
formed on the photosensitive drums 2a, 2b, 2c, and 2d to develop
(visualize) the electrostatic latent images as toner images. The
developing method performed by the developing devices 4a, 4b, 4c,
and 4d includes a two-component contact developing method. In this
developing method, a developer containing a magnetic carrier mixed
with toner particles is conveyed by a magnetic force, and image
development is performed in a contact state with the photosensitive
drums 2a, 2b, 2c, and 2d.
[0033] Primary transfer rollers 34a, 34b, 34c, and 34d serving as
transfer portions are constituted of an elastic member and abutted
against the photosensitive drums 2a, 2b, 2c, and 2d through an
endless belt-like intermediate transfer belt 31 in each transfer
nip portion. In this embodiment, the primary transfer rollers 34a,
34b, 34c, and 34d are used as the transfer portions; however, there
may be used a transfer blade which is subjected to high pressure
when transferring a toner image to a transfer material and is
abutted against the intermediate transfer belt 31.
[0034] The drum cleaning devices 5a, 5b, 5c, and 5d remove and
collect a transfer residual toner remaining on the respective
surfaces of the photosensitive drums 2a, 2b, 2c, and 2d.
[0035] In the exposure device 6, a laser beam modulated in response
to a time series electric digital pixel signal of image information
is output from a laser outputting portion (not illustrated) . The
exposure device 6 exposes the surfaces of the photosensitive drums
2a, 2b, 2c, and 2d through, for example, a polygon mirror (not
illustrated) rotating at high speed. According to this
constitution, electrostatic latent images of the respective colors
corresponding to the image information are formed on the surfaces
of the photosensitive drums 2a, 2b, 2c, and 2d charged by the
chargers 3a, 3b, 3c, and 3d.
[0036] The sheet conveyance unit 20 includes a sheet cassette 21, a
cassette conveyance roller 22, which is a conveying portion,
registration rollers 23a and 23b, which are conveying portions, a
manual paper feed tray conveyance roller 24, and a manual paper
feed tray 71. The sheet conveyance unit 20 selects the sheet P in
the sheet cassette 21 or on the manual paper feed tray 71 to convey
the selected sheet P, and conveys the sheet P to the secondary
transfer portion Te which is a transfer portion. The secondary
transfer portion Te transfers the visible image, formed by the
image forming portions 1a, 1b, 1c, and 1d, to the sheet P.
[0037] In the intermediate transfer unit 30, the intermediate
transfer belt 31 is stretched between a drive roller 32 and a
tension roller 33, and the intermediate transfer belt 31 is driven
by the drive roller 32 to be rotated (moved) in the arrow direction
(the counterclockwise direction). The intermediate transfer belt 31
is formed of a dielectric resin such as a polycarbonate, a
polyethylene terephthalate resin film, or a polyvinylidene fluoride
resin film. Further, an intermediate transfer belt cleaning device
80 is provided in a position facing the tension roller 33 through
the intermediate transfer belt 31. The intermediate transfer belt
cleaning device 80 includes a cleaning blade 81 formed of an
elastic body abutted against the intermediate transfer belt 31 at a
predetermined pressure and a conveying screw 82 which conveys
residual toner removed from the intermediate transfer belt 31 by
the cleaning blade 81. The residual toner is conveyed to a toner
collection vessel (not illustrated) by the conveying screw 82.
[0038] The fixing unit 40 is provided on the downstream side of the
secondary transfer portion Te. The fixing unit 40 is a fixing
portion having a fixing roller 42 including a heat source and a
pressure roller 41. The fixing unit 40 fixes the visible image on
the sheet P by heat, transferred at the secondary transfer portion
Te, to the sheet P. Further, a sheet discharging unit 50 which is a
sheet discharging device is provided on the downstream side in the
conveyance direction of the sheet P. The sheet discharge tray 60 is
disposed on the further downstream side in the sheet passing
direction of the sheet discharging unit 50 and above the
intermediate transfer unit 30. The sheet P discharged by the sheet
discharging unit 50 is stacked on the sheet discharge tray 60
serving as a sheet stacking portion.
[0039] Next, an image forming operation performed by the image
forming apparatus 100 is described. When an image formation
starting signal is given, the photosensitive drums 2a, 2b, 2c, and
2d of the image forming portions 1a, 1b, 1c, and 1d, which are
driven and rotated at a predetermined process speed, are uniformly
negatively charged by the chargers 3a, 3b, 3c, and 3d. Then, the
exposure device 6 converts an image signal of an output image into
an optical signal by means of a laser outputting portion (not
illustrated), and the laser beam which is the optical signal
obtained by conversion scans and exposes each surface of the
charged photosensitive drums 2a, 2b, 2c, and 2d to form the
electrostatic latent image.
[0040] Subsequently, the electrostatic latent image formed on the
photosensitive drum 2a is first adhered with yellow toner by the
developing device 4a to which a developing bias having the same
polarity as the charged polarity (negative electrode polarity) of
the photosensitive drum 2a is applied, and the electrostatic latent
image is developed as the toner image. Then, the yellow toner image
is transferred onto the intermediate transfer belt 31 at a primary
transfer portion Ta by the primary transfer roller 34a to which a
transfer bias (opposite-polarity (straight polarity) against the
toner) is applied.
[0041] The intermediate transfer belt 31 transferred with the
yellow toner image is moved to the image forming portion 1b by the
drive roller 32. Likewise, a magenta toner image, formed on the
photosensitive drum 2b by the above same method, is superposed on
the yellow toner image on the sheet P, and then transferred at a
primary transfer portion Tb constituted of the image forming
portion 1b and the primary transfer roller 34b. Hereinafter,
likewise, a cyan toner image and a black toner image, respectively
formed on the photosensitive drums 2c and 2d of the image forming
portions 1c and 1d, are sequentially superposed on the yellow and
magenta toner images superposed and transferred onto the
intermediate transfer belt 31, at primary transfer portions Tc and
Td. According to this constitution, a full color toner image is
formed on the intermediate transfer belt 31.
[0042] The sheet P conveyed from the sheet cassette 21 or the
manual paper feed tray 71 is conveyed to the secondary transfer
portion Te by the registration rollers 23a and 23b according to the
timing when the lead end of the toner image on the intermediate
transfer belt 31 is moved to the secondary transfer portion Te. The
full color toner image is then transferred onto the sheet p,
conveyed to the secondary transfer portion Te, by the secondary
transfer roller 36 to which the transfer bias (opposite-polarity
(straight polarity) against the toner) is applied.
[0043] The sheet P formed with the full color toner image is
conveyed to the fixing unit 40 serving as a fixing portion which is
an image heat fixing device. After the full color toner image is
heated and pressurized by a fixing nip between the fixing roller 42
and the pressure roller 41 to be thermally fixed onto the surface
of the sheet P, the sheet P is discharged into the sheet discharge
tray 60, provided outside the apparatus, by the sheet discharging
unit 50, whereby a series of the image forming operation is
terminated.
[0044] FIG. 2 is an enlarged side elevational view illustrating a
configuration of the sheet discharging unit 50 as viewed from the
downstream side in the conveyance direction of the sheet P. As
illustrated in FIG. 2, the sheet discharging unit 50 which is a
sheet discharging device includes a pair of sheet discharge rollers
including a sheet discharge drive roller 51 and a sheet discharge
driven roller 52. A sheet discharge drive roller 51 is corresponds
to a one sheet discharge roller. A sheet discharge driven roller 52
is corresponds to an other sheet discharge roller. The sheet
discharge drive roller 51 transmits a driving force of a drive
source (not illustrated). The sheet discharge driven roller 52
faces the sheet discharge drive roller 51 and applies pressure on
the sheet discharge drive roller 51 to be biased. The sheet
discharging unit 50 discharges the sheet P, which has passed
through the fixing unit 40 and, on which a toner image is fixed by
heat, outside an image forming apparatus body 100a by the rotation
of the sheet discharge drive roller 51 and the sheet discharge
driven roller 52.
[0045] The sheet discharge drive roller 51 includes a metal shaft
51b, which is a rotating shaft, and an elastic layer 51a is
provided on the metal shaft 51b. The elastic layer 51a is formed of
a resin material such as a foamed rubber material or a foamed
elastomer, which is a foamed material, and, for example, urethane,
NBR, and EPDM are used therein. In this case, in order to reliably
grip and convey the sheet, the hardness of the rubber material is
preferably 20 to 60 (when the hardness is measured by Asker C
hardness meter). The sheet discharge drive roller 51 is connected
to a drive source (not illustrated), and is rotatably constituted
as a drive roller. The elastic layer 51a is continuously formed in
the direction along the metal shaft 51b.
[0046] Meanwhile, the sheet discharge driven roller 52 includes a
metal shaft 52b, which is a rotating shaft, and an elastic layer
52a is provided on the metal shaft 52b. Like the sheet discharge
drive roller 51, the elastic layer 52a of the sheet discharge
driven roller 52 is formed of a resin material such as a foamed
rubber material or a foamed elastomer, which is a foamed material,
and, for example, urethane, NBR, and EPDM are used therein. At
least one elastic layer of the sheet discharge drive roller 51 and
the sheet discharge driven roller 52 maybe formed of a foamed
material, the total amount of heat conducted from the sheet P is
reduced and the sheet P is stacked on the sheet discharge tray 60
with flexibility, whereby curling of the discharged sheet P is
reduced. Further, it is preferable that the sheet discharge driven
roller 52 is formed of the same material as in the sheet discharge
drive roller 51 or a material having the same heat conductivity as
that of the sheet discharge drive roller 51 to reduce curling of
the discharged sheet P. Furthermore, as in the sheet discharge
drive roller 51, in order to reliably grip and convey the sheet,
the hardness of the rubber material of the sheet discharge driven
roller 52 is preferably 20 to 60 (when the hardness is measured by
Asker C hardness meter). The elastic layer 52a is continuously
formed in the direction along the metal shaft 52b. The sheet
discharge driven roller 52 includes pressure springs 54 disposed at
the both ends of the metal shaft 52b through a bearing 53, and the
pressure springs 54 apply a predetermined pressure to the sheet
discharge drive roller 51. The pressure of the pressure spring 54
applied to the sheet discharge drive roller 51 is preferably set to
about 0.1 to 1 kg so that the conveying performance for the sheet P
can be ensured.
[0047] The elastic layer 51a of the sheet discharge drive roller 51
has a substantially symmetrical shape with respect to a center line
a of the center in the width direction of the sheet P. The elastic
layer 51a is constituted of a straight portion 61, which is a
cylindrical portion without a change of the outer diameter, and
taper portion 62 having a tapered shape with a taper amount d1 from
the both end portions of the straight portion 61b to the both end
portions of the elastic layer 51a. A region of the center line a is
at least referred to as a central portion of the sheet discharge
drive roller 51. A region of the straight portion 61 also
corresponds to the central portion of the sheet discharge drive
roller 51. In the elastic layer 52a of the sheet discharge drive
roller 51, the outer diameter D1 of the central portion is set to
be larger than the outer diameter D2 of the both end portions 62a
which are the both end portions in the width direction.
[0048] When the length L in the axial direction of the straight
portion 61 is too long, the effect of the difference between the
outer diameter D1 of the straight portion 61 of the elastic layer
51a and the outer diameters D2 of the both end portions 62a: D1-D2
may be reduced. On the other hand, if the length L is too short, a
balance of holding the sheet P is likely to be disrupted in the
axial direction, thereby leading to skew feeding of the sheet P, or
normal discharging may not be able to be performed. Thus, the
length L in the axial direction of the straight portion 61 is
preferably larger than 0 but equal to or less than 1/3 with respect
to the length L1 (see, FIG. 3) in the width direction of the sheet
P with a passable maximum size. Namely, it is preferable that
0<L.ltoreq.1/3L1 is established.
[0049] FIG. 3 is a schematic perspective view illustrating the
configurations of the sheet discharging unit 50 and the sheet
discharge tray 60. As illustrated in FIG. 3, the sheet discharge
tray 60 is disposed on the downstream side in the passing direction
of the sheet P of the sheet discharging unit 50. In this case, when
the sheet P is discharged, the sheet P is conveyed on a center
basis regardless of the size of the sheet P.
[0050] According to the sheet discharging unit 50 of the present
embodiment, the sheet discharge drive roller 51 has the difference
between the outer diameter D1 of the straight portion 61 and the
outer diameters D2 of the both end portions 62a: D1-D2. Therefore,
the circumferential speed of the sheet discharge drive roller 51 is
different between the straight portion 61 and the both end portions
62a. While the circumferential speed of the straight portion 61 is
high, each circumferential speed of the both end portions 62a is
lower than that of the straight portion 61. Thus, the sheet
discharge speed of the discharged sheet P is different between the
central portion Pb and the both end portions Pc. As illustrated by
the arrow C in FIG. 3, the central portion Pb of the lead end
portion of the sheet P is discharged at a higher speed than the
both end portions Pc of the lead end portion of the sheet P,
whereby the central portion Pb of the lead end portion of the sheet
P is discharged earlier than the both end portions Pc of the lead
end portion of the sheet P. Since the both end portions Pc of the
lead end portion of the sheet P are pulled by the central portion
Pb of the lead end portion of the sheet P, the both end portions Pc
is discharged in a slightly lifted state to follow the shape of the
taper portions 62. Thus, when the sheet P1 is discharged, the lead
end portion of the sheet P1 can be discharged while only the
central portion Pb of the lead end portion of the sheet P1 is in
contact with the sheet P0 already discharged on the sheet discharge
tray 60.
[0051] As described above, according to the image forming apparatus
100 of the first embodiment, the outer diameters of the elastic
layer 51a of the sheet discharge drive roller 51 are formed to be
substantially symmetrical to each other with the central portion in
the width direction of the sheet P as the center, and the outer
diameter D1 of the straight portion 61 is set to be larger than the
outer diameters D2 of the both end portions 62a. The elastic layer
51a has the smooth taper shapes from the straight portion 61 to the
both end portions 62a. Thus, the discharge speed of the sheet P is
set to be high in the straight portion 61, but the discharge speeds
in the both end portions 62a are set to be lower than the discharge
speed in the straight portion 61. Therefore, the sheet P is nipped
and discharged by the pair of sheet discharge rollers in a convex
shape (U-shaped curve) that the central portion Pb is downward, and
the both end portions Pc of the lead end portion of the sheet P are
slightly lifted upward in the width direction perpendicular to the
discharging direction of the pair of sheet discharge rollers.
According to this constitution, the rigidity of the sheet in the
sheet discharging direction can be increased, and the contact area
between the lead end of the discharged sheet P1 and the already
discharged sheet P0 can be reduced in size as much as possible.
Consequently, it is possible to prevent such a trouble that the
discharged sheet P1 is caught by the already discharged sheet P0 to
push out the sheet P0. Further, the sheet jamming is prevented.
Furthermore, the dischargeability and the loading performance of
the discharged sheet P can be enhanced. In addition, it is possible
to prevent, as much as possible, such a phenomenon that the lead
end portion of the discharged sheet P is caught to be curled due to
the surface property of the sheet P, which has been already
discharged on the sheet discharge tray 60, or a minimal burr, which
is generated at the end of the sheet P when the sheet P is cut. The
sheet P may be nipped and discharged by the pair of sheet discharge
rollers in a reverse convex shape (inverted U-shaped curve) to
increase the rigidity of the sheet in the sheet discharging
direction.
[0052] The sheet discharge drive roller 51 and the sheet discharge
driven roller 52 respectively have the elastic layers on the outer
circumferential sides, whereby when the sheet P is passed between
the sheet discharge drive roller 51 and the sheet discharge driven
roller 52 while being pressed therebetween, the straight portion 61
of the sheet discharge drive roller 51 can be elastically depressed
according to the thickness of the sheet P. The elastic layer 51a
can be evenly in contact with the sheet P. According to this
constitution, there is no difference in the amount of heat of the
toner image on the sheet P conducted by the surfaces of the sheet
discharge drive roller 51 and the sheet discharge driven roller 52,
and thus, the occurrence of the uneven brightness of an image can
be prevented.
[0053] The elastic layer 51a is continued in the direction along
the metal shaft 51b. The edge portion of the elastic layer 51a is
not pressed against a softened sheet, and thus, the traces of the
rollers are not left on the sheet, whereby the image with fine
printing image quality is output.
[0054] The sheet discharge drive roller 51 and the sheet discharge
driven roller 52 respectively have the foam elastic layers 51a and
52a, whereby each heat capacity of the sheet discharge drive roller
51 and the sheet discharge driven roller 52 is set to be reduced,
and, thus, the total amount of heat conducted from the sheet P is
reduced. The discharged sheet P is not cooled more than necessary,
so the discharged sheet P does not become hard. After the sheet P
is stacked on the sheet discharge tray 60 with flexibility, the
sheet P is stacked on a flat stacking surface of the sheet
discharge tray 60 and is cooled, whereby curling of the discharged
sheet P is reduced. Furthermore, the amount of heat conducted from
the sheet P is uniformed on the front and rear sides of the sheet
P, whereby corrugation or curling of the discharged sheet P is
reduced.
[0055] The sheet discharge drive roller 51 in which the outer
diameter of the straight portion 61 is larger than the outer
diameters of the both end portions 62a is disposed above the sheet
discharge driven roller 52. Thus, the sheet P is discharged by
being pressed from the central position in the width direction of
the upper surface of the sheet P. Consequently, when the sheet P
passes between the sheet discharge drive roller 51 and the sheet
discharge driven roller 52, the sheet P is likely to be discharged
while being curled into a reversed U shape. The central position in
the width direction of the discharged sheet P1 is hardly caught by
the already discharged sheet P0. On the other hand, if the straight
portion 61 and the taper portions 62 are formed in the sheet
discharge driven roller 52, the sheet P may be inconveniently
discharged while being curled into a U shape.
[0056] Since the length of the straight portion 61 is equal to or
less than 1/3 of the length L1 in the width direction of the sheet
P with a passable maximum size, the gripping force for the sheet P
of the sheet discharge drive roller 51 and the sheet discharge
driven roller 52 is suitably maintained. When the sheet P with high
stiffness, like a heavy paper, is used, the entire region in the
width direction of the sheet P can be reliably nipped. Therefore,
the uneven brightness can be prevented. If the length of the
straight portion 61 is more than 1/3 of the maximum width of the
passable sheet P, there hardly occurs a difference in the discharge
speed between the straight portion 61 in the width direction of the
sheet P and the both end portions 62a. When the straight portion 61
is not provided, a balance of holding the sheet P is likely to be
disrupted in the axial direction, thereby leading to skew feeding
of the sheet P, or normal discharging may not be able to be
performed.
[0057] As described above, according to the first embodiment of the
present invention, based on a simple constitution, the image
forming apparatus 100 can be reduced in size and cost while
maintaining high quality printed image and enhancing the
dischargeability of the sheet P.
[0058] It goes without saying that this embodiment is not limited
to a developing system and a transferring system used in the image
forming apparatus described in this embodiment, but widely
effective in a discharge portion of an image forming apparatus.
Second Embodiment
[0059] FIG. 4 is an enlarged side elevational view illustrating a
configuration of a sheet discharging unit 150, used in the image
forming apparatus according to the second embodiment of the present
invention, as viewed from the downstream side in the conveyance
direction of the sheet P. The sheet discharging unit 150 is
different from the sheet discharging unit 50 of the first
embodiment in the use of a sheet discharge drive roller 151 instead
of the sheet discharge drive roller 51. In the sheet discharging
unit 150, the same components and effects as those in the sheet
discharging unit 50 are denoted by the same reference numerals, and
the description is suitably omitted. Also in the second embodiment,
the image forming apparatus similar to that of the first embodiment
can be used, and thus the description of the image forming
apparatus is omitted.
[0060] As illustrated in FIG. 4, the sheet discharging unit 150 is
constituted of a sheet discharge drive roller 151 and a sheet
discharge driven roller 52. The sheet discharge drive roller 151 is
constituted of a metal shaft 51b, which is a rotating shaft, and an
elastic layer 151a wound around the metal shaft 51b. The elastic
layer 151a has a crown shape with a maximum outer diameter D1 in
the central portion in the width direction of the sheet P. The
elastic layer 151a is formed of a foam rubber material, which is a
foamed material, and, for example, urethane, NBR, and EPDM are used
therein. In this case, in order to reliably grip and convey the
sheet P, the hardness of the rubber material is preferably 20 to 60
(when the hardness is measured by Asker Chardness meter). The sheet
discharge drive roller 151 is connected to a drive source (not
illustrated), and is rotatably constituted as a drive roller.
[0061] Meanwhile, the sheet discharge driven roller 52 is
constituted of a metal shaft 52b and an elastic layer 52a wound
around the metal shaft 52b. As in the sheet discharge drive roller
151, the elastic layer 52a of the sheet discharge driven roller 52
is formed of a foam rubber material, and, for example, urethane,
NBR, and EPDM are used therein. Further, it is preferable that the
sheet discharge driven roller 52 is formed of the same material as
in the sheet discharge drive roller 151 or a material having the
same heat conductivity as that of the sheet discharge drive roller
151. Further, as in the sheet discharge drive roller 151, the
hardness of the rubber material of the sheet discharge driven
roller 52 is preferably 20 to 60 (when the hardness is measured by
Asker C hardness meter). The sheet discharge driven roller 52
includes pressure springs 54 disposed at the both ends of the metal
shaft 52b through a bearing 53, and the pressure springs 54 apply a
predetermined pressure to the sheet discharge drive roller 151. The
pressure of the pressure spring 54 applied to the sheet discharge
drive roller 151 is preferably set to about 0.1 to 1 kg so that the
conveying performance for the sheet P can be ensured.
[0062] The elastic layer 151a of the sheet discharge drive roller
151 has a substantially symmetrical shape with respect to the
center line a of the center in the width direction of the sheet P,
and the outer shape is a smooth crown shape with a crown amount
d2.
[0063] The image forming apparatus of the second embodiment can
obtain an effect similar to the case of the first embodiment. The
elastic layer 151a of the sheet discharge drive roller 151 has a
crown shape with the maximum outer diameter D1 in the central
portion in the width direction of the sheet P. Therefore, as an
additional effect, the elastic layer 151a of the sheet discharge
drive roller 151 can be smoothly deformed in accordance with the
shape of the sheet P in the width direction of the sheet P.
[0064] Note that the above constitution can be changed as follows.
Among the elastic layers 51a, 151a, and 52a of the sheet discharge
drive rollers 51 and 151 and the sheet discharge driven roller 52
of the first and second embodiments, a tube-like resin film may be
wound around the outer circumference of at least one of the elastic
layers 51a, 151a, and 52a disposed on the printed surface side of
the sheet. According to this constitution, even if the toner is not
completely fusion bonded to the sheet P in the fixing unit 40, the
toner is prevented from being removed by the elastic layers 51a,
151a, and 52a.
[0065] The elastic layers 51a and 151a of the sheet discharge drive
rollers 51 and 151 of the first and second embodiments may be set
to have a hardness higher than that of the elastic layer 52a of the
sheet discharge driven roller 52. According to this constitution,
each hardness of the elastic layers 51a and 151a of the sheet
discharge drive rollers 51 and 151 is higher than the hardness of
the elastic layer 52a of the sheet discharge driven roller 52.
Thus, it is possible to prevent such a phenomenon that the elastic
layers 51a and 151a of the sheet discharge drive rollers 51 and 151
are gridded by the sheet discharge driven roller 52. Consequently,
the difference between the outer diameter D1 of the straight
portion 61 and the outer diameters D2 of the both end portions 62a:
D1-D2, in the sheet discharge drive rollers 51 and 151 is
maintained for a long period of time.
[0066] Further, in the first and second embodiments, the sheet
discharging device according to the present invention is
incorporated in the image forming apparatus; however, the sheet
discharging device maybe a finisher which can be connected from the
outside of the image forming apparatus.
[0067] While the present invention has been described with
reference to exemplary embodiments, it is to be understood that the
invention is not limited to the disclosed exemplary embodiments.
The scope of the following claims is to be accorded the broadest
interpretation so as to encompass all such modifications and
equivalent structures and functions.
[0068] This application claims the benefit of Japanese Patent
Application No. 2008-201811, filed Aug. 5, 2008, No. 2009-162516,
filed Jul. 9, 2009 which are hereby incorporated by reference
herein in their entirety.
* * * * *