Note: Descriptions are shown in the official language in which they were submitted.
2~77813
B0989001X
ELECTROPHOTOGRAPHIC MACHI~E WITH EFFICIENT TRA~SFER
This invention relates to image producing machines such as
electrophotographic printers and copiers and more
particularly to improving the transfer of toner from a
developed image to an image receiving surface.
BACK~ROUND OF TH~ _NVENTIQN
Image producing machines such as electronic printers and
copiers are frequently of the electrophotographic type. In
electrophotographic machines, a print is produced by
creating an image of the print on a photoreceptive surface,
developing the image and then fusing the image to print
material. In machines which utilize plain bond paper the
electrophotographic process is of the transfer type where a
photoreceptive material is placed around a rotating drum or
arranged as a belt to be driven by a system of rollers. In
the typical transfer process, photoreceptive material is
passed under a stationary charge generating station to place
a relatively uniform electrostatic charge, usually several
hundred volts, across the entirety of the photoreceptive
surface. Next, the photoreceptor is moved to an imaging
station where, in an electrophotographic printer, it
receives light rays which are modulated in accordance with
the data to be printed. The light generator may produce
laser beams, it may be an array of light-emitting diodes, or
it may be any other suitable light source. The light rays
are directed to the photoreceptor and cause it to bear a
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B0989001X 2
charge pattern which is a latent image of the information
used to modulate the light rays. Modulation is usually
derived from a character generator which is driven by image
pattern data frequently produced by a computer and held in
digitized form in memory. Similarly, in an
electrophotographic copier, light rays are reflected from an
original document to be copied in order to modulate the
light rays in accordance with data on that original. The
reflected light rays are then directed to the imaging
station to create an electrostatic image of the original on
the photoreceptive surface.
After producing an image on the photoreceptor, the next step
in the electrophotographic process is to move the image to a
developing station where developing material called toner is
placed on the image. This material may be in the form of a
colored powder which carries a charge and is
electrostatically attracted to those areas which it is
desired to develop.
The photoreceptor, with a developed image, is moved from the
developer to a transfer station where image receiving media,
usually paper, is juxtaposed to the developed image. A
charge is placed on the backside of the paper so that when
the paper is stripped from the photoreceptor, the toner
material is held on the paper and removed from the
photoreceptor. Any toner remaining on the photoreceptor
after transfer is removed by a cleaning station before the
photoreceptor is reused. The paper is sent to a fusing
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B0989001X 3
station for permanently bonding the tranferred toner to the
paper.
In the transfer operation, it is essential that the print
paper be tightly pressed against the photoreceptive surface.
Should there be any small air gaps or bubbles between the
paper and the photoreceptive surface, poor transfer will
result and will show up as a transfer void on the print.
There may also be residual images on the next print if the
cleaning operation is unsuccessful in adequately cleaning
the untransferred toner from the photoreceptive surface. In
prior art machines, the means of attaching the image
receiving paper to the photoreceptive surface usually has
been a transfer corona generator, that is, a means for
generating electrical charge which is deposited on the
backside of the image receiving paper with a polarity
opposite to the polarity of the charge on the toner. The
result is to attract the paper to the photoreceptive surface
and thereby obtain good transfer. However, air gaps may
sometimes be present due to creases in the paper or due to
paper distortions caused by the fusing operation. These
distortions are sometimes present where fusing heat is
accumulated in areas of heavy toner deposition as opposed to
lesser heating in white background areas. These differences
in heating paper can cause paper distortions which then
create poor transfer when the same sheet is returned to the
transfer station for the application of print to the reverse
side of the paper. Another problem occurs with the use of
perforated forms or labels or fanfold paper which contain
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B0989001X 4
perforations along which the paper is folded prior to use.
Such folding causes a crease along the perforation which can
result in the paper bulging away from the photoreceptor
surface near the crease. These bulges can cause difficulty
in flattening the sheet onto the photoreceptive surface near
the perforations, and as a result it may become necessary to
inhibit printing near perforations.
This invention is designed to increase the efficiency of
transfer by mechanically flattening the sheet inside the
transfer corona and pressing the paper against the
photoreceptive surface. In that manner, charge deposited on
the backside of image receiving media will attach the media
to the transfer surface without the presence of air gaps or
bubbles either at the leading and trailing edges of cut
sheet at or near perforation or fanfold paper, or in the
body of the paper.
U.S. Patent No. 4,101,212 relates to the provision of a
sheet depressing member in the form of a roller or a plate
to forcibly depress the sheet onto the surface of the
photosensitive recording element. The depressing member,
which may be made of MYLAR, is positioned outside of the
transfer corona such that it depresses the paper against the
photoconductive surface prior to entry of the paper into the
transfer zone. As such, this technique is only marginally
effective.
78~3
B0989001X s
U.S. Patent No. 3,620,617 relates to the provision of a
MYLAR (trademark of DuPont and Co.) flap partially covering
the transfer corona station. The flap is attached to the
shield of the transfer corona, and at no time does it bear
against the print paper. The flap is positioned to receive
a negative potential so as to provide an electrostatic field
between the flap and the photoconductor. The purpose of the
flap is to provide a uniform electromagnetic transfer field
thereby eliminating streaking.
U.S. Patent No. 4,673,280 relates to the provision of a
stationary metal paper guide placed at the voltage of the
corona shield. The guide does not bear against the backside
of copy paper but the lip of the guide does extend into the
region of corona emission. The purpose of the lip is to
limit the application of the transfer field to a
predetermined angular extent around the photoconductor drum
and the purpose of applying voltage to the guide is to
prevent charge from leaking laterally off the paper while it
is guided toward the photoconductor drum. This is another
attempt at producing more efficient transfer.
U.S. Patent No. 4,110,027 relates to a printer in which
continuous fanfold paper is used. Two rollers are used with
one roller positioned prior to and the second roller
positioned subsequent to the transfer corona to hold paper
to the photoconductor drum during transfer. These rollers
do not bear against the backside of the copy paper at any
point in the area of corona emission.
Z~ 88
~0989001~ 6
It is an object oE this invention to obtain improved-
efficiency in the transfer of a developed image to image
receiving material.
It is a further object of this invention to prevent or
minimize the formation of air gaps near the leading and
trailing edges of cut sheet print receiving material, or
near the perforations present on fanfold type image
receiving material.
It is another object of this invention to prevent the
formation of air bubbles within the body of image receiving
material.
It is still another object of this invention to provide a
paper support member to press paper against the
photoreceptive surface within the region of peak charge
distribution under the transfer corona, such that as the
paper leaves the support member it immediately receives
charge from the transfer corona, thus attaching the paper to
the photoreceptive surface without the formation of air
bubbles.
It is another object of the invention to provide support
members of more than one length so that papers of different
size may be firmly pressed against the photoreceptive
surface at the transfer station.
2(~7~88
B0989001X 7
It is yet another object of the invention to provide a paper
support member which can be moved to a home position out of
contact with the photoreceptive surface when image receiving
material is not present at the transfer station.
It is another object of the invention to provide a support
member made of metal such as spring steel which in turn is
covered by a plastic wear resistant insulating member.
z~7a~
BO989001X 8
SUMMARY OF THE INVENTIO~
This invention relates to the positioning of a flexib~e
support member near the transfer station of an
electrophotographic machine employing a transfer corona
generator. The end of the support member must extend into
the region of corona emission in the transfer zone and
should be positioned near the region of peak current
distribution for maximum benefit. The purpose of the
flexible support member is to bear against the backside of
image receiving media in order to press the media against
the photoreceptive surface. By extending the flexible
support member into the transfer zone, media is held against
the support surface such that it receivas charge from the
transfer corona as it leaves the line of release from the
mechanical pressure exerted on it by the support member.
Thus media is electrostatically attached to the
photoreceptive surface before air gaps have a chance to
form. In that manner, voids or air bubbles between the
media and the photoreceptive surface are eliminated, thereby
improving transfer and decreasing the amount of residual
toner left on the photoreceptive surface after transfer.
2~ 7788
B0989001X g
BRIEF DESCRIPTION OF THE DRAWING
The above-mentioned and other features and objects of this
invention and the manner of attaining them will become more
apparent and the invention itself will best be understood by
reference to the following description of embodiments of the
invention taken in conjunction with the accompanying
drawings, the description of which follows.
FIG. 1 is an illustration of an electrophotographic machine
in which the principles of the instant invention can be
gainfully employed.
FIG. 2 shows the transfer station of the machine of FIG. 1
with the flexible support member of the instant invention
positioned within the area of transfer corona emission.
FIG. 3 shows a perspective view of the support member shown
in FIG. 3.
FIG. 4 illustrates the support member, together with the
photoreceptive surface.
2~3'7788
BO989001X 10
DESCRIPTION_OF A PREFERRED EMBODIMENT
FIG. 1 illustrates an electrophotographic copier or printer
which can benefit from use of the instant invention. Copy
or print image receiving material, such as paper, which may
be either cut sheet paper or fanfold paper, is loaded into
input bin lO from which it is sent along a relatively
straight through paper path to exit bin 11. Transfer of the
image from photoreceptive surface 12 to the image receiving
material is accomplished at transfer station 13.
In the machine, shown in FIG. 1, photoreceptive material 12
is mounted on the surface of a revolving drum 14 which moves
in the direction A. Charge corona generators 15 place a
relatively uniform electrostatic charge across the
photoreceptive surface 12 as it rotates to thereunder. An
image is placed on the photoreceptive surface through the
application of image bearing light rays 16. Note that in
the particular machine configuration of FIG. 1, the imaging
station is located at the same position as the transfer
station 13.
As the photoreceptive material (photoconductor3 continues to
rotate, the photoreceptive material 12, now bearing an
image, moves to a developer station 17 where toner is placed
on the image. The now de~eloped image passes under the two
charge coronas 15, which are turned off, to a pretransfer
lamp 18 and on to the transfer station 13 where the
developed image is transferred to image receiving material.
r7 ~ ~3 8
B0989001X 11
Such material has been sent to the transfer station from
input bin lO and is controlled such that the leading edge of
the image receiving material mates with the leadiny edge of
the image. As the image receiving material is stripped away
from the photoconductor, the toner is transferred to the
image receiving material. After transfer, the
photoreceptive surface continues to rotate to the developer
17, which is now used as a cleaning station to clean away
residual toner left after the transfer operation. The
photoreceptive material is then sent to the charge corona 15
for the beginning of a new cycle of a image reproduction.
The machine of FIG. l, described above, is a two cycle
machine, that is, a machine that requires two revolutions of
the photoreceptive surface in order to produce one print.
On the first cycle of the drum revolution, as explained
above, the photoreceptive surface is charged, imaged, and
developed. On the second cycle, the developed image is
transferred to copy paper and residual toner is cleaned from
the photoreceptive surface.
The machine shown 1n FIG. 1 contains certain other elements
located around the drum, including a detector 19 for sensing
the failure of paper to detach from the drum after transfer.
A preclean erase lamp 20, an edge erase lamp 21, and a
sensor 22 for determining the density of toner developed on
the photoreceptive surface are also located next to the
drum. An electrostatic probe 23 is utilized for sensing the
charge level placed on the photoreceptive surface by charge
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B0989001X 12
corona 15, and an interimage erase lamp 24 is used to erase
charge prior to development in those areas which are not
touched by light imaging rays.
After transfer, the image receiving material is transported
on the underside of a vacuum transport 26 to a fusing
station 27 for fusing the toner into the image receiving
material. If cut sheet paper is being used, duplex copies
may be produced, that is, a print with an image on both
sides of the paper, by returning the paper along a duplex
path 28 to the transfer station 13 for application of an
image to the reverse side.
FIG. 2 is an illustration of the transfer station 13 showing
a paper inlet guide 30. Paper or other image receiving
material is directed to the transfer station on the
underside of the paper inlet guide 30 as shown by arrow B.
A transfer corona generator is present at the transfer
station 13 with the transfer zone defined by the side plate
31 on the inlet side and the side plate 32 on the exit side.
Corona wires 33 and 34 extend across the photoreceptive
surface 12 and through the application of voltages to these
wires, a corona emission occurs which causes a flow of ions
from the wires to the charged photoreceptive surface 12 as
it moves through the transfer zone. With the interposition
of image receiving material between the corona wires and the
photoreceptive surface ions strike the backside of the copy
paper and electrostatically attach the copy paper to the
surface 12.
z~.;o77a~
B0989001X 13
A flexible member 40 made from a plastic insulating material
such as polyetherimide is mounted on a flexible metal member
41 which is made from a material such as spring steel. The
combination of the metal member 41 and the insulating member
40 comprises the flexible support member of this invention,
providing a constant spring rate and precluding set, thereby
minimizing the stress on the plastic member 40. The metal
member 41 is positioned to make contact with a copy sheet
and is the primary member for bearing against the backside
of paper to eliminate transfer voids. Plastic insulating
member 40 also makes contact with the paper. Metal member 41
is fastened to shaft 44. A similar combination of a metal
member 43 with plastic member 42 is also present with the
metal member 43 also fastened to shaft 44. The presence of
two flexible support members is for purposes of providing
for two different size copy sheets. For example, member 40
is slightly less wide than 11 inches and is used to bear
against the backside of 11 inch paper. Member 41 is
slightly less wide than 3 inches and is used together with
member 40 to bear against the backside of 14 inch paper.
FIG. 3 shows a perspective view of shaft 44 with flexible
steel members 41 and 43 fastened thereto. Plastic members
40 and 42 are fastened to steel members 41 and 43. Shaft 44
contains an indented area 45 through which members 42 and 43
take a position slightly more removed from photoconductor 12
2~.r~37 ~8&
B0989001X 14
as shown in FIG. 2. Note that the view of members 40-44
shown in FIG. 2 is taken along section line 2-2 in FIG. 3.
FIG. 3 also shows that plastic insulating members 40 and 42
completely cover metal members 41 and 43 in order to shield
these members from corona emission and thereby prevent
arcing.
FIG. 4 shows the flexible members 40 and 42, together with
shaft 44 in position across the photoreceptive surface 12.
In working position, members 40 and 42 are in contact with
the backside of image receiving material. Metal members 41
and 43 are hidden from view in FIG. 4, but in the working
position are also in contact with the image receiving
material.
Shaft 44 is connected to stepper motor 46 such that
rotation of motor 46 moves the flexible support member 40
from a home posit:ion to a working position in which members
40-43 are in contact with the backside of image receiving
media (14 inch paper~. At the home position, members 40-43
are out of contact with the photoreceptive surface 12, so
that the surface can pass underneath the flexible support
members without touching. The tip 47 of member 40 is that
portion of the insulating member which bears against image
receiving material. Tip 47 defines a "line of release" from
which the image receiving material moves out o the
influence of the mechanical pressure exerted by member 40 on
the material.
7~8~
~0989001X 15
In operation, for 11 inch paper, as copy paper moves across
photoreceptive surface 12 into the transfer zone, stepper
motor 46 moves the flexible support members 40 and ~1
against the backside of the copy paper such that contact is
made just after the leading edge of the paper has moved
beyond the tip 47 of flexible insulating member 40. If 14
inch paper is in use, stepper motor 46 moves ~lightly
further so that flexible insulating member 42 together with
member 43 come into contact with the copy paper. Since
movement of the flexible support members 40-43 into pressure
exerting position does not occur until the leading edge of
copy paper is present, the members 40-43 do not contact the
photoreceptive surface directly thereby preventing damage to
that surface. Likewise, there is no contamination of the
support member with toner.
In addition to eliminating transfer voids in the body of a
sheet, the support member has the additional advantage of
improving transfer at the leading and trailing edges of the
sheet, thus reducing the possibility of paper leaving the
photoreceptive surface and crashing into the exit sidewall
32 of the transfer corona. The support member also makes it
possible to operate under a wider latitude of temperature
and humidity conditions by attaching the paper more firmly
to the photoconductor. It should be noted, that in addition
to the preferred embodiment described above, a series of
holes in the member may be added so that some electrostatic
tacking of the paper to the photoconductor can begin even
before the paper moves beyond the tip of the support
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B0989001X 16
members. It should also be no-ced that the dimension of the
support member must be slightly narrower than the paper
dimension to allow for paper registration tolerances. As a
consequence, the edges of paper may be subjected to
electrostatic tacking prior to the time that the body of the
sheet leaves the tip of the support member. As a result,
the edges may become tacked to the photoreceptive surface
too early, causing paper distortions to be trapped in the
copy sheet. To prevent such a condition, a section may be
added to the support member to block the transfer current to
the sheet at the edges. Such a section must be positioned
so that it does not directly contact the photoreceptive
surface.
It is preferred that the tip 47 of insulating members 40 and
42 be located near the region of peak corona current, that
is, directly under the corona wire 33 as shown in FIG. 2.
In that manner, ions produced by corona wire 33 strike the
backside of the image receiving material immediately upon
its release from support members 40 and 42, thereby
electrostatically attaching the material to the
photoconductor before any air gaps are allowed to form.
However, as long as the support member extends into the
transfer zone the benefit of attaching the image receiving
material to the photoconductor will be achieved to a degree.
The maximum benefit is achieved when the tip 47 is
positioned directly under the corona wire.
As noted above, the metal members 41 and 43 are the
primary members for mechanically bearing down on the paper
7~
B0989001X 17
to eliminate voids. Insulating members 40 and 42 should
also touch the paper, but usually do not apply significant
mechanical force. This is the arrangement of the preferred
embodiment since plastic insulating material may produce a
triboelectric effect when forcibly engaged with the paper.
Such an effect electrically charges the paper and may cause
premature transfer of toner from the photoreceptive surface
to the paper which can create a problem in background areas.
Metal members 41 and 43 avoid that problem and also give
considerably longer wear which increases the replacement
interval. Nevertheless, in some machines the triboelectric
effect may not be significant and in such case the
advantages of this invention could be realized by plastic
member contact only.
It has also been found that plastic material with
relatively high glass transisiton temperatures are
desirable, that is to say, polyetherimide is more desirable
as the insulating member than is polyester, even though
polyester wears longer. The combination of crystallinity
and relatively low glass transition temperature for
polyester may lead to a softening or melting of abraded wear
products which then tend to collect at the edge of the
member thus lengthening it in an irregular manner. In some
applications this may not be a problem and polyester can be
a suitable alternative.
While it is desirable for both the insulating member
and the metal member to contact the surface, worst case
tolerances may allow for initial contact by the plastic
member with metal member contact occurring as the plastic
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B0989001X 18
member wears in. Such tolerances may also allow for metal
member contact only, but that would chance a small increase
in air gaps at the leading edge.
It may be surprising that a metal member in contact
with paper does not bleed away transfer charge on the paper,
especially under high humidity conditions. Such bleed-off,
however, is time dependent and at sufficiently high process
speeds does not detrimentally affect image transfer.
While the invention has been particularly shown and
described with reference to a preferred embodiment which
shows photoconductor mounted on a drum surface, it will be
understood by those skilled in the art that a photoconductor
mounted in a belt arrangement is also within the ambit of
the invention. Also, it should be understood that the
flexible nature of the support member can be achieved in
various ways. For example, the preferred embodiment
described above is made from flexible plastic. An
inflexible material could be used with flexibility given to
the member by using springs, torsion motors, or other
flexible mounting arrangements. The foregoing and other
changes in form and details may be made therein without
departing from the spirit and scope of the invention.
