U.S. patent number 3,729,311 [Application Number 05/162,897] was granted by the patent office on 1973-04-24 for electrostatic transfer method.
This patent grant is currently assigned to Xerox Corporation. Invention is credited to Michael J. Langdon.
United States Patent |
3,729,311 |
Langdon |
April 24, 1973 |
ELECTROSTATIC TRANSFER METHOD
Abstract
An improved method of successively transferring a plurality of
powder images, such as toner images of different colors, from an
electrostatic plate onto a single copy sheet by employing an
electrically biased transfer drum to bring the copy sheet in
contact with the electrostatic plate during successive transfer
steps. During transfer of the first toner image, the transfer drum
is biased at a first potential and during the transfer of all
succeeding toner images, the transfer drum is biased at a potential
greater than the first potential, the potential applied to the
transfer drum during any one succeeding transfer step being of
greater value than the potential applied thereto during the
immediately preceding transfer step.
Inventors: |
Langdon; Michael J. (Penfield,
NY) |
Assignee: |
Xerox Corporation (Stamford,
CT)
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Family
ID: |
22587577 |
Appl.
No.: |
05/162,897 |
Filed: |
July 15, 1971 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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830426 |
Jun 4, 1969 |
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Current U.S.
Class: |
430/47.1;
399/314 |
Current CPC
Class: |
G03G
15/0131 (20130101) |
Current International
Class: |
G03G
15/01 (20060101); G03g 013/22 (); G03g 013/16 ();
G03g 015/22 () |
Field of
Search: |
;96/1.2,1.4 ;117/17.5
;355/4,14,17 |
References Cited
[Referenced By]
U.S. Patent Documents
Other References
andrus et al., "Principles of Image Transfer and Fixation"
Xerography and Related Processes, Focal Press (1965) pp.
391-398..
|
Primary Examiner: Van Horn; Charles E.
Parent Case Text
This is a continuation-in-part of application Ser. No. 830,426
filed June 4, 1969, and now abandoned.
Claims
What is claimed is:
1. The method of producing a copy of a color original upon a sheet
of final support material including
reducing the original color subject matter into a finite number of
color components,
xerographically recording each color component as a charged toner
image upon an image retaining plate,
electrically attracting each toner image from said plate to a sheet
of final support material whereby at least a portion of each
successive image is transferred onto said sheet of final support
material in registration over a previously transferred image,
and
altering the electrical force of attraction for each successive
image transfer step to compensate for changes in the electrical
characteristics of the image receiving sheet of final support
material due to the presence of a previously transferred image
thereon so as to produce optimum image transfer during each
successive toner image transfer operation.
2. The method of claim 1 including the further step of fixing said
toner image to said receiving body upon the completion of the last
transfer step.
3. In a xerographic process for reproducing color originals wherein
the original is separated into a number of color components and
each of the components is then recorded as a series of charged
toner images on a moving photosensitive plate, the plate being
adapted to move the color component images seriatim through an
image transfer zone, the improvement comprising,
arranging an electrically biased transfer member to move through
the transfer zone wherein the transfer member electrically
communicates with the charged toner images supported on the plate
surface so as to attract the images from the plate towards said
transfer member,
securing a sheet of final support material to said transfer member
to receive the images attracted thereto,
advancing the sheet on said transfer member repeatedly through the
transfer zone in registration with each of the color component
images supported on the plate surface whereby at least a portion of
each successively transferred toner image placed on the support
sheet is superimposed over a previously transferred image, and
increasing the bias potential applied to the transfer member for
each successive transfer step to compensate for changes in the
electrical characteristics of the image receiving support sheet
caused by the addition of previously transferred charged toner
images thereto whereby each color component image is efficiently
transferred from said plate to said receiving body to faithfully
recreate the original input scene information.
4. The method of claim 3 further including the step of fixing the
color component images to the final support sheet upon the
completion of the final transfer operation.
5. The method of claim 3 further including the step of fixing the
color component toner images to the final support sheet upon the
completion of the final transfer operation.
6. The method of creating a full color copy of a color original
upon a sheet of final support material including
separating the original color information into primary color
components,
recording a first color component as a charged toner image upon an
image retaining plate,
placing a sheet of final support material in overlying contact with
said first color component image,
establishing an electrical field within the contact zone to attract
the first color component image from said plate to said sheet of
final support material,
removing said sheet from said plate and recording a second color
component as a charged toner image on said plate,
replacing the support sheet in overlying contact with the second
color component image on said plate whereby at least a portion of
said second color component image is superimposed in registration
over said first image,
establishing a second electrical field within the contact zone to
attract the second color component image in superposition over said
first color component image, said second electrical field being of
a strength greater than said first electrical field to compensate
for changes in the electrical characteristics of the final support
material caused by the deposition of the first color component
image thereon,
removing the sheet from said plate and repeating the image
recording and transfer steps to produce a full color rendition of
the original subject matter on said final support sheet.
7. The method of claim 6 wherein the original subject matter is
separated into the primary color components of red, green and blue
and images thereof recorded with cyan, magenta and yellow
toners.
8. The method of claim 7 wherein the strength of the electrical
transfer field is increased uniformly after the completion of each
image transfer step.
Description
BACKGROUND OF THE INVENTION
This invention relates to a method of transferring toner images
from an electrostatic plate to a copy sheet and, more particularly,
to the successive transfer of powder images onto the same copy
sheet.
In a copying system such as those which employ transfer or powder
imaging, a uniform electrostatic charge is generated on the surface
of a photoconductive plate and the plate is exposed to a light
image conforming to the information to be copied. A latent
electrostatic image is created which is then developed with a
finely divided powder material, referred to herein as toner. The
toner image thus created is transferred from the surface of the
plate to a copy sheet thereby forming a copy of the information
being reproduced.
One technique used to transfer the toner image from the surface of
the plate to the copy sheet is to place the copy sheet between the
plate and a transfer drum having a conductive core and a relatively
non-conductive surface material and apply an electrical potential
to the core of the transfer drum as it rotates to bring the copy
sheet in contact with the plate. The application of potential to
the transfer drum forms a field between the transfer drum and plate
which causes the toner image to be attracted to the copy sheet. As
a result of this attraction, the toner remains on the copy sheet
when the latter is removed from the plate. This transfer technique
works well to transfer toner images onto clean copy sheets over a
range of voltages, however, when it is desirable to transfer more
than one toner image to the same copy sheet in successive transfer
steps, the transfer efficiency decreases as toner builds up on the
copy sheet. As a result of this decrease in transfer efficiency,
only portions of toner images are transferred to the copy sheet in
succeeding transfer steps while the remainder of the toner images
remain on the electrostatic plate. It is believed that transfer
efficiency decreases as toner builds up on the copy sheet because
the layer of toner already of the sheet decreases the attractive
force on the transfer drum during succeeding transfer steps.
In copying systems in which a plurality of different color toner
images are transferred to a single copy sheet in successive
transfer steps, this decrease in transfer efficiency described
above becomes critical. For instance, multicolor copies can be made
by forming toner images of different colors and successively
transferring the toner images onto the copy sheet on top of one
another to form a multicolor copy composed of the different color
images. If transfer efficiency is allowed to drop off in the manner
described above as successive toner images of different colors are
transferred to the copy sheet, the colors which are transferred
first will become dominant and will cause an unbalance of color in
the final multicolor image on the copy sheet. The method of
transferring a plurality of toner images to a copy sheet described
herein eliminates the problem of decreasing transfer efficiency as
the toner builds up on the copy sheet. In multicolor copying
systems in which a plurality of color toner images are transferred
to a single copy sheet to form a composite multicolor copy, the
employment of the present transfer method enables each color image
to be transferred with approximately the same transfer efficiency
thereby assuring that the final toner image on the copy sheet
maintains good color balance.
The method of transfer disclosed herein applies a successively
increasing electrical potential to the conductive core of a
transfer drum as successive transfer steps are carried out. The
effect of the increased potential is to offset the decrease of the
attractive force on a toner image being transferred due to the
presence of toner already on the copy sheet. As a result, the
transfer efficiency of each successive transfer step remains
substantially constant regardless of the amount of toner already on
the copy sheet.
Accordingly, it is a further object of the invention to improve the
transfer of a plurality of toner images from an electrostatic plate
to the same copy sheet.
It is a further object of the invention to improve the transfer of
plurality of toner images of different colors from electrostatic
plate to the same copy sheet.
It is a further object of the invention to improve the transfer of
toner images from an electrostatic plate onto a copy sheet over a
toner image already n the copy sheet.
It is a further object of the invention to maintain the same
transfer efficiency during successive transfer steps in a
multi-transfer process in which a plurality of toner images are
transferred to the same copy sheet on top of one another.
SUMMARY OF THE INVENTION
The present invention is a method of transferring a plurality of
toner images onto the same copy sheet. A copy sheet is brought into
contact with each toner image after it is formed on an
electrostatic plate by a transfer drum having a conductive core and
a relatively non-conductive material on its surface. The electrical
potential applied to the core of the transfer drum during each
successive transfer step is increased over that of the immediately
preceding transfer step so that substantially the same attractive
force attracts each toner image throughout the multiple transfer
process thereby assuring good efficiency in all transfer steps.
BRIEF DESCRIPTION OF THE DRAWINGS
For a better understanding of the invention as well as other
objects and further features thereof, reference is had to the
following detailed description to be used in conjunction with the
accompanying drawings wherein:
FIG. 1 is a schematic illustration of the invention in a color
copying machine.
FIG. 2 is a cut-away view of the transfer drum.
FIG. 3 is a partial section view of FIG. 2 taken through section
3--3.
FIG. 4 is a partial section view of FIG. 2 taken through section
3--3.
FIG. 5 is a partial section view of FIG. 2 taken through section
4--4.
FIG. 6 is a view of the cam and follower which together control the
operation of the register stops and gripper fingers on the transfer
drum.
FIG. 7 is a rear view of the transfer drum and supporting
mechanisms.
FIG. 8 is a detailed view of the mechanism which controls the
position of the cam shown in FIG. 6.
FIG. 9 is a side view of the transfer drum and supporting
apparatus.
FIG. 10 is a block diagram of the control device which steps up the
electrical potential on the transfer drum.
DESCRIPTION OF THE PREFERRED EMBODIMENT
The apparatus described herein is an electrostatic reproduction
system which reproduces a multicolor copy from a multicolor
original such as a document. Referring to FIG. 1 there is shown a
color copying machine which reproduces an original in color
xerographically. The apparatus for reproducing multicolor images
includes an electrostatic drum or photoreceptor 90, the surface of
which can include a photoconductive material overlying a conductive
material, which rotates through various xerographic processing
stations; charging station 10, exposing station 20, developing
station 30, transfer station 40, and cleaning station 50. The
photoreceptor rotates with shaft 91 in the direction indicated by
the arrow through the stations mentioned above in a processing
cycle, the approximate positions of the various stations being
shown by the brackets next to the drum surface in FIG. 1.
The photoreceptor makes a plurality of revolutions; for instance,
two or three revolutions, through the processing stations in order
to carry out a multicolor copying cycle. During each revolution, a
latent electrostatic image corresponding to one of the colors in
the original is formed on the surface on the photoreceptor and
developed with a finely divided, pigmented material such as toner
of the corresponding color, and, then, the toner image is
transferred from the photoreceptor to a copy sheet at the transfer
station. The toner images, each of a different color, that are
formed on the photoreceptor in each revolution of a copying cycle
are transferred to the copy sheet in registration with one another
and the composite toner image resulting on the copy sheet after the
copying cycle has been completed is a multicolor copy of the
original.
In the machine shown in FIG. 1, the developing station contains
three separate developing assemblies 31, 32, and 33. Although each
developing assembly is mechanically similar, the color of the toner
applied to the surface of the photoreceptor 90 by each developing
assembly is different. In the apparatus shown, for example, the
toner colors in the developing assemblies 31, 32, and 33 are
yellow, cyan, and magenta, respectively. The three toner colors can
be developed in any convenient color order and the different color
images formed can be placed on the copy sheet in any order. The
developing assemblies are selectively operated during a copying
cycle so that only one of the assemblies applies toner to the
surface of the photoreceptor during each revolution. Thus, in the
system shown in FIG. 1, during the first revolution yellow toner is
applied to the surface of the photoreceptor by developing assembly
31 while developing assemblies 32 and 33 remain in an inoperative
condition. Then, during the second revolution of the photoreceptor,
cyan toner is applied to the surface of the photoreceptor by
developing assembly 32 while developing assemblies 31 and 33 remain
in an inoperative condition. Finally, during the third revolution
of the photoreceptor, magenta toner is applied to the surface of
the photoreceptor by developing assembly 33 while developing
assemblies 31 and 32 remain inoperative. In this manner, toner
images of each of the three developing colors used in the apparatus
shown in FIG. 1, yellow, cyan, and magenta, are formed on the
photoreceptor and then transferred to the copy sheet during
successive revolutions of the photoreceptor.
A latent electrostatic image is formed on the surface of the
photoreceptor during each revolution by first placing a uniform
charge on its surface and then exposing the charged surface to a
light image corresponding to the particular color toner being
applied to the photoreceptor by a developing assembly during that
revolution. Any suitable device 21 can be used to form the light
images. Similarly, any suitable charging means can be utilized at
station 10 to charge the surface of the photoreceptor such as the
corona charging device indicated by reference number 11. Exposing
station 20, in addition to having means to expose the photoreceptor
to light images as described above, can include an inter-image
erasing device which dissipates, or erases, the charge on the
surface of the photoreceptor between latent images. This apparatus
can be any suitable device for dissipating the charge on the
photoreceptor such as an electroluminescent panel 22 which is
activated only between latent images. The inter-image erasing
device is a desirable feature when a latent image formed on the
photoreceptor does not completely cover the periphery of the
photoreceptor since these areas would otherwise completely develop
out as the photoreceptor passes through the developing station and
cause a waste of toner.
The developing station 30 includes three identical developing
apparatus 31, 32, and 33 which apply toner particles to the latent
image on the photoreceptor surface. These three developing
assemblies, all of which are normally in an inoperative condition,
are brought into an operative condition selectively in accordance
with the color toner to be placed on the photoreceptor during any
particular revolution. The housing of developing assembly 32 is
broken away so that the internal elements of the assembly can be
seen. The members which apply toner to the photoreceptor are
magnetic brushes 35 which bring magnetic developer, a mixture of
magnetic carrier particles and toner particles, into contact with
the surface of the photoreceptor 90. The developing assembly is
contained within housing 39 and is replenished with toner particles
from supply container 34 as the toner is used. The developer is
moved to the upper portion of the housing, which contains magnetic
transporter 37 and movable gate 38, by agitator 36. Upon reaching
the upper portion of the housing the developer is attracted to the
magnetic transporter which rotates in the clockwise direction to
convey the developer toward the vicinity of gate 38.
Depending upon the position of gate 38, the developer is either
passed onto magnetic brushes 35 to be applied to the photoreceptor
or is dropped from the transporter directly into the lower portion
of the housing without touching the magnetic brushes. The
developing assembly is in an inoperative condition when the gate 38
is in the position shown in the solid lines. In this position the
gate acts as a guide to direct the developer onto the magnetic
transporter away from the magnetic brushes and down directly into
the lower portion of the housing. When the gate is in the position
shown in dotted lines the developing assembly is placed in the
operative condition. In the inoperative position the gate acts as a
scrapper and guide which frees the developer from the magnetic
transporter 37 and directs it onto the magnetic brushes 35. The
developer is brought into contact with the surface of the
photoreceptor by the upper magnetic brush where it is again brought
into contact with the surface of the photoreceptor. The toner
particles in the developer are attracted from the carrier particles
to the photoreceptor as the developer is placed adjacent the
photoreceptor by the magnetic brushes thereby developing the latent
image thereon.
Each developing assembly operates in the manner described above,
the latent image on the photoreceptor being developed by that
particular developing assembly which has its gate in the position
shown in dotted lines. Due to the simplicity of the gate device
described above, each developing assembly can be maintained in a
standby condition since its agitator, magnetic transporter and
magnetic brushes can continue to rotate even through the assembly
is not applying toner particles to the photoreceptor.
After the toner image is formed on the surface of the photoreceptor
it is transferred from the photoreceptor to a copy sheet in the
transfer station 40. Transfer drum 42 is adapted to convey a copy
sheet through the transfer station in contact with and in
registration with the toner image on the photoreceptor. In sheet
feeding apparatus 70, an individual copy sheet is fed to the
transfer drum as needed from a stack of sheets 72 by feed roller 71
which moves the sheets through guides 73 and onto the surface of
the transfer drum. The copy sheet is fastened to the transfer drum
42 by a series of grippers and the drum carries the copy sheet in
three revolutions through the transfer station to transfer the
plurality of color images to the copy sheet. The transfer drum has
the same size circumference as the photoreceptor and both rotate at
the same speed, therefore, once the copy sheet is aligned with the
grippers on the drum it is also in registration with the
photoreceptor during all three transfer steps. The transfer station
40 includes pretransfer corona charging device 41 which adjusts the
electrostatic charge on the toner particles to prepare the toner
image from transfer to the copy sheet. The transfer drum 42 has a
conductive core with a layer of relatively non-conductive material
on its periphery. An electrical bias is applied to the conductive
core of the transfer drum during the transfer step to create an
electrostatic field between the photoreceptor and a copy sheet
which urges the toner image from the photoreceptor to the copy
sheet.
After making a plurality of revolutions on the transfer drum the
copy sheet is stripped from the surface of the transfer drum by
fingers 64 and conveyed into a fusing apparatus 60 by belt
conveyors 62 and 63 where the fusing housing 61 fixes the toner
image to the copy sheet. After the toner image is fixed to the copy
sheet, the copy sheet is guided into vacuum transport assembly 80
by guides 65. The vacuum transport assembly includes a plurality of
belts 83 and holes 82 to impose a vacuum between the belts. The
vacuum and belt arrangement carries the copy sheet from the area of
guides 65 towards storage tray 84 where it is stored.
After each toner image is transferred to the copy sheet, the
surface of the photoreceptor is cleaned in preparation for
subsequent revolutions in station 50. Cleaning station 50 includes
a precleaning corona charging device 51 and a brush cleaning device
52 which act together to remove any residue toner remaining on the
surface of the photoreceptor after transfer has taken place. Any
toner that is removed by brush 52 is withdrawn from the brush into
a filter bag apparatus 53 where it is held separate from the
reproduction system.
Referring to FIG. 2, the transfer drum has a hollow, conductive
core 102 with a layer 104 of insulating material. The cylindrical
core 102 is made of any suitable conductive material such as
aluminum and has a thin wall in order that the drum is lightweight
and that the various mechanisms inside the drum have adequate room
to function. The end plates 103 and 105, which are supported by and
turn with main transfer drum shaft 101, are made of any suitable
insulating material such as insulating plastic which effectively
insulates the conductive core 102 from other parts of the machine.
The outside surface of the conductive core contains a layer 104 of
any suitable relatively non-conductive material, such as a
rubber-like insulating material, upon which the copy sheet is
supported as it is brought through the transfer station. The layer
104 is preferably made of a rubber-like material so that it yields
and, consequently, is not likely to mar the photoreceptor if it
should be brought into contact with it.
A ring 116, made of a conductive material such as brass, is
fastened to end plate 105 in a concentric manner with shaft 101 and
functions to transmit an electrical bias from stationary brush 117
to the conductive core of the drum. The brass ring 116 is secured
to the end plate 105 by a plurality electrically conducting, metal
screws 118. The metal screws pass through the end plate and are
threaded into support ring 119 which, in turn, is in contact with
the conductive core 102. The electrical bias on brush 117 passes
through the ring 116, the screws 118, and support ring 119 to
conductive core 102. The other end plate of the transfer drum end
plate 103, also has a support ring 120 and is fastened thereto by
screws 125.
The transfer drum contains three sets of registration stops 113 and
gripper fingers 112 which are keyed to shaft 111 and which operate
to register and grip the leading edge of the copy sheet before the
drum takes it through the transfer station. Shaft 111 and frame 124
are mounted on the support plates 130 and 132, shaft 111 being able
to rotate about its longitudinal axis in response to the movement
of the follower arm 106. Arm 123, together with a spring 115 (shown
in FIG. 3) which biases the shaft 111 in one direction, arms 126
which control the movement of register stops 113, and arms 131
which support gripper fingers 112 are supported by and turn with
support shaft 111.
The transfer drum also contains frame 124 which is fastened to
support plates 130 and 132. In addition to supporting bracket 122,
to which the spring on the end of arm 133 is fastened (shown in
FIG. 3), the frame 124 contains slots 129 which guide the up and
down movement of register stops 113. The movement of shaft 111 is
controlled by a stationary cam 201 (shown in FIG. 6) in conjunction
with follower 107 which rotates with the transfer drum. Follower
107, as it rotates with the transfer drum, moves about the
periphery of fixed cam 201. The movement of the followers, in turn,
causes arm 106 to move, and, since arm 106 is supported by and
keyed to shaft 111, the shaft 111 rotates in response to the
movement of the arm 106.
Referring to FIG. 3, which is a partial view of the transfer drum
in FIG. 2, taken through section 3--3, spring 115, acting through
arm 123, biases shaft 111 in the counterclockwise direction.
Internal frame 124 has spring support 122 attached to it which is
suspended towards the center of the transfer drum and away from the
free end of arm 123. The spring 115 is attached between the free
end of arm 123 and the tip of the spring support 122. Since arm 123
is keyed to shaft 111, the spring 115 acts to bias the shaft in the
counterclockwise direction thereby biasing the register stops 113
in their furthest downward position (see FIG. 4) and the gripper
fingers 112 in their most clockwise direction (see FIG. 5).
Referring to FIG. 4, which is a partial view of the transfer drum
shown in FIG. 2 taken through section 4--4, register stops 113 are
guided in a straight line movement in a plane which is
substantially normal to the surface of the transfer drum by the
slot 140 in the transfer drum and the slot 129 in the frame 124.
Arm 126, which is keyed to shaft 111, turns with the shaft to move
register stops 113 either up or down depending on the direction of
rotation of shaft 111. Slot 127, which is positioned in the end
portion of arm 126, is adapted to enable pin 128, which is fastened
to register stop 113, to slide within it thereby transferring the
rotary motion of the arm 126 to the linear motion of the register
stop. As shaft 111 rotates in the counterclockwise direction, arm
126 rotates in the counterclockwise direction, thereby driving
register stop 113 in a linear path towards the center of the
transfer drum. Then, when shaft 111 rotates in the clockwise
direction arm 126 rotates in the clockwise direction thereby
driving register stop 113 away from the center of the transfer drum
to bring it above the surface of the transfer drum. In FIG. 4, the
register stops 113 are shown in their furthest downward
position.
Referring to FIG. 5, which is a partial view of the transfer drum
shown in FIG. 2 taken through section 5--5, the gripper fingers 112
are supported by arm 131 which, in turn, is keyed to shaft 111. As
shaft 111 turns in the counterclockwise direction, gripper fingers
112 rotate in the counterclockwise direction to bring the angled
tip of the gripper finger 112 down and into contact with the
surface of the transfer drum. Then, when the shaft 111 rotates in
the clockwise direction arm 131 and gripper fingers 112 rotate in
the clockwise direction to bring the tip of the gripper finger 112
above and away from the surface of the transfer drum. The tip
portion of the gripper fingers 112 move through slots 145 in the
transfer drum when shaft 131 turns in either direction. In FIG. 5,
the gripper fingers 112 are shown in their extreme counterclockwise
position.
The movement of shaft 111 regulates the operation of the gripper
fingers and register stops in unison to register, grip and, then,
release the leading edge of a copy sheet. In operation, the shaft
111 first rotates in the clockwise direction from its biased
position to bring the register stops and gripper fingers just above
the surface of the transfer drum. A copy sheet is then fed onto the
surface of the transfer drum until its leading edge is in
registration with the register stops. When the leading edge of the
copy sheet has been properly registered, shaft 111 rotates in the
counterclockwise direction to bring the register stops below the
surface of the transfer drum and the gripper fingers into the
position where they press the leading edge of the copy sheet
against the surface of the transfer drum. The transfer drum then
continues to rotate through its plurality of consecutive
revolutions during which the plurality of toner images are
transferred from the photoreceptor to the copy sheet.
After the plurality of toner images have been transferred to the
copy sheet and the copy sheet is to be removed from the transfer
drum, shaft 111 rotates again in the clockwise direction, but to a
greater extent than during the registering and gripping step. As a
result the gripper fingers release the copy sheet and the register
stops push the leading edge of the copy away from the transfer drum
surface to such an extent that the leading edge also clears the tip
of the gripper fingers. (This is possible since the register stops
are joined to arms 126 at a greater distance from shaft 111 than
the gripper fingers are joined from the shaft ). At this time
stripper fingers 64 (shown in FIG. 1) are brought near the surface
of the transfer drum and as the transfer drum continues to rotate,
the copy sheet is completely separated from the drum surface and
conveyed into the fuser 60 (also shown in FIG. 1). The register
stops and gripper fingers then rotate counterclockwise with shaft
111 to return to the positions where they are biased by spring
115.
Referring to FIG. 1, transfer drum 42 has misfeed detector 43
adjacent it between sheet feeder 73 and the transfer station 40.
The purpose of the misfeed detector is to detect when a copy sheet
is improperly registered with the register stops, is not gripped
properly by the gripper fingers, or has not been fed into the
transfer drum at all. The detector is represented merely by a box
adjacent the transfer drum in FIG. 1 since any suitable detector
device can be used. For instance, a series of photocells can be
placed adjacent the surface of the transfer drum with associated
light sources, and, depending on where the leading edge of the
sheet is positioned relative to the stops or where the gripper
finger is located relative to the copy sheet, i.e., over or under
the sheet, or whether or not a copy sheet is on the transfer drum
at all, certain photocells are activated. The various combinations
of photocell signals can then be "read" by a logic circuit to
detect if a copy sheet is on the transfer drum and is properly
aligned. If a copy sheet is properly aligned on the transfer drum,
it is allowed to pass through the transfer station where a
plurality of toner images are transferred to it. However, if the
logic circuit determines that the copy sheet is not properly
aligned, it generates a signal which is directed to a solenoid 320
(see FIG. 7) which activates the transfer drum withdrawal
apparatus. The withdrawal apparatus rotates the transfer drum away
from the photoreceptor thereby preventing the toner image from
being transferred to a misaligned copy sheet or to the transfer
drum surface when no sheet is on the drum. The apparatus which
enables the transfer drum to rotate away from the photoreceptor is
described in detail below.
Referring to FIG. 2, the rotation of shaft 111 is controlled by cam
201 and follower 107. The shaft 111, arm 106, and follower 107
rotate with the transfer drum while cam 201 remains in the
stationary position. As a result of this arrangement the follower
travels around the periphery of the cam and rotates arm 106 in
response to the peripheral shape of the cam. The relationship
between the cam and follower can be seen best in FIG. 6. Transfer
drum 42, along with shaft 111, arm 106 and follower 107, is driven
in the clockwise direction by shaft 101, as shown by the arrow. Cam
201, on the other hand, remains stationary relative to shaft 101
and, when engaged with the follower 107, forces the follower to
move away from and towards the center of the transfer drum by cam
portions 155 and 160. Through arm 106, cam portion 155 causes the
registration stops and gripper fingers to register and grip the
leading edge of the copy sheet while cam portion 160 causes the
register stops and gripper fingers to release the leading edge of
the sheet and drive it away from the surface of the transfer
drum.
Referring to FIG. 7, cam 201 has a hub 312 which is adapted to
slide along shaft 101 so that it can move between an operative
position, where it is engaged by follower 107, and an inoperative
position, where it does not engage the follower. In this figure the
cam and hub are shown in the operative position which places the
hub against the end plate of the transfer drum 42. In its operative
position, the cam 201 engages the follower and operates the
registration stops and gripper fingers as described above. When it
is desirable to move the cam out of engagement with the follower,
for instance, when the copy is to be brought continuously through a
plurality of revolutions on the transfer drum to effect transfer of
a plurality of toner images to it, the cam is moved to the left
thereby bringing it to its inoperative position.
Shaft 101, the shaft that supports and drives transfer drum 42, is
mounted for rotation in frame members 304 and 308. Gear 351 and
pulley 352 rotate on shaft assembly 350 which, in turn, is
supported for rotation by frame 354. Frame 308 is adapted to rotate
about shaft assembly 350 so that when the transfer drum 42 is moved
away from photoreceptor 90, gears 351 and 353 remain in a meshed
relationship to one another. A belt (not shown) driven by a
suitable drive means rotates pulley 352 and gear 351 on shaft
assembly 350. The gear 351 drives gear 353 which, in turn, drives
the transfer drum 42 through shaft 101.
The movement of cam 201 between its inoperative position and its
operative position, which is shown in FIG. 7, is controlled by the
position of arm 203. When a signal is received by solenoid 211 to
move the cam 201 to the right to its operative position, pin 206 is
moved by the solenoid to the left. Arm 203, a second arm (not
visible) is on the other side of shaft 101, pivots about pin 204
thereby causing the tip 205 of the arm to force the cam 201 and hub
312 into its operative position against the action of spring 202.
When the cam is in the operative position and a signal is received
by the solenoid 211 to move it onto the inoperative position, pin
206 is allowed to move to the right causing arm 203 to rotate
clockwise about pin 204. Due to the action of spring 202, (a second
spring, not visible, is on the other side of shaft 101), which is
fastened to hub 312, the cam 201 moves to the left and out of
engagement with the follower.
FIG. 8 shows the view of the transfer drum in which part of the
frame 306 is broken away from the mechanism which moves the cam so
that the various elements of the mechanism can be clearly seen.
Solenoid 211 moves pin 206 through solenoid arm 210, the arm moving
either to the right and left depending on the signal received by
the solenoid. Pin 204 is supported for rotation by an appendage of
arm 304 and spring 202 is fastened to arm 304 to maintain a
mechanical bias on the cam toward the inoperative position.
Referring to FIG. 9, stripper fingers 64 on the top of the transfer
drum when activated, are placed near the drum surface to help strip
a copy sheet from the transfer drum after all the images have been
transferred to the copy sheet. Referring to FIG. 7, stripper
fingers 64 are supported for rotation by shaft 360 which, in turn,
is supported for rotation in frames 306 and 308. The stripper
fingers are activated by solenoid 211 also. Referring to FIG. 8,
arm 275, which is keyed to shaft 360, rotates the shaft 360 to
bring the stripper fingers in and out of position adjacent the
surface of the transfer drum. Arm 220, which is supported to pivot
on frame bracket 209, is in the shape of an L, the lower extremity
of the arm having a slot 376 in which pin 206 travels. As solenoid
arm 210 moves to the left bringing pin 206 into the clockwise or
downward direction, the stripper fingers are brought adjacent the
transfer drum surface. The linkage (not shown) between arm 275 and
the horizontal extremity of arm 220 is similar to the linkage
between pin 206 and slot 276, the arm 275 having a pin at its end
which slides in a slot in the end of the horizontal extremity of
arm 220. Through this linkage, arm 275 is forced down by arm 220
thereby rotating shaft 360 to bring the stripper fingers adjacent
the surface of the transfer drum. Then, when the signal to solenoid
arm 210 ceases, spring 202 causes arm 210 to move pin 206 to the
right, the associated action of arms 220 and 275 and shaft 360
driving the stripper fingers away from the surface of the transfer
drum. As a result of the linkage described above, the stripper
fingers can come into contact with the transfer drum only during
the time that cam 201 is in its operative position.
During the rotation of shaft 101, it is necessary to assure that
cam 201 does not turn with the shaft, especially when the cam is in
its operative position. Keeper arm 313 is intended for this
purpose. The lower position of the keeper are 313 is fastened to
hub 312 while its upper portion has a slot 314 which is supported
by pin 315. As the hub 312 moves along shaft 101, the keeper arm
313 moves with it, the upper portion of the keeper arm always
sliding on pin 315 which is supported by arm 304. Due to this slot
and pin arrangement, cam 201 always remains in a stationary
position relative to the follower arm 106 even though the shaft 101
and the transfer drum 42 are continuously rotating.
Referring to FIG. 9, transfer drum 42 is brought away from the
photoreceptor when a copy sheet is misfed onto its surface by the
feeding mechanism. In such a circumstance misfeed detector 43
identifies a misaligned copy sheet or absence of a copy sheet and
activates a mechanism which rotates arms 304 and 308 in the
counterclockwise direction to carry the transfer drum as well as
all of its collateral elements in the counterclockwise direction.
When a misfeed signal is received by solenoid 301, solenoid arm 320
moves to the right or away from the transfer drum, bringing cross
bar 302, to the right also. Cross bar 302 forces arms 304 and 308
in the counterclockwise direction against the action of spring 377
thereby moving both arms 304 and 308, and the transfer drum, in the
counterclockwise direction about shafts 375 and 350. Spring 377
mechanically biases the transfer drum into contact with or in close
proximity to the surface of the photoreceptor 90 so that a copy
sheet on the surface of the transfer drum contacts the surface of
the photoreceptor during the transfer step. By this withdrawal
mechanism, the copy sheet can be withdrawn from the transfer
station if misaligned or the transfer drum brought away from the
transfer station if no copy sheet is on it so that the toner image
cannot be transferred to the surface of the transfer drum. This
arrangement prevents toner images from being transferred onto the
transfer drum itself or onto the copy sheet in any other manner
than in perfect register therewith.
Referring to FIG. 7, shaft 375 is mounted in frame 306 and supports
arm 304 so that arm 304 can turn on the shaft when cross bar 302
forces the arm 304 to bring the transfer drum away from the
photoreceptor. Shafts 350 and 375 have common centerlines and, as a
consequence, as cross bar 302 forces frames 304 and 308 to rotate,
the drive means for the transfer drum, including gears 351 and 353,
moves in tact thereby maintaining the drive relationship. In this
manner the position of any point on the transfer drum surface
relative to any point on the photoreceptor surface is preserved
even though the transfer drum is withdrawn from the photoreceptor
because of a misfeeding of the copy sheet. Gear 351 is driven
directly off the main shaft of the machine by a belt (not shown)
through pulley 352.
The arm 304 can be rotated manually by engaging arm 380 with latch
381. Arm 380 can be rotated in the counterclockwise direction
(refer to FIG. 9) by causing shaft 382 to rotate in the
counterclockwise direction. Shaft 382 turns counterclockwise by
pivoting arm 383 counterclockwise. Arm 383 is supported on brackets
384 and rotates about an axis coincident with shaft 382. As arm 383
pivots in the counterclockwise direction, connecting bars 385,
which connect arm 383 and cross bar 302, are forced away from the
transfer drum bringing arm 304 in the counterclockwise direction.
The combination of the movement of arms 304 and 380 position arm
380 so as to engage the latch 381. Once the latch and arm are in
this position, the transfer drum cannot return toward the
photoreceptor until latch 381 is released manually. This manual
latch feature is especially desirable when maintenance is to be
carried out on the transfer drum.
As mentioned above in conjunction with FIG. 1, the photoreceptor
makes a plurality of revolutions in order to complete a copying
cycle and a toner image of one color is transferred to the sheet
during each revolution. The copy sheet is fed onto the surface of
the transfer drum while the drum rotates at the same speed as the
photoreceptor. In order to register and grip the leading edge of
the copy sheet with the register stops and gripper fingers as the
transfer drum rotates, the copy sheet must be fed at a faster rate
of speed than the speed at which the surface of the transfer drum
is moving. Referring to FIG. 1 register rollers 74 feed the leading
edge of the copy sheet onto the surface of the transfer drum 42 at
a speed which is slightly faster than the speed at which the drum
surface is moving and at a time when the register stops and gripper
fingers are passing through the 6 o'clock position of the transfer
drum. After the leading edge of the copy sheet is registered on the
register stops, the gripper fingers grip the copy sheet and the
copy sheet is carried through the transfer station 40 for three
revolutions. During the time that register stops and gripper
fingers pass through the 6 o'clock position of the transfer drum
until just after gripper fingers grip the copy sheet, cam 106
(refer to FIG. 2) assumes its operative position in which it is
engaged by the follower 107. After the gripper fingers have secured
the copy sheet to the transfer drum surface, cam 201 is immediately
moved to its inoperative position. Then, the transfer drum carries
the copy sheet through three revolutions during which yellow, cyan
and magenta toner images are transferred to the copy sheet. Of
course, the cam 201 could be maintained in its operative position
for each revolution of the transfer drum, or for any number of
revolutions of the transfer drum, if it were desired to do so.
After the leading edge of the copy passes through the transfer
station for the final time, cam 201 is moved back into its
operative position and the gripper fingers are operated to release
the copy sheet, the register stops are activated to force the copy
sheet away from the surface of the transfer drum and the stripper
fingers are operated to separate the copy sheet from the drum and
direct its leading edge towards the fusing apparatus. The stripper
fingers are moved adjacent the surface of the transfer drum when
the copy sheet is placed on the transfer drum also, due to the
operative position of the cam during this time. However, the
stripping fingers do not interfere with copy sheet being fed onto
the drum during this time since they are located at approximately
the 1 o'clock position of the drum while the leading edge of copy
sheet is placed on the drum at approximately the 6 o'clock
position.
The transfer of toner images from the photoreceptor to the copy
sheet takes place at the transfer station, generally designated 40.
During the transfer operation, the copy sheet is secured against
the outer periphery of the transfer drum in a manner herein
described and the sheet's image receiving surface brought into
synchronous moving contact with the photoreceptor within the
transfer region. During transfer, the core of the transfer drum is
electrically connected to a suitable DC source of power capable of
biasing the core to a predetermined potential. Since the outer
surface of the transfer drum is coated with a layer of
non-conductive material, little or no current flows between the
photoconductor and the transfer roll. As a result, a relatively
strong electrostatic force field is created within the transfer
region between the two contacting members. By proper selection of
the core polarity, the force field is made to electrically transfer
the negatively charged toner particles from the photoreceptor to
the receiving surface of the copy sheet.
Conventionally, in most xerographic devices, a toner image is
transferred from the photoconductive plate surface to a sheet of
final support material using a fixed transfer bias somewhere in the
range of between +700 and +2,000 volts. However, the efficiency of
the system is considerably reduced when a second image is
transferred over the first utilizing this original fixed bias
setting. It is believed that the decrease in transfer efficiency is
caused by an increase in the resistivity of the receiving body due
to the introduction of toner material into the system by the
transferring of the first image onto the support sheet. Similarly,
when a third image is transferred over the first two images, as is
the case in most color processes, the loss of transfer efficiency
becomes even more pronounced.
Attempts to compensate for the loss in efficiency by placing the
fixed bias at a relatively high level, that is, at a level
sufficiently high enough to effectively transfer the third toner
image, have not proven to be successful. When utilizing the
apparatus herein disclosed it has been found that the bias required
to attain this result is so high as to actually inhibit the
transfer of the first toner image and in some cases even that of
the second image. Although the exact phenomena involved is not
fully understood at this time, it is believed the combination of a
high bias and a relatively low resistivity in the receiving body
during the initial transfer steps results in an air breakdown
occurring within the transfer region. This causes the polarity of
at least some of the toner particles making up the xerographic
image to be reversed and these particles, rather than being
attracted towards the copy sheet, are repelled back to the
photoconductor during the transfer operation.
Regardless of the phenomena involved, it has been found that there
exists some optimum biasing potential range within which each
successive toner image can be efficiently transferred and that this
range is dictated basically by the amount of new toner material
added to the receiving copy sheet surface during the previous
transfer steps.
The apparatus of the present invention overcomes this undesirable
decrease in transfer efficiency where multiple images are to be
placed on a single support sheet by changing the operating bias
level on the transfer drum between each successive transfer
operation. The transfer bias placed on the transfer drum is
sequentially increased between each image transfer operation in
order to compensate for an increase in the system's resistivity due
to the addition of new toner material onto the copy sheet. The
increase in bias is stepped in a manner such that each image
transfer is accomplished within a range whereby the efficiency of
all three image transfer steps is achieved under optimum
conditions. For example, good transfer was obtained during all
three transfer steps when the initial voltage imposed on the
transfer drum was +3,000 volts during the first revolution to
transfer the yellow toner image than increase to a +3,500 volts
during the second revolution to transfer the cyan toner image in
superposition over the yellow image, and finally increase to a
+4,000 volts on the final revolution so as to transfer the magenta
toner image in superposition over the first two toner images.
The apparatus for stepping up the voltages on the transfer drum can
be any suitable apparatus which functions to increase the voltage
in each successive revolution of a copy cycle. For example, FIG. 10
illustrates a control apparatus in block diagram form which is
suitable for this purpose. During each revolution of the
photoreceptor, two electrical signals are generated from the
transfer drum to indicate that the transfer step is about to begin.
Any suitable device can be used to generate the signals; for
instance, two cams can be placed on the shaft driving the
photoreceptor which have follower arms which, in turn, close a
switch as transfer begins to allow current to pass through the
switch. The signal generating devices illustrated in clock form in
FIG. 10 are two such cam switches in conjunction with a power
source. One switch is a transfer switch which is closed by its cam
and follower arrangement just as the leading edge of the copy sheet
enters the transfer station to give the "go" signal to the control
apparatus to apply a voltage to the transfer drum. The other switch
is a voltage control circuit switch which tells the voltage control
circuit that another revolution is being made by the
photoreceptor.
The voltage control circuit is a device which has three possible
output voltages. The exact output voltage of the control circuit
during each transfer step is dependent on how many signals have
already been received from the voltage control circuit switch
during the copying cycle. At the beginning of the copying cycle in
a three color system; that is, during the first revolution of the
photoreceptor, a first signal enters the voltage control circuit
and the resulting output of the control circuit is the lowest
voltage of its three possible output voltages. Then, during the
second revolution of the photoreceptor another signal is fed to the
voltage control circuit which results in the voltage control
circuit stepping up its output to a voltage which is higher than
that of the first revolution. Similarly, during the third and final
revolution of the photoreceptor, another signal is fed to the
voltage control circuit which results in the voltage control
circuit stepping up its output to the highest voltage of its three
possible output voltages. At this time, the copy cycle has been
completed and a new copy cycle begins with a subsequent revolution
of the photoreceptor. During the fourth revolution, or the first
revolution of a new copying cycle, the voltage control circuit
again generates the lowest voltage of its three possible output
voltages in response to another signal from the voltage control
circuit signal.
Each output voltage of the voltage control circuit is passed
through a relay control before being fed into the DC power supply.
The signal generated by the transfer switch is fed to the relay
control also. The relay control allows current to pass from the
voltage control circuit outputs to the DC power source only when
the "go" signal has been received from the transfer switch. The DC
power supply, in turn, delivers a voltage to the electrical brush
which is a multiple of the voltages fed to it. Assuming that the
three output voltages of the voltage control circuit are 30, 35,
and 40 volts, and the DC power source generates a voltage which is
10 times the voltage it receives from the voltage control circuit,
the electrical brush will deliver to the core of the transfer drum
3,000, 3,500 and 4,000 volts in the first, second, and the third
revolutions, respectively, of the photoreceptor.
It is intended that appropriate drive means be associated with the
color copying system described herein, and such drive means used
can be any suitable type. For instance, the main shaft 91 of the
machine can be driven by a main machine motor and the various
processing stations around the photoreceptor driven therefrom by a
suitable gearing arrangement. In addition, appropriate control
circuits can be applied throughout the machine in order to assure
that it functions as described above.
In addition to the apparatus outlined above, many other
modifications and/or additions to this invention will be readily
apparent to those skilled in the art upon reading this disclosure,
and these are intended to be encompassed within the invention
disclosed and claimed herein.
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