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Electrostatographic printers are known in which a
toner image is electrostatically formed on a photoreceptive
image bearing member. The toner image is transferred to a
receiving substrate, typically paper or other print
receiving material. The toner image is subsequently fused
to the substrate.
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In known color electrostatographic printer with an
intermediate transfer member, a plurality of image bearing
members are used to develop multiple color toner images.
Each color toner image is electrostatically transferred
sequentially from the image bearing members and registered
to an intermediate transfer member. The composite toner
image is then electrostatically transferred from the
intermediate transfer member to the final substrate. Such
systems that use electrostatic transfer to transfer the
composite toner image from the intermediate to the final
substrate and then subsequently fix the image on the
substrate in a fusing system have transfer limitations.
For example, there are limitations due to stresses
introduced with rougher paper stock, foils, paper moisture
content variations, etc. Also, the need to
electrostatically transfer a full layered color composite
toner image to the substrate creates additional high
stresses for electrostatic transfer.
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Stressful system conditions can include for example
systems that may wish to use papers allowed to condition at
wide ranges of relative humidity, and systems that may wish
to image onto a large range of paper roughness and widths.
Such stresses can have significant effect on transfer due
to the effect on the electrostatic fields used in
electrostatic transfer, and they can also have significant
effect on paper transport. In addition with direct to
paper transfer, fibers, talc and other particulate or
chemical contaminants can readily directly transfer from
the paper to the imaging modules during direct contact in
the electrostatic transfer zones. This can tend to
contaminate the imaging drums, development systems, cleaner
systems, etc., and can lead to early failure of the imaging
systems. This is especially true for certain stressful
paper types including for example certain types of recycled
papers. Due to all these and other problems, systems that
use direct transfer to the final media generally have
narrow media latitude for obtaining and/or for maintaining
high print quality.
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Alternatively, a toner image is formed on a
photoreceptor. The toner image is transferred to a
transfuse member. The transfuse member is employed to
generally simultaneously transfer and fuse the toner image
to a substrate. The transfuse member preferably has good
release properties for efficient transfer of the toner
image to the substrate. However, materials having
acceptable release properties can have unacceptably short
component life therefore resulting in increased costs for
replacement and increased printer down time. In addition,
materials having acceptable release properties can fail to
exhibit additional desirable transfer properties such as
improved conformability for good transfer to rougher
substrates.
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Briefly stated, a printing apparatus has an image
producing station, an intermediate transfer member for
receiving a toner image from the image producing station,
and a transfuse member having a support surface for
receiving a toner image. An optional release agent
management system having a release agent applicator applies
a layer of release agent to the support surface. A toner
image is subsequently transferred over the release agent
and onto the support surface. The toner image is then
transferred to a substrate and preferably simultaneously
fused to the substrate to form a document. An intermediate
transfer member cleaner in accordance with the invention,
engages the surface of the intermediate transfer member to
clean release agent from the intermediate transfer member
transferred from the transfuse member to the intermediate
transfer member.
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In one preferred embodiment, an electrostatographic
printing machine with a release agent management system
engaging a transfuse member in accordance with the
invention has multiple toner image producing stations, each
forming a developed toner image of a component color. The
developed toner images are electrostatically transferred at
the first transfer nip to an intermediate transfer member
to form a composite toner image thereon. The composite
toner image is then transferred at a second transfer nip to
a support surface formed by a transfuse member. The
transfuse member preferably has improved conformability and
other properties for improved transfusion, generally
simultaneous transfer and fusion, of the composite toner
image to a substrate. The composite toner image and the
substrate are brought together in the third transfer nip to
generally simultaneously transfer the composite toner image
and fuse the composite toner image to the substrate to form
the final document.
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A release agent management system applies a release
agent to the surface of the transfuse member prior to the
second transfer nip. The release agent improves transfer
of the composite toner image from the transfuse member to
the substrate. The release agent could include silicone
and other types of oil, wax, and even water. The release
agent is metered at a preestablished rate onto the surface
of the transfuse member. The release agent is at least
partially transferred to the substrate, along with the
toner image, in the third transfer nip.
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Alternately a transfuse member made of silicone rubber
has natural release properties due to the internal silicone
oils present in the material. In this case only a small
amount or no external release agent management system is
required. Regardless whether the release agent is
internal, external or both, the release agent can be
transferred from the transfuse member to the transfer
member.
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Toner image producing stations, in particular those
employing photoreceptors, are susceptible to contamination
from oils. A cleaning station in accordance with the
invention, engages the surface of the intermediate member
to clean release agent transferred from the transfuse
member to the intermediate member. The cleaning station is
positioned downstream in the process direction from the
second transfer nip and prior to the first transfer nips.
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The cleaning station is preferably formed of a
cleaning blade slidingly engaging the surface of the
intermediate transfer member. The cleaning blade removes
toner, debris and particular release oil from the
intermediate member. In addition, a web cleaner preferably
engages the surface of the intermediate member to further
clean release oil from the surface of the intermediate
member.
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One preferred material for the transfuse member is
silicone. Silicone typically has natural release
properties from the silicone oils present in the material.
However, once these silicone oils are depleted, the
transfuse member exhibits reduced release properties and
rapid decrease in transfuse member quality leading to
failure. Therefore the release management system
preferably replaces the natural silicone oils at a rate
generally equal to the rate of loss the silicone oils
during the printing process. Alternatively the rate of
application of the silicone oils can be less than the rate
of loss the silicone oils to still result in increased
transfuse member operational life relative to a system
having no application of the release agent.
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An alternative preferred material for the top most
surface of the transfuse member is Viton™ (Trademark of
E.I. DuPont for a series of fluoroelastomers based on the
copolymer of vinylidene fluoride and hexafluoropropylene).
Viton™ exhibits improved transfuse member properties with
a generally extended operational life. However, Viton™
can provide insufficient release of the toner image. The
release agent management system preferably meters at a
preestablished rate a release agent onto the topmost
surface of the transfuse member. An initial quantity of
release agent, preferably a silicone oil, is applied to the
Viton™ coated transfuse member. The release agent is then
applied at the rate the release agent is transferred to the
substrate or otherwise lost in the printing process.
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The intermediate transfer member buffers the image
bearing member from the third transfer nip. In particular,
the intermediate transfer member can buffer the image
bearing member from release agents on the transfuse member.
The release agent can be inherent in the topmost layer of
the transfuse member, such as silicone oil in a silicone
top most layer, and/or can be applied to the transfuse
member by a release agent management system. The cleaning
station cleans the release agent from the intermediate
member regardless of source, including release agents
naturally occurring in the transfuse member or applied by
a release agent management system.
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The release agent management system preferably has a
release agent applicator formed of a web impregnated with
a release agent. The web is brought into contact with the
transfuse member to transfer the release agent to the
surface of the transfuse member. An applicable system
employed with a fuser roller is disclosed in
US-A-5,749,038. Alternately, the release agent management
system can have a roll configuration release agent
applicator. Applicable systems employed with fuser rolls
are disclosed in US-A-4,214,549, and US-A-4,254,732.
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A particular embodiment in accordance with this
invention will now be described with reference to the
accompanying drawings; in which:-
- Figure 1 is a schematic side view of a duplex cut
sheet electrostatographic printer having an intermediate
transfer member cleaning station in accordance with the
invention;
- Figure 2 is an enlarged schematic side view of the
transfer nips of the printer of Figure 1;
- Figure 3 is an enlarged schematic side view of the
intermediate transfer member cleaning station and the
release agent management system of the printer of Figure 2;
- Figure 4 is an enlarged schematic side view of the
intermediate transfer member cleaning station and an
alternate embodiment release agent management system of the
printer of Figure 2;
- Figure 5 is an enlarged schematic side view of a
further alternate transfer member cleaning station of the
printer of Figure 2;
- Figure 6 is a graphical representation of residual
toner as a function of transfuse member temperature;
- Figure 7 is a graphical representation of crease as a
function of transfuse member temperature for given
representation of residual substrate temperature; and,
- Figure 8 is a graphical representation of oil on copy
as a function of copy count.
-
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With reference to Figures 1 and 2, a multi-color cut
sheet duplex electrostatographic printer 10 has an
intermediate transfer belt 12. The intermediate transfer
belt 12 is driven over guide rollers 14, 16, 18, and 20.
The intermediate transfer belt 12 moves in a process
direction shown by the arrow A. For purposes of
discussion, the intermediate transfer member 12 defines a
single section of the intermediate transfer member 12 as a
toner area. A toner area is that part of the intermediate
transfer member which receives the various processes by the
stations positioned around the intermediate transfer member
12. The intermediate transfer member 12 may have multiple
toner areas; however, each toner area is processed in the
same way.
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The toner area is moved past a set of four toner image
producing stations 22, 24, 26, and 28. Each toner image
producing station 22, 24, 26, 28 operates to place a color
toner image on the toner image of the intermediate transfer
member 12. Each toner image producing station 22, 24, 26,
28 operates in the same manner to form developed toner
image for transfer to the intermediate transfer member 12.
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The image producing stations 22, 24, 26, 28 are
described in terms of a photoreceptive system, but it is
readily recognized by those of skill in the art that
ionographic systems and other marking systems can readily
be employed to form developed toner images. Each toner
image producing station 22, 24, 26, 28 has an image bearing
member 30. The image bearing member 30 is a drum or belt
supporting a photoreceptor.
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The image bearing member 30 is uniformly charged at a
charging station 32. The charging station is of well-known
construction, having charge generation devices such as
corotrons or scorotrons for distribution of an even charge
on the surface of the image bearing member 30. An exposure
station 34 exposes the charged image bearing member 30 in
an image-wise fashion to form an electrostatic latent image
at the image area. For purposes of discussion, the image
bearing member defines an image area. The image area is
that part of the image bearing member which receives the
various processes by the stations positioned around the
image bearing member 30. The image bearing member 30 may
have multiple image areas; however, each image area is
processed in the same way.
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The exposure station 34 preferably has a laser
emitting a modulated laser beam. The exposure station 34
raster scans the modulated laser beam onto the charged
image area. The exposure station 34 can alternately employ
LED arrays or other arrangements known in the art to
generate a light image representation that is projected
onto the image area of the image bearing member 30. The
exposure station 34 exposes a light image representation of
one color component of a composite color image onto the
image area to form a first electrostatic latent image.
Each of the toner image producing stations 22, 24, 26, 28
will form an electrostatic latent image corresponding to a
particular color component of a composite color image.
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The image area is advanced to a development station
36. The developer station 36 has a developer corresponding
to the color component of the composite color image.
Typically, therefore, individual toner image producing
stations 22, 24, 26, and 28 will individually develop the
cyan, magenta, yellow, and black that make up a typical
composite color image. Additional toner image producing
stations can be provided for additional or alternate colors
including highlight colors or other custom colors.
Therefore, each of the toner image producing stations 22,
24, 26, 28 develops a component toner image for transfer to
the toner area of the intermediate transfer member 12. The
developer station 36 preferably develops the latent image
with a charged dry toner powder to form the developed
component toner image. The developer can employ a magnetic
toner brush or other well known development arrangements.
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The image area having the component toner image then
advances to the pretransfer station 38. The pretransfer
station 38 preferably has a pretransfer charging device to
charge the component toner image and to achieve some
leveling of the surface voltage above the image bearing
member 30 to improve transfer of the component image from
the image bearing member 30 to the intermediate transfer
member 12. Alternatively the pretransfer station 38 can
use a pretransfer light to level the surface voltage above
the image bearing member 30. Furthermore, this can be used
in cooperation with a pretransfer charging device. The
image area then advances to a first transfer nip defined
between the image bearing member 30 and the intermediate
transfer member 12. The image bearing member 30 and
intermediate transfer member 12 are synchronized such that
each has substantially the same linear velocity at the
first transfer nip 40. The component toner image is
electrostatically transferred from the image bearing member
30 to the intermediate transfer member 12 by use of a field
generation station 42. The field generation station 42 is
preferably a bias roller that is electrically biased to
create sufficient electrostatic fields of a polarity
opposite that of the component toner image to thereby
transfer the component toner image to the intermediate
transfer member 12. Alternatively the field generation
station 42 can be a corona device or other various types of
field generation systems known in the art. A prenip
transfer blade 44 mechanically biases the intermediate
transfer member 12 against the image bearing member 30 for
improved transfer of the component toner image. The toner
area of the intermediate transfer member 12 having the
component toner image from the toner image producing
station 22 then advances in the process direction.
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After transfer of the component toner image, the image
bearing member 30 then continues to move the image area
past a preclean station 39. The preclean station employs
a preclean corotron to condition the toner charge and the
charge of the image bearing member 30 to enable improved
cleaning of the image area. The image area then further
advances to a cleaning station 41. The cleaning station 41
removes the residual toner or debris from the image area.
The cleaning station 41 preferably has blades to wipe the
residual toner particles from the image area. Alternately
the cleaning station 41 can employ an electrostatic brush
cleaner or other well known cleaning systems. The
operation of the cleaning station 41 completes the toner
image production for each of the toner image producing
stations 22, 24, 26, and 28.
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The first component toner image is advanced at the
image area from the first transfer nip 40 of the image
producing station 22 to the first transfer nip 40 of the
toner image producing station 24. Prior to entrance of the
first transfer nip 40 of the toner image producing station
24 an image conditioning station 46 uniformly charges the
component toner image to reduce stray, low or oppositely
charged toner that would result in back transfer of some of
the first component toner image to the subsequent toner
image producing station 24. The image conditioning
stations, in particular the image conditioning station
prior to the first toner image producing station 22 also
conditions the surface charge on the intermediate transfer
member 12. At each first transfer nip 40, the subsequent
component toner image is registered to the prior component
toner images to form a composite toner image after transfer
of the final toner image by the toner image producing
station 28.
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The geometry of the interface of the intermediate
transfer member 12 with the image bearing member 30 has an
important role in assuring good transfer of the component
toner image. The intermediate transfer member 12 should
contact the surface of the image bearing member 30 prior to
the region of electrostatic field generation by the field
generation station 42, preferably with some amount of
pressure to insure intimate contact. Generally, some
amount of pre-nip wrap of the intermediate transfer member
12 against the image bearing member 30 is preferred.
Alternatively, the pre-nip pressure blade 44 or other
mechanical biasing structure can be provided to create such
intimate pre-nip contact. This contact is an important
factor in reducing high electrostatic fields from forming
at air gaps between the intermediate transfer member 12 and
the component toner image in the pre-nip region. For
example, with a corotron as the field generation station
42, the intermediate transfer member 12 should preferably
contact the toner image in the pre-nip region sufficiently
prior to the start of the corona beam profile. With a
field generation station 42 of a bias charging roller, the
intermediate transfer member 12 should preferably contact
the toner image in the pre-nip region sufficiently prior to
the contact nip of the bias transfer roller. "Sufficiently
prior" for any field generation device can be taken to mean
prior to the region of the pre-nip where the field in any
air gap greater than about 50mm between the intermediate
transfer member 12 and the component toner image has
dropped below about 4 volts/micron due to falloff of the
field with pre-nip distance from the first transfer nip 40.
The falloff of the field is partly due to capacitance
effects and this will depend on various factors. For
example, with a bias roller this falloff with distance will
be slowest with larger diameter bias rollers, and/or with
higher resistivity bias rollers, and/or if the capacitance
per area of the insulating layers in the first transfer nip
40 is lowest. Lateral conduction along the intermediate
transfer member 12 can even further extend the transfer
field region in the pre-nip, depending on the transfer belt
resistivity and other physical factors. Generally the
desired pre-nip contact is between about 2 to 10 mm for
resistivities within the desired range and with bias roller
diameters between about 12 mm and 50 mm.
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The field generation station 42 will preferentially
use very conformable bias rollers for the first transfer
nips 40 such as foam or other roller materials having an
effectively very low durometer ideally less than about 30
Shore A. In systems that use belts for the imaging
modules, optionally the first transfer nip 40 can include
acoustic loosening of the component toner image to assist
transfer.
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In the preferred arrangement, "slip transfer" is
employed for registration of the color image. For slip
transfer, the contact zone between the intermediate
transfer member 12 and the image bearing member 30 will
preferably be minimized subject to the pre-nip
restrictions. The post transfer contact zone past the
field generation station 42 is preferentially small for
this arrangement. Generally, the intermediate transfer
member 12 can optionally separate along the preferred bias
roller of the field generation station 42 in the post nip
region if an appropriate structure is provided to insure
that the bias roller does not lift off the surface of the
image bearing member due to the tension forces of the
intermediate transfer member 12. For slip transfer
systems, the pressure of the bias roller employed in the
field generation station 42 should be minimized. Minimized
contact zone and pressure minimizes the frictional force
acting on the image bearing member 30 and this minimizes
elastic stretch issues of the intermediate transfer member
12 between first transfer nips 40 that can degrade color
registration. It will also minimize motion interactions
between the drive of the intermediate transfer member 12
and the drive of the image bearing member 30.
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For slip transfer systems, the resistivity of the
intermediate transfer member 12 should also be chosen to be
high, generally within or even toward the middle to upper
limits of the most preferred range discussed later, so that
the required pre-nip contact distances can be minimized.
In addition, the coefficient of friction of the top surface
material on the intermediate transfer member should
preferentially be minimized to increase operating latitude
for the slip transfer registration and motion quality
approach.
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In an alternate embodiment the image bearing members
30, such as photoconductor drums, do not have separate
drives and instead are driven by the friction in the first
transfer nips 40. In other words, the image bearing
members 30 are driven by the intermediate transfer member
12. Therefore, the first transfer nip 40 imparts
sufficient frictional force on the image bearing member to
overcome any drag created by the development station 36,
cleaner station 41, additional subsystems and by bearing
loads. For a friction driven image bearing member 30, the
optimum transfer design considerations are generally
opposite to the slip transfer case. For example, the lead
in of the intermediate transfer member 12 to the first
transfer zone preferentially can be large to maximize the
friction force due to the tension of the intermediate
transfer member 12. In the post transfer zone, the
intermediate transfer member 12 is wrapped along the image
bearing member 30 to further increase the contact zone and
to therefore increase the frictional drive. Increased
post-nip wrap has a larger benefit than increased pre-nip
wrap because there will be increased pressure there due to
electrostatic tacking forces. As another example, the
pressure applied by the field generation device 42 can
further increase the frictional force. Finally for such
systems, the coefficient of friction of the material of the
top most layer on the intermediate transfer member 12
should preferentially be higher to increase operating
latitude.
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The toner area then is moved to the subsequent first
transfer nip 40. Between toner image producing stations
are the image conditioning stations 46. The charge
transfer in the first transfer nip 40 is normally at least
partly due to air breakdown, and this can result in non
uniform charge patterns on the intermediate transfer member
12 between the toner image producing stations 22, 24, 26,
28. As discussed later, the intermediate transfer member
12 can optionally include insulating topmost layers, and in
this case non uniform charge will result in non uniform
applied fields in the subsequent first transfer nips 40.
The effect accumulates as the intermediate transfer member
12 proceeds through the subsequent first transfer nips 40.
The image conditioning stations 46 "level" the charge
patterns on the belt between the toner image producing
stations 22, 24, 26, 28 to improve the uniformity of the
charge patterns on the intermediate transfer member 12
prior to subsequent first transfer nips 40. The image
conditioning stations 46 are preferably scorotrons and
alternatively can be various types of corona devices. As
previously discussed, the charge conditioning stations 46
additionally are employed for conditioning the toner charge
to prevent re-transfer of the toner to the subsequent toner
image producing stations. The need for image conditioning
stations 46 is reduced if the intermediate transfer member
12 consists only of semiconductive layers that are within
the desired resistivity range discussed later. As further
discussed later, even if the intermediate transfer member
12 includes insulating layers, the need for image
conditioning stations 46 between the toner image producing
stations 22, 24, 26, 28 is reduced if such insulating
layers are sufficiently thin.
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The guide roller 14 is preferably adjustable for
tensioning the intermediate transfer member 12.
Additionally, the guide roller 14 can, in combination with
a sensor sensing the edge of the intermediate transfer
member 12, provide active steering of the intermediate
transfer member 12 to reduce transverse wander of the
intermediate transfer member 12 that would degrade
registration of the component toner images to form the
composite toner image.
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Each toner image producing station positions component
toner image on the toner area of the intermediate transfer
member 12 to form a completed composite toner image. The
intermediate transfer member 12 transports the composite
toner image from the last toner image producing station 28
to pre-transfer charge conditioning station 52. When the
intermediate transfer member 12 includes at least one
insulating layer, the pretransfer charge conditioning
station 52 levels the charge at the toner area of the
intermediate transfer member 12. In addition the pre-transfer
charge conditioning station 52 is employed to
condition the toner charge for transfer to a transfuse
member 50. It preferably is a scorotron and alternatively
can be various types of corona devices. A second transfer
nip 48 is defined between the intermediate transfer member
12 and the transfuse member 50. A field generation station
42 and pre-transfer nip blade 44 engage the intermediate
transfer member 12 adjacent the second transfer nip 48 and
perform the same functions as the field generation stations
and pre-transfer blades 44 adjacent the first transfer nips
40. The composite toner image is transferred
electrostatically and with heat assist to the transfuse
member 50.
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The electrical characteristics of the intermediate
transfer member 12 are also important. The intermediate
transfer member 12 can optionally be constructed of a
single layer or multiple layers. In any case, preferably
the electrical properties of the intermediate transfer
member 12 are selected to reduce high voltage drops across
the intermediate transfer member. To reduce high voltage
drops, the resistivity of the back layer of the
intermediate transfer member 12 preferably has sufficiently
low resistivity. The electrical characteristics and the
transfer geometry must also be chosen to prevent high
electrostatic transfer fields in pre-nip regions of the
first and second transfer nips 40, 48. High pre-nip fields
at air gaps of around typically >50 microns between the
component toner images and the intermediate transfer member
12 can lead to image distortion due to toner transfer
across an air gap and can also lead to image defects caused
by pre-nip air breakdown. This can be avoided by bringing
the intermediate transfer member 12 into early contact with
the component toner image prior to the field generating
station 42, as long as the resistivity of any of the layers
of the intermediate transfer member 12 are sufficiently
high. The intermediate transfer member 12 also should have
sufficiently high resistivity for the topmost layer to
prevent very high current flow from occurring in the first
and second transfer nips 40, 48. Finally, the intermediate
transfer member 12 and the system design needs to minimize
the effect of high and/or non-uniform charge buildup that
can occur on the intermediate transfer member 12 between
the first transfer nips 40.
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The preferable material for a single layer
intermediate transfer member 12 is a semiconductive
material having a "charge relaxation time" that is
comparable to or less than the dwell time between toner
image producing stations, and more preferred is a material
having a "nip relaxation time" comparable or less than the
transfer nip dwell time. As used here, "relaxation time"
is the characteristic time for the voltage to drop across
the thickness of the layer of the intermediate transfer
member. The dwell time is the time that an elemental
section of the transfer member 12 spends moving through a
given region. For example, the dwell time between imaging
stations 22 and 24 is the distance between imaging stations
22 and 24, divided by the process speed of the transfer
member 12. The transfer nip dwell time is the width of the
contact nip created during the influence of the field
generation station 42, divided by the process speed of the
transfer member 12.
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The "charge relaxation time" is the relaxation time
when the intermediate transfer member is substantially
isolated from the influence of the capacitance of other
members within the transfer nips 40. Generally the charge
relaxation time applies for regions prior to or past the
transfer nips 40. It is the classic "RC time constant",
that is rkeo, the product of the material layer quantities
dielectric constant k times resistivity r times the
permitivity of vacuum eo. In general the resistivity of a
material can be sensitive to the applied field in the
material. In this case, the resistivity should be
determined at an applied field corresponding to about 25 to
100 volts across the layer thickness. The "nip relaxation
time" is the relaxation time within regions such as the
transfer nips 40. If 42 is a corona field generation
device, the "nip relaxation time" is substantially the same
as the charge relaxation time. However, if a bias transfer
device is used, the nip relaxation time is generally longer
than the charge relaxation time. This is because it is
influenced not only by the capacitance of the intermediate
transfer member 12 itself, but it is also influenced by the
extra capacitance per unit area of any insulating layers
that are present within the transfer nips 40. For example,
the capacitance per unit area of the photoconductor coating
on the image bearing member 30 and the capacitance per unit
area of the toner image influence the nip relaxation time.
For discussion, CL represents the capacitance per unit area
of the layer of the intermediate transfer member 12 and Ctot
represents the total capacitance per unit area of all
insulating layers in the first transfer nips 40, other than
the intermediate transfer member 12. When the field
generation station 42 is a bias roller, the nip relaxation
time is the charge relaxation time multiplied by the
quantity [1+ (Ctot/ CL)].
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The range of resistivity conditions defined in the
above discussion avoid high voltage drops across the
intermediate transfer member 12 during the transfers of the
component toner images at the first transfer nips 40. To
avoid high pre-nip fields, the volume resistivity in the
lateral or process direction of the intermediate transfer
member must not be too low. The requirement is that the
lateral relaxation time for charge flow between the field
generation station 42 in the first transfer nip 40 should
be larger than the lead in dwell time for the first
transfer nip 40. The lead in dwell time is the quantity
L/v. L is the distance from the pre-nip region of initial
contact of the intermediate transfer member 12 with the
component toner image, to the position of the start of the
field generation station 42 within the first transfer nip
40. The quantity v is the process speed. The lateral
relaxation time is proportional to the lateral resistance
along the belt between the field generating station 42 and
the pre-nip region of initial contact, and the total
capacitance per area Ctot of the insulating layers in the
first transfer nip 40 between the intermediate transfer
member 12 and the substrate of the image bearing member 30
of the toner image producing station 22, 24, 26, 28. A
useful expression for estimating the preferred resistivity
range that avoids undesirable high pre-nip fields near the
field generation stations 42 is: [rLVLCtot]>1. The quantity
rL is referred to as the "lateral resistivity" of the
intermediate transfer member 12. It is the volume
resistivity of the member divided by the thickness of the
member. In cases where the electrical properties of the
member 12 is not isotropic, the volume resistivity of
interest for avoiding high pre-nip fields is that
resistivity of the layer in the process direction. Also,
in cases where the resistivity depends on the applied
field, the lateral resistivity should be determined at a
field of between about 500 to 1500 volts/cm.
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Thus the preferred range of resistivity for the single
layer intermediate transfer member 12 depends on many
factors such as for example the system geometry, the
transfer member thickness, the process speed, and the
capacitance per unit area of the various materials in the
first transfer nip 40. For a wide range of typical system
geometry and process speeds the preferred resistivity for
a single layer transfer belt is typically a volume
resistivity less than about 1013 ohm-cm and a more preferred
range is typically <1011 ohm-cm volume resistivity. The
lower limit of preferred resistivity is typically a lateral
resistivity above about 108 ohms/square and more preferred
is typically a lateral resistivity above about 1010
ohms/square. As an example, with a typical intermediate
transfer member 12 thickness of around 0.01 cm, a lateral
resistivity greater than 1010 ohms/square corresponds to a
volume resistivity of greater than 108 ohm-cm.
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Discussion below will specify the preferred range of
electrical properties for the transfuse member 50 to allow
good transfer in the second transfer nip 48. The transfuse
member 50 will preferably have multiple layers and the
electrical properties chosen for the topmost layer of the
transfuse member 50 will influence the preferred
resistivity for the single layer intermediate transfer
member 12. The lower limits for the preferred resistivity
of the single layer intermediate transfer member 12
referred to above apply if the top most surface layer of
the transfuse member 50 has a sufficiently high
resistivity, typically equal to or above about 109 ohm-cm.
If the top most surface layer of the transfuse member 50
has a somewhat lower resistivity than about 109 ohm-cm, the
lower limit for the preferred resistivity of the single
layer intermediate transfer member 12 should be increased
in order to avoid transfer problems in the second transfer
nip 48. Such problems include undesirably high current
flow between the intermediate transfer member 12 and the
transfuse member 50, and transfer degradation due to
reduction of the transfer field. In the case where the
resistivity of the top most layer of the transfuse member
50 is less than about 109 ohm-cm, the preferred lower limit
volume resistivity for the single layer intermediate
transfer member 12 will typically be around greater than or
equal to 109 ohm-cm.
-
In addition, the intermediate transfer member 12
should have sufficient lateral stiffness to avoid
registration issues between toner image producing stations
22, 24, 26, 28 due to elastic stretch. Stiffness is the
sum of the products of Young's modulus times the layer
thickness for all of the layers of the intermediate
transfer member. The preferred range for the stiffness
depends on various systems parameters. The required value
of the stiffness increases with increasing amount of
frictional drag at and/or between the toner image producing
stations 22, 24, 26, 28. The preferred stiffness also
increases with increasing length of the intermediate
transfer member 12 between toner image producing stations,
and with increasing color registration requirements. The
stiffness is preferably >800 PSI-inches and more preferably
>2000 PSI-inches, (>90 Nm and more preferably >225 Nm).
-
One of the material candidates for the single layer
intermediate transfer member 12 is a polyamide that
achieves good electrical control via conductivity
controlling additives.
-
The intermediate transfer member 12 may also
optionally be multi-layered. The back layer, opposite the
toner area, will preferably be semi-conductive in the
discussed range. The preferred materials for the back
layer of a multi-layered intermediate transfer member 12
are the same as that discussed for the single layer
intermediate belt 12. Within limits, the top layers can
optionally be "insulating" or semiconductive. There are
certain advantages and disadvantages of either.
-
A layer on the intermediate transfer member 12 can be
thought of as behaving "insulating" for the purposes of
discussion here if the relaxation time for charge flow is
much longer than the dwell time of interest. For example,
a layer behaves "insulating" during the dwell time in the
first transfer nip 40 if the nip relaxation time of that
layer in the first transfer nip 40 is much longer than the
time that a section of the layer spends in traveling
through the first transfer nip 40. A layer behaves
insulating between toner image producing stations 22, 24,
26, 28 if the charge relaxation time for that layer is much
longer than the dwell time that a section of the layer
takes to travel between the toner image producing stations.
On the other hand, a layer behaves semiconducting in the
sense meant here when the relaxation times are comparable
or lower than the appropriate dwell times. For example, a
layer behaves semi conductive during the dwell time of the
first transfer nip 40 when the nip relaxation time is less
than the dwell time in the first transfer nip 40.
Furthermore, a layer on the intermediate transfer member 12
behaves semiconductive during the dwell time between toner
image producing stations 22, 24, 26, 28 if the relaxation
time of the layer is less than the dwell time between toner
image producing stations. The expressions for determining
the relaxation times of any top layer on the intermediate
transfer member 12 are substantially the same as those
described previously for the single layer intermediate
transfer member. Thus whether or not a layer on the multi-layered
intermediate transfer member 12 behaves
"insulating" or "semiconducting" during a particular dwell
time of interest depends not only on the electrical
properties of the layer but also on the process speed, the
system geometry, and the layer thickness.
-
A layer of the transfer belt will typically behave
"insulating" in most transfer systems if the volume
resistivity is generally greater than about 1013 ohm-cm.
Insulating top layers on the intermediate transfer member
12 cause a voltage drop across the layer and thus reduce
the voltage drop across the composite toner layer in the
first transfer nip 40. Therefore, the presence of
insulating layers requires higher applied voltages in the
first and second transfer nips 40, 48 to create the same
electrostatic fields operating on the charged composite
toner image. The voltage requirement is mainly driven by
the "dielectric thickness" of such insulating layers, which
is the actual thickness of a layer divided by the
dielectric constant of that layer. One potential
disadvantage of an insulating layer is that undesirably
very high voltages will be required on the intermediate
transfer member 12 for good electrostatic transfer of the
component toner image if the sum of the dielectric
thickness of the insulating layers on the intermediate
transfer member 12 is too high. This is especially true in
color imaging systems with layers that behave "insulating"
over the dwell time longer than one revolution of the
intermediate transfer member 12. Charge will build up on
such insulating top layers due to charge transfer in each
of the field generation stations 42. This charge buildup
requires higher voltage on the back of the intermediate
transfer member 12 in the subsequent field generation
stations 42 to achieve good transfer of the subsequent
component toner images. This charge can not be fully
neutralized between first transfer nips 40 with image
conditioning station 46 corona devices without also causing
undesirable neutralization or even reversal of the charge
of the transferred composite toner image on the
intermediate transfer member 12. Therefore, to avoid the
need for unacceptably high voltages on the back of the
intermediate transfer member 12, the total dielectric
thickness of such insulating top layers on the intermediate
transfer member 12 should preferably be kept small for good
and stable transfer performance. An acceptable total
dielectric thickness can be as high as about 50 mm and a
preferred value is <10 mm.
-
The top most layer of the intermediate transfer member
12 preferably has good toner releasing properties such as
low surface energy, and preferably has low affinity to oils
such as silicone oils. Materials such as PFA, TEFLON™,
and various fluoropolymers are examples of desirable
overcoating materials having good toner release properties.
One advantage of an insulating coating over the
semiconductive backing layer of the intermediate transfer
member 12 is that such materials with good toner releasing
properties are more readily available if the constraint of
needing them to also be semiconductive is removed. Another
potential advantage of high resistivity coatings applies to
embodiments that wish to use a transfuse member 50 having
a low resistivity top most layer, such as <<109 ohm-cm. As
discussed, the resistivity for the intermediate transfer
member 12 of a single layer is preferably limited to
typically around >109 ohm-cm to avoid transfer problems in
the second transfer nip 48 if the resistivity of the top
most layer of the transfuse member 50 is lower than about
109 ohm-cm. For a multiple layer intermediate transfer
member 12, having a sufficiently high resistivity top most
layer, preferably >109 ohm-cm, the resistivity of the back
layer can be lower.
-
Semiconductive coatings on the intermediate transfer
member 12 are advantaged in that they do not require charge
leveling to level the charge on the intermediate transfer
member 12 prior to and between toner image producing
stations 22, 24, 26, 28. Semiconductive coatings on the
intermediate transfer member are also advantaged in that
much thicker top layers can be allowed compared to
insulating coatings. The charge relaxation conditions and
the corresponding ranges of resistivity conditions needed
to enable such advantages are similar to that already
discussed for the back layer. Generally, the
semiconductive regime of interest is a resistivity such
that the charge relaxation time is smaller than the dwell
time spent between toner image producing stations 22, 24,
26, 28. A more preferred resistivity construction allows
thick layers, and this construction is a resistivity range
such that the nip relaxation time within the first transfer
nip 40 is smaller than the dwell time that a section of the
intermediate transfer member 12 takes to move through the
first transfer nip 40. In such a preferred regime of
resistivity the voltage drop across the layer is small at
the end of the transfer nip dwell time, due to charge
conduction through the layer.
-
The constraint on the lower limit of the resistivity
related to the lateral resistivity apply to the
semiconductive top most layer, to any semiconductive middle
layers, and to the semiconductive back layer of a multiple
layer intermediate transfer member 12. The preferred
resistivity range for each such layer is substantially the
same as discussed for the single layer intermediate
transfer member 12. Also, the additional constraint on the
resistivity related to transfer problems in the second
transfer nip 48 apply to the top most layer of a multiple
layer intermediate transfer member 12. Preferably, the top
most semiconductive layer of the intermediate transfer
member 12 should be typically >109 ohm-cm when the top most
layer of the transfuse member 50 is typically somewhat less
than 109 ohm-cm.
-
Transfer of the composite toner image in the second
transfer nip 48 is accomplished by a combination of
electrostatic and heat assisted transfer. The field
generation station 42 and guide roller 74 are electrically
biased to electrostatically transfer the charged composite
toner image from the intermediate transfer member 12 to the
transfuse member 50.
-
The transfer of the composite toner image at the
second transfer nip 48 can be heat assisted if the
temperature of the transfuse member 50 is maintained at a
sufficiently high optimized level and the temperature of
the intermediate transfer member 12 is maintained at a
considerably lower optimized level prior to the second
transfer nip 48. The mechanism for heat assisted transfer
is thought to be softening of the composite toner image
during the dwell time of contact of the toner in the second
transfer nip 48. The toner softening occurs due to contact
with the higher temperature transfuse member 50. This
composite toner softening results in increased adhesion of
the composite toner image toward the transfuse member 50 at
the interface between the composite toner image and the
transfuse member. This also results in increased cohesion
of the layered toner pile of the composite toner image.
The temperature on the intermediate transfer member 12
prior to the second transfer nip 48 needs to be
sufficiently low to avoid too high a toner softening and
too high a resultant adhesion of the toner to the
intermediate transfer member 12. The temperature of the
transfuse member 50 should be considerably higher than the
toner softening point prior to the second transfer nip to
ensure optimum heat assist in the second transfer nip 48.
Further, the temperature of the intermediate transfer
member 12 just prior to the second transfer nip 48 should
be considerably lower than the temperature of the transfuse
member 50 for optimum transfer in the second transfer nip
48.
-
The temperature of the intermediate transfer member 12
prior to the second transfer nip 48 is important for
maintaining good transfer of the composite toner image. An
optimum elevated temperature for the intermediate transfer
member 12 can allow the desired softening of the composite
toner image needed to permit heat assist to the
electrostatic transfer of the second transfer nip 48 at
lower temperatures on the transfuse member 50. However,
there is a risk of the temperature of the intermediate
transfer member 12 becoming too high so that too much
softening of the composite toner image occurs on the
intermediate transfer member prior to the second transfer
nip 48. This situation can cause unacceptably high
adhesion of the composite toner image to the intermediate
transfer member 12 with resultant degraded second transfer.
Preferably the temperature of the intermediate transfer
member 12 is maintained below or in the range of the Tg
(glass transition temperature) of the toner prior to the
second transfer nip 48.
-
The transfuse member 50 is guided in a cyclical path
by guide rollers 74, 76, 78, 80. Guide rollers 74, 76
alone or together are preferably heated to thereby heat the
transfuse member 50. The intermediate transfer member 12
and transfuse member 50 are preferably synchronized to have
the generally same velocity in the transfer nip 48.
Additional heating of the transfuse member is provided by
heating rollers 74 and 76, and further the transfuse member
50 can be heated by the addition of a heating station 82.
The heating station 82 is preferably formed of radiant
lamps positioned internally to the path defined by the
transfuse member 50. Alternatively the heating station 82
can be a heated shoe contacting the back of the transfuse
member 50 or other heat sources located internally or
externally to the transfuse member 50. The transfuse
member 50 and a pressure roller 84 define a third transfer
nip 86 therebetween.
-
To assure acceptable release of the toner from the
transfuse member 50, an optional release agent applicator
88 applies a uniform, controlled quantity of a releasing
material or agent, such as a silicone oil, to the surface
of the transfuse member 50. (See Fig. 3) The releasing
agent is applied to the surface of the transfuse member
prior to the second transfer nip. The toner image is
transferred onto the surface of the transfer member having
the release agent. The releasing agent serves to assist in
the subsequent release of the composite toner image from
the transfuse member 50 to the substrate in the third
transfer nip 86. The release agent forms a weak boundary
layer that aids in separation of the toner image from the
transfuse member 50. Silicone oil typically has a low
surface energy therefore spreading easily on the surface of
materials having a relatively higher surface energy.
Silicone oil is additionally tolerant of the heat in the
third transfer nip. A transfuse member having an outer
most or topmost layer of silicone will have natural release
properties from the silicone oil present in the material.
However, this silicone oil will be depleted overtime
leading to a decrease in release properties and therefore
decreased transfer efficiency of the toner image to the
substrate. In addition the transfuse member will
eventually fail. With reference to figure 8, disclosing a
transfuse system having a transfuse member with a top most
layer of silicone, and without a release agent management
system, line 490 calculated from the data shown discloses
the amount of silicone oil per copy decreases as the copy
count increases. The decrease in oil on the copies is an
indicator of the depletion of natural oil in the silicone
of the transfuse member. This decrease results in degraded
release of the toner image form the transfuse member and
transfer to the substrate, and eventual failure of the
transfuse member. The release agent applicator applies a
preestablished amount of release agent, typically a
silicone oil, to reduce or eliminate the loss of the
natural silicone oils during the printing process. The
application rate is preferably the rate of loss of the
release agent to typically the substrate. This application
rate results in neither an increase nor a decrease of
silicone oils present on the transfuse member. Release
agents can be absorbed by substrates, such as paper, at
rates of about .1-.2 mg/sheet of substrate. Therefore, at
a steady operating state, the preferred application rate,
represented by line 491 is generally the transfer rate of
release agent to the substrate at a given process rate. A
slower process rate typically results in increase
absorption of release agent by the substrate. Initially
the application rate may need to be increased to fully coat
the transfuse member and other associated components. The
application rate can also be higher if additional release
agent is desired for additional purposes. However, a
relatively high amount of release agent is generally not
preferred due to the potential for the additional release
agent to be transferred to the intermediate member and
ultimately to a photoreceptor.
-
The release agent applicator 88 is preferably of a web
configuration for application of relatively low levels of
release agent (See Fig. 3). The release agent applicator
88 has a web 289 impregnated with release agent. The web
289 is fed off of a supply roll 290 and urged or biased
against the surface of the transfuse member 50 by a nip
roll 291. Release agent is transferred from the web 289 to
the surface of the transfuse member by frictional contact
of the relatively slower surface speed of the web 289
against the relatively higher surface speed of the
transfuse member 50. After contact with transfuse member
50, the web is directed around a wrap roll 292 and spooled
onto a take up roll 293. The nip roll 291 and take up roll
are preferably rotatably driven to move the web 289 past
the transfuse member 50. The supply roll 290 is preferably
undriven. The web 289 can additionally serve to clean the
residual toners, paper debris, and other contaminants on
the surface of the transfuse member 50.
-
For application of relatively higher levels of release
agent, a release agent applicator 188 of a roll
configuration can be employed in place of the release agent
applicator 88. (See Fig. 4) The release agent applicator
188 has a metering roll 190 partially immersed in a bath of
release agent 193. The release agent 193 is contained in a
sump 192 and is replenished as depleted. The metering roll
190 rollingly engages a donor roll 189 interposed between
the metering roll 190 and the transfuse member 50. The
metering roll 190 and donor roll 189 are preferably idler
rolls whereby the rotation of the metering roll 190 and
donor roll 189 are derived from the rolling contact of the
donor roll 190 with the moving transfuse member 50.
-
Release agent 193 coats the surface of the rotating
metering roll 193 and is transferred to the donor roll 189
at the nip defined therebetween. A wick 194 submersed in
the sump 192 and slidingly engaging the surface of the
metering roll 190 disturbs the air layer on the surface of
the metering roll 190 to thereby assist in application of
the release agent to the metering roll 190. The metering
roll 190 is preferably formed of a steel surface roll.
-
A wiper blade 191 contacts the metering roll 190 to
meter the quantity of release agent on the surface of the
metering roll 190 to a preestablished thickness to result
in the preferred rate of release agent applied to the
transfuse member 50. The release agent transferred to the
donor roll 189 is further transferred to the transfuse
member 50 at the nip defined therebetween. The donor roll
189 preferably has a conformable surface, such as silicone,
for improved transfer of the release agent 193 to the
transfuse member 50. Other arrangements of oil application
include oil rolls or webs. This includes both fibrous or
microporous sleeved rolls and webs.
-
Transfuse members 50 having a top most layer of
Viton™ will typically require a higher rate of application
of release agent to provide sufficient release of the toner
image from the transfuse member. There is essentially no
natural excretion of natural oils from Viton™. Therefore
additional release agent is preferably applied to ensure
complete coating of the top most surface of the transfuse
member 50. The application rate is preferably from .2-10
mg/sheet of substrate, but can be higher.
-
The transfuse member 50 is preferably constructed of
multiple layers. The transfuse member 50 must have
appropriate electrical properties for being able to
generate high electrostatic fields in the second transfer
nip 50. To avoid the need for unacceptably high voltages,
the transfuse member 50 preferably has electrical
properties that enable sufficiently low voltage drop across
the transfuse member 50 in the second transfer nip 48. In
addition the transfuse member 50 will preferably ensure
acceptably low current flow between the intermediate
transfer member 12 and the transfuse member 50. The
requirements for the transfuse member 50 depend on the
chosen properties of the intermediate transfer member 12.
In other words, the transfuse member 50 and intermediate
transfer member 12 together have sufficiently high
resistance in the second transfer nip 48.
-
The transfuse member 50 will preferably have a
laterally stiff back layer, a thick, conformable rubber
intermediate layer, and a thin outer most layer.
Preferably the thickness of the back layer will be greater
than about 0.05 mm. Preferably the thickness of the
intermediate conformable layers and the top most layer
together will be greater than 0.25 mm and more preferably
will be greater than about 1.0 mm. The back and
intermediate layers need to have sufficiently low
resistivity to prevent the need for unacceptably high
voltage requirements in the second transfer zone 48. The
preferred resistivity condition follows previous
discussions given for the intermediate transfer member 12.
That is, the preferred resistivity range for the back and
intermediate layer of a multiple layer transfuse member 50
ensures that the nip relaxation time for these layers in
the field generation region of the second transfer nip 48
is smaller than the dwell time spent in the field
generation region of the second transfer nip 48. The
expressions for the nip relaxation times and the nip dwell
time are substantially the same as the ones discussed for
the single layer intermediate transfer member 12. Thus the
specific preferred resistivity range for the back and
intermediate layers depends on the system geometry, the
layer thickness, the process speed, and the capacitance per
unit area of the insulating layers within the transfer nip
48. Generally, the volume resistivity of the back and
intermediate layers of the multi-layer transfuse member 50
will typically need to be below about 1011 ohm-cm and more
preferably will be below about 108 ohm-cm for most systems.
Optionally, the back layer of the transfuse member 50 can
be highly conductive such as a metal.
-
Similar to the multiple layer intermediate transfer
member 12, the top most layer of the transfuse member 50
can optionally behave "insulating" during the dwell time in
the transfer nip 48 (typically >1012 ohm-cm) or
semiconducting during the transfer nip 48 (typically 106 to
1012 ohm-cm). However, if the top most layer behaves
insulating, the dielectric thickness of such a layer will
preferably be sufficiently low to avoid the need for
unacceptably high voltages. Preferably for such insulating
behaving top most layers, the dielectric thickness of the
insulating layer should typically be less than about 50m
and more preferably will be less than about 10m. If a very
high resistivity insulating top most layer is used, such
that the charge relaxation time is greater than the
transfuse member cycle time, charge will build up on the
transfuse member 50 due to charge transfer during the
transfer nip 48. Therefore, a cyclic discharging station
77 such as a scorotron or other charge generating device
will be needed to control the uniformity and reduce the
level of cyclic charge buildup.
-
The transfuse member 50 can alternatively have
additional intermediate layers. Any such additional
intermediate layers that have a high dielectric thickness
typically greater than about 10 microns will preferably
have a sufficiently low resistivity such to ensure low
voltage drop across the additional intermediate layers.
-
The transfuse member 50 preferably has a top most
layer formed of a material having a low surface energy, for
example silicone elastomer, fluoroelastomers such as
Viton™, polytetrafluoroethylene, perfluoralkane, and other
fluorinated polymers. The transfuse member 50 will
preferably have intermediate layers between the top most
and back layers constructed of a Viton™ or preferably
silicone with carbon or other conductivity enhancing
additives to achieve the desired electrical properties.
The back layer is preferably a fabric modified to have the
desired electrical properties. Alternatively the back
layer can be a metal such as stainless steel.
-
The transfuse member 50 can optionally be in the form
of a transfuse roller (not shown), or is preferably in the
form of a transfuse belt. A transfuse roller for the
transfuse member 50 can be more compact than a transfuse
belt and it can also be advantaged relative to less
complexity of the drive and steering requirements needed to
achieve good motion quality for color systems. However, a
transfuse belt has advantages over a transfuse roller such
as enabling large circumference for longer life, better
substrate stripping capability, and generally lower
replacement costs.
-
The intermediate layer of the transfuse member 50 is
preferably thick to enable a high degree of conformance to
rougher substrates 70 and to thus expand the range of
substrate latitude allowed for use in the printer 10. In
addition the use of a relatively thick intermediate layer,
greater than about 0.25 mm and preferably greater than 1.0
mm enables creep for improved stripping of the document
from the output of the third transfer nip 86. In a further
embodiment, thick low durometer conformable intermediate
and top most layers such as silicone are employed on the
transfuse member 50 to enable creation of low image gloss
by the transfuse system with wide operating latitude.
-
The use of a relatively high temperature on the
transfuse member 50 prior to the second transfer nip 48
creates advantages for the transfuse system. The transfer
step in the second transfer nip 48 simultaneously transfers
single and stacked multiple color toner layers of the
composite toner image. The toner layers nearest to the
transfer belt interface will be hardest to transfer. A
given separation color toner layer can be nearest the
surface of the intermediate transfer member 12 or it can
also be separated from the surface, depending on the color
toner layer to be transferred in any particular region.
For example, if a toner layer of magenta is the last
stacked layer deposited onto the transfer belt, the magenta
layer can be directly against the surface of the
intermediate transfer member 12 in some color print regions
or else stacked above cyan and/or yellow toner layers in
other color regions. If transfer efficiency is too low, a
high fraction of the color toners that are close to the
intermediate transfer member 12 will not transfer but a
high fraction of the same color toner layers that are
stacked onto another color toner layer will transfer. Thus
for example, if the transfer efficiency of the composite
toner image is not very high, the region of the composite
toner image having cyan toner directly in contact with the
surface of the intermediate transfer member 12 can transfer
less of the cyan toner layer than the regions of the
composite toner image having cyan toner layers on top of
yellow toner layers. The transfer efficiency in the second
transfer nip 48 is >95% therefore avoiding significant
color shift.
-
With reference to Figure 6 disclosing experimental
data on the amount of residual toner left on the
intermediate transfer member 12 as a function of the
transfuse member 50 temperature. Curve 90 is with electric
field, pressure and heat assist and curve 92 is without
electric field assist but with pressure and heat assist.
A very low amount of residual toner means very high
transfer efficiency. The toner used in the experiments has
a glass transition temperature range Tg of around 55°C.
Substantial heat assist is observed at temperatures of the
transfuse member 50 above Tg. Substantially 100% toner
transfer occurs when operating with an applied field and
with the transfuse member 50 temperature above around
165°C, well above the range of the toner Tg. Preferential
temperatures will vary depending on toner properties. In
general, operation well above the Tg is found to be
advantageous for the heat assist to the electrostatic
transfer for many different toners and system conditions.
-
Too high a temperature of the transfuse member 50 in
the second transfer nip 48 can cause problems due to
unacceptably high toner softening on the intermediate
transfer member side of the composite toner layer. Thus
the temperature of the transfuse member 50 prior to the
second transfer nip 48 must be controlled within an optimum
range. The optimum temperature of the composite toner
image in the second transfer nip 48 is less than the
optimum temperature of the composite toner image in the
third transfer nip 86. The desired temperature of the
transfuse member 50 for heat assist in the second transfer
nip 48 can be readily obtained while still obtaining the
desired higher toner temperatures needed for more complete
toner melting in the third transfer nip 86 by using pre-heating
of the substrate 70. Transfer and fix to the
substrate 70 is controlled by the interface temperature
between the substrate and the composite toner image.
Thermal analysis shows that the interface temperature
increases with both increasing temperature of the substrate
70 and increasing temperature of the transfuse member 50.
-
At a generally constant temperature of the transfuse
member 50 in the second and third transfer nips 48, 86, the
optimum temperature for transfer in the second transfer nip
48 is controlled by adjusting the temperature of the
intermediate transfer member 12, and transfuse in the third
transfer nip 86 is optimized by preheating of the substrate
70. Alternatively, for some toner formulations or
operation regimes no preheating of the substrate 70 is
required.
-
The substrate 70 is transported and registered by a
material feed and registration system 69 into a substrate
pre-heater 73. The substrate pre-heater 73 is preferably
formed a transport belt transporting the substrate 70 over
a heated platen. Alternatively the substrate pre-heater 73
can be formed of heated rollers forming a heating nip
therebetween. Alternately, the substrate pre-heater 73 can
be formed of radiant heaters. The substrate 70 after
heating by the substrate preheater 73 is directed into the
third transfer nip 86.
-
Figure 7 discloses experimental curves 94, 96 of a
measure of fix called crease as a function of the
temperature of the transfuse member 50 for different pre-heating
temperatures of a substrate. Curve 94 is for a
preheated substrate and a curve 96 for a substrate at room
temperature. The results disclose that the temperature of
the transfuse member 50 for similar fix level decreases
significantly at higher substrate pre-heating curve 94
compared to lower substrate pre-heating curve 96. Heating
of the substrate 70 by the substrate pre-heater 73 prior to
the third transfer nip 86 allows optimization of the
temperature of the transfuse member 50 for improved
transfer of the composite toner image in the second
transfer nip 48. The temperature of the transfuse member
50 can thus be controlled at the desired optimum
temperature range for optimum transfer in the second
transfer nip 48 by controlling the temperature of the
substrate 70 at the corresponding required elevated
temperature needed to create good fix and transfer to the
substrate 70 in the third transfer nip 86 at this same
controlled temperature of the transfuse member 50.
Therefore cooling of the transfuse member 50 prior to the
second transfer nip 48 is not required for optimum transfer
in the second transfer nip 48. In other words the
transfuse member 50 can be maintained at substantially the
same temperature in both the second and third transfer nips
48, 86.
-
Furthermore, the over layer, the intermediate and
topmost layers, of the transfuse member 50 can be
relatively thick, preferably greater than about 1.0 mm,
because no substantial cooling of the transfuse member 50
is required prior to the second transfer nip 48.
Relatively thick intermediate and topmost layers of the
transfuse member 50 allows for increased conformability.
The increased conformability of the transfuse member 50
permits printing to a wider latitude of substrates 70
without a substantial degradation in print quality. In
other words the composite toner image can be transferred
with high efficiency to relatively rough substrates 70.
-
In addition, the transfuse member 50 is preferably at
substantially the same temperature in both the second and
third transfer nips 48, 86. However, the composite toner
image preferably has a higher temperature in the third
transfer nip 86 relative to the temperature of the
composite toner image in the second transfer nip 48.
Therefore the substrate 70 has a higher temperature in the
third transfer nip 86 relative to the temperature of the
intermediate transfer member 12 in the second transfer nip
48. Alternatively, the transfuse member 50 can be cooled
prior to the second transfer nip 48, however the
temperature of the transfuse member 50 is maintained above,
and preferably substantially above the Tg of the composite
toner image. Furthermore, under certain operating
conditions, the top surface of the transfuse member 50 can
be heated just prior to the second transfer nip 48.
-
The composite toner image is transferred and fused to
the substrate 70 in the third transfer nip 86 to form a
completed document 72. Heat in the third transfer nip 86
from the substrate 70 and transfuse member 50, in
combination with pressure applied by the pressure roller 84
acting against the guide roller 76 transfer and fuse the
composite toner image to the substrate 70. The pressure in
the third transfer nip 86 is preferably in the range of
about 40 - 500 psi (275 - 3450 KPa), and more preferably in
the range 60 psi to 200 psi (414 to 1380 KPa). The
transfuse member 50, by combination of the pressure in the
third transfer nip 86 and the appropriate durometer of the
transfuse member 50 induces creep in the third transfer nip
to assist release of the composite toner image and
substrate 70 from the transfuse member 50. Preferred creep
is greater than 4%. Stripping is preferably further
assisted by the positioning of the guide roller 78 relative
to the guide roller 76 and pressure roller 84. The guide
roller 78 is positioned to form a small amount of wrap of
the transfuse member 50 on the pressure roller 84. The
geometry of the guide rollers 76, 78 and pressure roller 84
form the third transfer nip 86 having a high pressure zone
and an adjacent low pressure zone in the process direction.
The width of the low pressure zone is preferably one to
three times, or more preferably about two times the width
of the high pressure zone. The low pressure zone
effectively adds an additional 2-3% of relative creep and
thereby improves stripping. Additional stripping
assistance can be provided by stripping system 87,
preferably an air puffing system. Alternatively the
stripping system 87 can be a stripping blade or other well
known systems to strip documents from a roller or belt.
Alternatively, the pressure roller can be substituted with
other pressure applicators such as a pressure belt.
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After stripping, the document 72 is directed to an
optional selectively activatable simplex or duplex glossing
station 110 and thereafter to a sheet stacker or other well
know document handing system (not shown). The printer 10
can additionally provide duplex printing by directing the
document 72 through an inverter 71 where the document 72 is
inverted and reintroduced at about the middle of the pre-transfer
heating station 73 for printing on the opposite
side of the document 72.
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Preferably, a cooling station 66 cools the
intermediate transfer member 12 after second transfer nip
48 in the process direction. The cooling station 66
preferably transfers a portion of the heat on the
intermediate transfer member 12 at the exit side of the
second transfer nip 48 to a heating station 64 at the
entrance side of the second transfer nip 48. Alternatively
the cooling station 66 can transfer a portion of the heat
-absorbed from the intermediate transfer member 12 at the
exit side of the second transfer nip 48 to the substrate
prior to the third transfer nip 86. Alternatively the heat
sharing can be implemented with multiple heating stations
64 and cooling stations 66 to improve heat transfer
efficiency.
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A cleaning station 54 engages the intermediate
transfer member 12. The cleaning station 54 preferably
removes release agent, typically an oil, that may be
deposited onto the intermediate transfer member 12 from the
transfuse member 50 at the second transfer nip. For
example, if a preferred silicone top most layer is used for
the transfuse member 50, some silicone oil present in the
silicone material can transfer from the transfuse member 50
to the intermediate transfer member 12 and eventually
contaminate the image bearing members 30. In addition the
cleaning station 54 removes residual toner remaining on the
intermediate transfer member 12. The cleaning station 54
also cleans release agents deposited on the transfuse
member 50 by the release agent management system 88 that
can contaminate the image bearing members 30.
The cleaning station 54 preferably has first and second
cleaning members 254, 255 to remove release agent and
residual solids from the intermediate transfer member.
While the preferred cleaning station 54 has first and
second cleaning members 254, 255, a single cleaning member
in certain operational environments can provide adequate
removal of release agent from the intermediate member 12 to
prevent contamination of the image bearing members 30. The
first cleaning member 254 is preferably a blade cleaner
(see figure 3). The blade cleaner is oriented transverse
to the process direction of the intermediate member 12.
The blade cleaner is urged against the surface of the
intermediate member 12 to scrape residual toner, debris and
particularly release agent, typically silicone oil, from
the surface of the transfuse member 50.
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The second cleaning member 255 is a web cleaner having
a spool of flexible micro-porous absorbent web material
256. The web material 256 preferably has high affinity and
it absorbs release agent through capillary action.
Examples of the web material 256 are porous paper, cotton
fabric, -open cell foam, microporous materials (i.e. Gore-Tex),
nonwoven fabric (i.e. BMP-like fabric) and other
natural and synthetic absorbent materials. The web
material 256 is spooled off a supply reel 257 and urged
against the surface of the intermediate member 12 by a nip
roller 258. The web material 256 absorbs release agent
through capillary action and further collects residual
solids residing on the intermediate member 50. The web
material 256 is indexed at a relatively slow rate in a
direction opposite the process direction. The web material
256 is indexed as it becomes saturated with oil or the
pores become clogged with solids. The web material 256 is
spooled up on a take up reel 260. Alternatives of the nip
roller 258 could be a pinched roller, or skid pad.
Alternately, the web material 256 can be continuously
moved. The cleaning station 54 is preferably positioned
relatively far from the second transfer nip 48 and
relatively close to the first transfer nip, in the process
direction. This spacing provides additional time for drop
formation of any release agent transferred to the
intermediate member 50. Drops of release agent are
typically cleaned with higher efficiency by the cleaning
station 54 than a film of release agent on the intermediate
member 50. The cleaning system further cools the
intermediate transfer member before it contacts with the PR
drums.
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Both sides of the material 256 web can effectively be
used by adding more nip rollers to the system. As it shows
in figure 5, the web material 256 is spooled off a supply
reel 257 and urged against the surface of the intermediate
member 12 by a nip roller 258. The web material 256 is
directed around roller 261, and a second nip roller 264.
The second nip roller 264 also serves as the take-up reel.
The roller 261 can be optional depending on the space
requirement of the web system. Alternately an additional
take-up reel (not shown) can be added to separate the
function of the second nip roller 264 from the take-up
reel. The disclosed embodiment not only enables the
effective use of both side of the web material 256, it also
creates effective cleaning of the intermediate transfer
member 12 by web cleaning it twice prior to the first
transfer nips.
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In a further embodiment, the first cleaning member 254
having a blade is combined with a second cleaning member
formed of a wick cleaner 270. The wick cleaner 270 has a
wick 271 contacting the surface of the intermediate member
12 to absorb release agent on the surface of the
intermediate transfer member 12. The wick 271 absorbs the
release agent from the surface of the intermediate transfer
member and transfers the release agent to a reservoir 272.
The wick 271 can be susceptible to contamination or
clogging from solids on the intermediate member 12. The
wick 271 is therefore less preferred in some operational
environments due to the difficulty of periodically bringing
a clean portion of the wick 271 into contact with the
intermediate transfer member 12. The operational life of
the wick cleaner 270 can be extended by indexing of the
wick 271.
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Although the preferred cleaning system embodiment
having blade member prior to the web, the orders of the two
cleaning members could be reversed in some applications.
The two members of the cleaning system 54 is not limit to
the combination of blade and web; it could be a brush
cleaner and a blade, a brush cleaner and a web, etc.
Another embodiment is a first blade, web, and a second
blade. The first blade takes up a majority of the
contaminants and protects the web from getting dirty too
fast. The web does the further cleaning and creates some
leveling of oil uniformity on the transfer member. The
second blade performs final leveling of the oil to meet the
system requirements. As practice in prior art, a toner
disturber can be installed prior to the cleaning system.
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The toner on the intermediate member is also a good
absorber of the release agent. The toner disturber or the
cleaner moves the toner on the intermediate member to
further enhance oil absorbing process and to ensure the oil
uniformity on the intermediate member. The toner
refreshing process required for the sticky cleaner 58 can
also refresh the intermediate member 12 and the cleaning
system 54 to correct oil non-uniformity problem. Oil non-uniformity
might cause transfer problems in local regions
and results in image defects.
-
All the cleaning systems proposed for the intermediate
member could also be implemented on the image bearing
member 30 to further reduce the release agent contamination
to the image bearing member 30 and other imaging subsystems.
-
The cleaning system can be implemented whenever there
is release agent in the intermediate member, independent of
the source of the release agent. In a two member transfuse
system having an image bearing member and a transfuse
member, the release agent could come from the internal or
external oil of the transfuse member. In a xerographic
printing system with an intermediate transfer member,
substrates on the duplex mode could transfer the release
agent from a fusing station back to the intermediate
member.
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A cleaning system 58 engages the surface of the
transfuse member 50 past the third transfer nip 86 to
remove any residual toner and contaminants from the surface
of the transfuse member 50. Preferably the cleaning system
58 includes a cleaning roller having a sticky surface
created by partially melted toner. The cleaning roller is
preferably heated by the transfuse member 50 to thereby
maintain the toner on the cleaning roller in a partially
melted state. The cleaning roller is maintained in a
pressure arrangement of 10-50 psi (69 - 345 KPa) against
the roll 80. Alternatively the cleaning roller can be
opposed by a pressure roller (not shown) located on the
underside of the transfuse member. The operating
temperature range is sufficiently high to melt the toner,
but sufficiently low to prevent toner layer splitting. The
partially melted toner is maintained within the optimum
temperature range for cleaning by the temperature of the
transfuse member 50 in combination with any necessary
heating or cooling of the cleaning roller.
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The transfuse member 50 is driven in the cyclical path
by the pressure roller 84. Alternatively drive is provided
or enhanced by driving guide roller 74. The intermediate
transfer member 12 is preferably driven by the pressured
contact with the transfuse member 50, and further torque
assisted by roll 16. Drive to the intermediate transfer
member 12 is preferably derived from the drive for the
transfuse member 50, by making use of adherent contact
between intermediate transfer member 12 and the transfuse
member 50. The adherent contact causes the transfuse
member 50 and intermediate transfer member 12 to move in
synchronism with each other in the second transfer nip 48.
Adherent contact between the intermediate transfer member
12 and the toner image producing stations 22, 24, 26, 28
may be used to ensure that the intermediate transfer member
12 moves in synchronism with the toner image producing
stations 22, 24, 26, 28 in the first transfer zones 40.
Therefore the toner image producing stations 22, 24, 26, 28
can be driven by the transfuse member 50 via the
intermediate transfer member 12. Alternatively, the
intermediate transfer member 12 is independently driven.
When the intermediate transfer member is independently
driven, a motion buffer (not shown) engaging the
intermediate transfer member 12 buffers relative motion
between the intermediate transfer member 12 and the
transfuse member 50. The motion buffer system can include
a tension system with a feedback and control system to
maintain good motion of the intermediate transfer member 12
at the first transfer nips 40 independent of motion
irregularity translated to the intermediate transfer member
12 at the second transfer nip 48. The feedback and control
system can include registration sensors sensing motion of
the intermediate transfer member 12 and/or sensing motion
of the transfuse member 50 to enable registration timing of
the transfer of the composite toner image to the substrate
70.
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An optional gloss enhancing station 110 is preferably
positioned down stream in the process direction from the
third transfer nip 86 for selectively gloss enhancing the
gloss properties of documents 72. The gloss enhancing
station 110 has opposed fusing members 112, 114 defining a
gloss nip 116 there between, which can be simplex or
duplex. The gloss nip 116 is adjustable to provide the
selectability of the gloss enhancing. In particular, the
fusing members are cammed whereby the transfuse nip is
sufficiently large to allow a document to pass through
without substantial contact with either fusing member 112,
114 that would cause glossing. When the operator selects
gloss enhancement, the fusing members 112, 114 are cammed
into pressure relation and driven to thereby enhancement
the level of gloss on documents 72 passed through the gloss
nip 116. The amount of gloss enhancement is operator
selectable by adjustment of the temperature of the fusing
members 112, 114. Alternatively, the gloss level can be
controlled by adjustment of the nip pressure. Increasing
the nip pressure increases the nip width, and hence the
dwell. Over a relatively short dwell period, the heat
transferred to the paper is roughly proportional to the
square root of the dwell which is directly proportional to
the nip width. Higher temperatures of the fusing members
112, 114 will result in increased gloss enhancement.
US-A-5,521,688, describes a gloss enhancing station with a
radiant fuser.
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The separation of fixing and glossing functions
provides operational advantages. Separation of the fixing
and glossing functions permits operator selection of the
preferred level of gloss on the document 72. The
achievement of high gloss performance for color systems
generally requires relatively higher temperatures in the
third transfer nip 86. It also typically requires
materials on the transfuse member 50 having a higher heat
and wear resistance, as well as a higher durometer such as
Viton™. Excessive wear can result in differential gloss
caused by changes in surface roughness of the transfuse
member due to wear. The higher temperature requirements
and the use of more heat and wear resistant materials
generally results in the need for high oil application
rates by the release agent management system 88. In
transfuse systems such as the printer 10 increased
temperatures and increased amounts of oil on the transfuse
member 50 could possibly create contamination problems of
the photoreceptors 30. Printers having a transfuse system
and needing high gloss use a thick nonconformable transfuse
member, or a relatively thin transfuse member. However, a
relatively nonconformable transfuse member and a relatively
thin transfuse member fail to have the high degree of
conformance needed for good printing on, for example,
rougher paper stock.
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The use of the gloss enhancing station 110
substantially reduces or eliminates the need for gloss
creation in the third transfer nip 86. The reduction or
elimination of the need for gloss in the third transfer nip
86 therefore minimizes surface wear issues for color
transfuse member materials and enables a high life
transfuse member 50 with readily available silicone or
other similar soft transfuse member materials. It allows
the use of relatively thick layers on the transfuse member
50 with resultant gain in operating life for the transfuse
member materials and with resultant high conformance for
imaging onto rougher substrates. It reduces the
temperature requirements for the transfuse materials set
with further gain in transfuse material life, and it can
substantially reduce the oil requirements in the third
transfer nip 86.
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The gloss enhancing station 110 is preferably
positioned sufficiently close to the third transfer nip 86,
so the gloss enhancing station 110 can utilize the
increased document temperature that occurs in the third
transfer nip 86. The increased temperature of the document
72 reduces the operating temperature needed for the gloss
enhancing station 110. The reduced temperature of the
gloss enhancing station 110 improves the life and
reliability of the gloss enhancing materials.
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Use of a highly conformable silicone transfuse member
50 is an example demonstrated as one important means for
achieving good operating fix latitude with low gloss.
Critical parameters are sufficiently low durometer for the
top most layer of the transfuse member 50, preferably of
rubber, and relatively high thickness for the intermediate
layers of the transfuse member 50, preferably also of
rubber. Preferred durometer ranges will depend on the
thickness of the composite toner layer and the thickness of
the transfuse member 50. The preferred range will be about
25 to 55 Shore A, with a general preference for about 35 to
45 Shore A range. Therefore preferred materials include
many silicone material formulations. Thickness ranges of
the middle and upper most layers of the transfuse member 50
will preferably be greater than about 0.25 mm and more
preferably greater than 1.0 mm. Preference relative to low
gloss will be for generally thicker layers to enable
extended toner release life, conformance to rough
substrates, extended nip dwell time, and improved document
stripping. In an optional embodiment a small degree of
surface roughness is introduced on the surface of the
transfuse member 50 to enhance the range of allowed
transfuse material stiffness for producing low transfuse
gloss. Especially with higher durometer materials and/or
low thickness layers there will be a tendency to reproduce
the surface texture of the transfuse member. Thus some
surface roughness of the transfuse member 50 will tend
toward low gloss in spite of high stiffness. Preference
will be transfuse member surface gloss number <30 GU.
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A narrow operating temperature latitude for good fix
with low gloss in transfuse has been demonstrated at
relatively high toner mass/area conditions. Toner of size
about 7 microns requiring toner masses about 1 mg/cm2
requires a temperature of the transfuse member 50 between
110-120C and preheating of the paper to about 85C to
achieve gloss levels of <30 GU while simultaneously
achieving acceptable crease level below 40. However, low
mass/area toner conditions have shown increased operating
transfuse system temperature range for fix and low gloss.
The use of small toner having high pigment loading, in
combination with a conformable transfuse member 50, allows
low toner mass/area for color systems therefore extending
the operating temperature latitude for low gloss in the
third transfer nip 86. Toner of size about 3 microns
requiring toner masses about 0.4 mg/cm2 requires a
temperature of the transfuse member 50 between 110-150C,
and paper preheating to about 85C, to achieve gloss levels
of <30 GU while simultaneously achieving acceptable crease
level below 40.
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The gloss enhancing station 110 preferably has fusing
members 112, 114 of Viton™. Alternatively hard fusing
members such as thin and thick Teflon™ or expanded PTFE
sleeves/overcoatings on rigid rollers or on belts, or else
such overcoatings over rubber underlayers, are alternative
options for post transfuse gloss enhancing. The fusing
members 112, 114, preferably have an top most fixing layer
stiffer than that used for the top most layer of the
transfuse member 50, with a high level of surface
smoothness (surface gloss preferably>50 GU and more
preferably >70 GU). The topmost surface can be
alternatively textured to provide a texture to the
documents 72. The gloss enhancing station 110 preferably
includes a release agent management application system (not
shown). The gloss enhancing station can further include
stripping mechanisms such as an air puffer, or stripper
fingers to assist stripping of the document 72 from the
fusing members 112, 114.
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Optionally the toner formulation may include wax or
encapsulated release agents to reduce the oil requirements
for the gloss enhancing station 110.
-
The gloss enhancing station 110 is described in
combination with the printer 10 having an intermediate
transfer member 12 and a transfuse member 50. However, the
gloss enhancing station 110 is applicable with all printers
having transfuse systems producing documents 72 with low
gloss. In particular this can include transfuse systems
that employ a single transfer/transfuse member.
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As a system example, the transfuse member 50 is
preferably 120 C in the third transfer nip 86, and the
substrate 70 is preheated to 85 C. The result is a
document 72 having a gloss value 10-30 GU. The fusing
members are preferably heated to 120C. The temperature
and/or dwell of the fusing members 112, 114 is preferably
adjustable so different degrees or levels of glossing can
be applied to different print runs or different prints
within a run dependent on operator choice. Higher
temperatures and dwell of the fusing members 112, 114
increase the gloss enhancement while lower temperatures and
dwell will the reduce the amount of gloss enhancement on
the documents 72.
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The glossing members 112, 114 are preferably fusing
rollers, but can alternatively the glossing members 112,
114 can be fusing belts. The top most surface of each
glossing member 112, 114 is relatively non-conformable,
preferably having a durometer above 55 Shore A. The gloss
enhancing station 110 provides gloss enhancing past the
printer 10 employing a transfuse system that operates with
low gloss in the third transfer nip 86. The printer 10
preferably forms documents 72 having 10-30 Gardner Gloss
Units (GU) after the third transfer nip 86. The gloss on
the documents 72 will vary with toner mass per unit area.
The gloss enhancing unit 110 preferably increases the gloss
of the documents 72 to greater than about 50 GU on Lustro
Gloss™ paper distributed by SD Warren Company.