FIELD OF THE INVENTION
[0001] The present invention relates generally to imaging apparatus and techniques and more
particularly to apparatus and techniques for transfer of images from an image-bearing
surface to a substrate via an intermediate transfer medium.
BACKGROUND OF THE INVENTION
[0002] Various techniques for electrostatic image transfer are known in the patent literature.
U.S. Patent 4,684,238 describes intermediate transfer apparatus in which a plurality
of liquid images, which include a liquid carrier having toner particles dispersed
therein, are attracted from a photoconductive member to an intermediate belt. Liquid
carrier is removed from the intermediate belt by vacuum apparatus and the toner particles
are compacted on the intermediate belt in image configuration. Thereafter, the toner
particles are transferred from the intermediate belt to the copy sheet in image configuration
by electrostatic attraction.
[0003] U.S. Patent 4,690,539 shows a system similar to that shown in U.S. Patent 4,684,238
which is suitable for multi-color multiple-pass electrophoretic image transfer.
[0004] In U.S. Patents 3,318,212 and 3,893,761 there are described methods and devices in
which a powder image being transported on a resiliently deformable intermediate support
surface is softened and thus rendered sticky while present on that surface and then
is transferred and fixed onto a paper receiving support under the influence of pressure.
[0005] U.S. Patent 4,015,027 describes an electrophotographic toner transfer and fusing
method wherein a heated roller or belt is employed for pressure transfer of dry toner
images from an intermediate transfer medium onto paper. At column 11, line 29 - column
12 line 38 there appears a detailed discussion of heating of images upon transfer
thereof as proposed therein and as taught in the prior art including specifically
U.S. Patent 3,591,276 to Byrne.
[0006] Reference is made to Figs. 5A - 5C, 6A - 6C, 7A and 7C of U.S. Patent 4,015,027.
It is seen that in nearly all cases described, the toner is heated to at least its
melting point during the transfer stage. In a technique proposed in U.S. Patent 4,015,027
and exemplified by Fig. 6(a), the toner is heated to at least its melting point prior
to the transfer zone. In the transfer zone, the toner cools below its melting point
during transfer and fusion.
[0007] A belt construction characterized in that it has a very low heat capacitance and
a thickness of between 15 and about 200 microns is proposed in U.S. Patent 4,015,027.
In one embodiment the belt comprises a 50 micron layer of aluminized Kapton having
a 25 micron coating of silicon rubber. Another embodiment employs a 12.5 micron layer
of stainless steel instead of the Kapton together with a silicon rubber coating. A
reflecting layer is incorporated in the belt to reduce heating thereof.
[0008] U.S. Patent 4,796,048 describes a system for transferring a liquid toner image from
a photoconductive member to an intermediate transfer member for subsequent transfer
to a copy sheet. In several of the examples the liquid toner image is heated to remove
solvent associated with the toner image. The toner particles are melted to thermally
offset the image to the copy sheet.
[0009] U. S. Patent 4,708,460 describes a system for transferring a liquid toner image from
a photoconductive member to an intermediate transfer member for subsequent transfer
to a copy sheet. The liquid toner image is heated by radiant heat on the intermediate
transfer member to vaporize some of the liquid carrier and to partially melt the toner
particles, decreasing their viscosity. During transfer to the final substrate heat
substantially vaporizes the remainder of the liquid carrier from the image and fuses
the image to the copy sheet.
SUMMARY OF THE INVENTION
[0010] The present invention seeks to provide improved imaging apparatus.
[0011] There is therefor provided apparatus for transfer of a liquid toner image (containing
carrier liquid and toner particles which solvate the carrier liquid at a solvation
temperature above room temperature) from an image bearing surface to a substrate,
the apparatus comprising: an intermediate transfer member arranged in operative association
with the image bearing surface, first transfer means operative for transferring the
image from the image bearing surface onto the intermediate transfer member, and heating
apparatus operative for heating the image on the intermediate transfer member to a
temperature above the solvation temperature, below the melting point of the toner
particles and below the boiling point of the carrier liquid prior to transfer of the
image to the substrate so as to cause the image to adhere to the substrate.
[0012] In accordance with a preferred embodiment of the invention the apparatus also comprises
second transfer means operative for transferring the heated image from the intermediate
transfer member to a substrate, the second transfer means being operative for cooling
the intermediate transfer member sufficiently such that the adhesion of the image
thereto is less than the cohesion of the image. In a preferred embodiment of the invention
the second transfer means in conjunction with the substrate is operative to cool the
image sufficiently such that the adhesion of the image to the intermediate transfer
member is less than the cohesion of the image.
[0013] The first transfer means includes, in a preferred embodiment of the invention, apparatus
for transferring multiple images from the image bearing surface onto the intermediate
transfer member.
[0014] In a preferred embodiment of the invention the toner particles in the liquid toner
image are pigmented.
[0015] Further in a preferred embodiment of the invention the heating apparatus is operative
to heat the image such that the image remains at a temperature above the solvation
temperature until contact of the image with the substrate.
[0016] Further in accordance with a preferred embodiment of the invention, the intermediate
transfer member comprises a thin walled cylinder preferably with a thickness of less
than 125 microns. In alternative preferred embodiments the wall thickness may be less
than 50, less than 30 or less than 7 microns. In a preferred embodiment of the invention
the thin walled cylinder includes metallic material. In a preferred embodiment the
thin walled cylinder comprises a layer of polymer material and a thin release layer.
[0017] In a preferred embodiment of the invention, the intermediate transfer member includes
a relatively heat conductive inner layer and a relatively heat insulative outer layer.
[0018] In a preferred embodiment of the invention the intermediate transfer member has a
low effective heat capacity such that the surface temperature of the intermediate
transfer member is substantially reduced during transfer of an image therefrom onto
substrate.
[0019] There is additionally provided a method for transfer of a liquid toner image (containing
carrier liquid and toner particles which solvate the carrier liquid at a solvation
temperature above room temperature) from an image bearing surface to a substrate,
including the steps of: transferring the image from the image bearing surface onto
an intermediate transfer member, and heating the image on the intermediate transfer
member to a temperature above the solvation temperature, below the melting point of
the toner particles and below the boiling point of said carrier liquid prior to transfer
of the image to the substrate so as to cause the image to adhere to the substrate.
[0020] The method includes, in a preferred embodiment of the invention, the step of cooling
the intermediate transfer member sufficiently such that the adhesion of the image
thereto is less than the cohesion of the image. In a preferred embodiment of the invention
the image is cooled sufficiently such that the adhesion of the image to the intermediate
transfer member is less than the cohesion of the image.
[0021] In a preferred embodiment of the invention the step of transferring the image from
the image bearing surface is repeated a plurality of times, each transfer corresponding
to an image of a different color.
[0022] The method preferably includes the step of transferring the heated image from the
intermediate transfer member to the substrate, wherein the step of transferring the
image from the intermediate transfer member onto the substrate is operative to cool
the image to below the solvation temperature.
[0023] There is also provided in accordance with a further preferred embodiment of the present
invention apparatus for transfer of an image from an image bearing surface onto a
substrate including an intermediate transfer member positioned in operative association
with the image bearing surface, means for transferring an image from the image bearing
surface onto the intermediate transfer member, and means for transferring the image
from the intermediate transfer member onto a substrate and being operative for heating
the intermediate transfer member and the image so as to cause the image to adhere
to the substrate and for cooling the intermediate transfer member sufficiently such
that the adhesion of the image thereto is less than the cohesion of the image.
[0024] There is also provided in accordance with yet a further preferred embodiment of the
present invention apparatus for transfer of multiple images from an image bearing
surface onto a substrate including an intermediate transfer member positioned in operative
association with the image bearing surface, means for transferring multiple images
from the image bearing surface onto the intermediate transfer member, and means for
transferring the multiple images from the intermediate transfer member onto a substrate
and being operative for heating the intermediate transfer member and the image so
as to cause the image to adhere to the substrate and for cooling the intermediate
transfer member sufficiently such that the adhesion of the image thereto is less than
the cohesion of the image.
[0025] There is further provided in accordance with an additional preferred embodiment of
the present invention apparatus for transfer of an image from an image bearing surface
onto a substrate including an intermediate transfer member positioned in operative
association with the image bearing surface, means for transferring an image from the
image bearing surface onto the intermediate transfer member, and means for transferring
the image from the intermediate transfer member onto a substrate and wherein the intermediate
transfer member includes a thin walled cylinder of thickness less than 125 microns.
[0026] There is also provided in accordance with yet a further embodiment of the present
invention apparatus for transfer of multiple images from an image bearing surface
onto a substrate including an intermediate transfer member positioned in operative
association with the image bearing surface, means for transferring multiple images
from the image bearing surface onto the intermediate transfer member, and means for
transferring the multiple images from the intermediate transfer member onto a substrate
and wherein the intermediate transfer member includes a thin walled cylinder of thickness
less than 125 microns.
[0027] There is additionally provided in accordance with yet a further embodiment of the
present invention apparatus for transfer of an image from an image bearing surface
onto a substrate including an intermediate transfer member positioned in operative
association with the image bearing surface; means for transferring an image from the
image bearing surface onto the intermediate transfer member, and means for transferring
the image from the intermediate transfer member onto a substrate and wherein the intermediate
transfer member includes a relatively heat conductive inner layer and a relatively
heat insulative outer layer.
[0028] There is also provided in accordance with an additional embodiment of the present
invention apparatus for transfer of multiple images from an image bearing surface
onto a substrate including an intermediate transfer member positioned in operative
association with the image bearing surface, means for transferring multiple images
from the image bearing surface onto the intermediate transfer member, and means for
transferring the multiple images from the intermediate transfer member onto a substrate
and wherein the intermediate transfer member includes a relatively heat conductive
inner layer and a relatively heat insulative outer layer.
[0029] There is further provided in acordance with a further preferred embodiment of the
present invention an intermediate transfer member for transfer of an image from an
image bearing surface onto a substrate and including a thin walled cylinder having
a thickness less than 125 microns.
[0030] There is also provided in accordance with an additional preferred embodiment of the
present invention an intermediate transfer member for transfer of an image from an
image bearing surface onto a substrate and including a relatively heat conductive
inner layer and a relatively heat insulative outer layer.
[0031] There is additionally provided in accordance with yet a further embodiment of the
present invention a method for transfer of an image from an image bearing surface
onto a substrate including the steps of positioning an intermediate transfer member
in operative association with the image bearing surface, transferring an image from
the image bearing surface onto the intermediate transfer member, and transferring
the image from the intermediate transfer member onto a substrate and including the
steps of heating the intermediate transfer member and the image so as to cause the
image to adhere to the substrate and cooling the intermediate transfer member sufficiently
such that the adhesion of the image thereto is less than the cohesion of the image.
[0032] There is additionally provided in accordance with a further preferred embodiment
of the present invention a method for transfer of multiple images from an image bearing
surface onto a substrate including the steps of positioning an intermediate transfer
member in operative association with the image bearing surface, transferring multiple
images from the image bearing surface onto the intermediate transfer member, and transferring
the multiple images from the intermediate transfer member onto a substrate including
the steps of heating the intermediate transfer member and the image so as to cause
the image to adhere to the substrate and cooling the intermediate transfer member
sufficiently such that the adhesion of the image thereto is less than the cohesion
of the image.
BRIEF DESCRIPTION OF THE DRAWINGS
[0033] The present invention will be understood and appreciated more fully from the following
detailed description taken in conjunction with the drawings in which:
Fig. 1 is a generalized schematic sectional illustration of imaging apparatus constructed
and operative in accordance with a preferred embodiment of the present invention;
Figs. 2A, 2B and 2C are illustrations of transfer of an image from an intermediate
transfer element onto a substrate;
Fig. 3 is a generalized illustration of viscosity as a function of temperature;
Fig. 4A is a side sectional illustration of a heated thin-walled intermediate transfer
element constructed and operative in accordance with a preferred embodiment of the
present invention;
Fig. 4B is a sectional illustration taken along the lines IV - IV of Fig. 4A;
Fig. 5A is a side sectional illustration of a heated thin-walled intermediate transfer
element constructed and operative in accordance with an alternative embodiment of
the present invention;
Fig. 5B is a sectional illustration taken along the lines V - V of Fig. 5A;
Fig. 6A is a side sectional Illustration of a heated thin-walled intermediate transfer
element constructed and operative in accordance with a further alternative embodiment
of the present invention;
Fig. 6B is a sectional illustration taken along the lines VI - VI of Fig. 6A;
Fig. 7A is a side sectional illustration of a heated thin-walled intermediate transfer
element constructed and operative in accordance with yet another embodiment of the
present invention;
Fig. 7B is a sectional illustration taken along the lines VII - VII of Fig. 7A;
Fig. 8 is a sectional illustration of a partially heated intermediate transfer element;
and
Fig. 9 is a graphical illustration of the temperature variation on a low thermal mass
intermediate transfer element in an arrangement such as that illustrated in Fig. 8.
DETAILED DESCRIPTION OF THE INVENTION
[0034] Referring to Fig. 1 there is shown electrostatographic imaging apparatus in which
the present invention may be employed and employing a liquid image forming composition.
In a general sense, the imaging apparatus may comprise an electrostatographic printing
machine or alternatively any other suitable type of imaging apparatus. Examples of
systems in which the present invention may be employed include electrophotography,
electrography, ionography, xero-printing, gravure-like printing and electrostatic
printing.
[0035] For convenience, the description which follows is presented in the context of an
electrophotographic system employing liquid toner, but without limiting the applicability
of the present invention.
[0036] A metal drum 10, having formed thereon a photoconductive surface 12, is mounted on
a shaft 14. Drum 10 is driven in the direction of arrow 16 such that the photoconductive
surface 12 moves past a corona discharge device 18 adapted to charge the photoconductive
surface 12. An image to be reproduced is focused by a lens 20 upon the photoconductive
surface 12. The areas of the photoconductive surface 12 struck by light conduct the
charge, or a portion thereof, to ground, thus forming an electrostatic latent image.
[0037] Developer liquid containing pigmented particles is circulated from any suitable source
into a gap 22 defined between a development electrode 24 and the photoconductive surface
12. The development electrode 24 may be appropriately biased as known to the art,
to assist in toning the electrostatic latent image as it passes into contact with
the developer liquid.
[0038] Charged toner particles suspended in a carrier liquid, both of which form part of
the developer liquid, travel by electrophoresis to the electrostatic latent image.
[0039] Excess liquid is removed from the developed image by metering apparatus which may
incorporate a reverse roller indicated generally at reference numeral 30.
[0040] Transfer of the image to a carrier sheet 40, such as paper, supported by a platen
roller 42, is effected by an intermediate transfer assembly 50 which is a subject
of the present invention.
[0041] The transfer assembly 50 comprises an intermediate transfer element 52, typically
in the form of a cylindrical roller. The intermediate transfer element 52 is preferably
an intermediate transfer element of the type illustrated in any of Figs. 4A - 7B.
[0042] Transfer of the image from the photoconductive surface 12 to the intermediate transfer
element 52 may take place in accordance with any suitable technique known in the prior
art. Examples of suitable techniques are electrostatic transfer, heat transfer, pressure
transfer, electrophoretic transfer and combinations thereof. A preferred transfer
method is electrophoretic transfer.
[0043] After the image is transferred from the photoconductive surface 12 to the intermediate
transfer element 52, continued rotation of the photoconductive surface 12 in the direction
of arrow 16 brings the surface past a conventional cleaning station 32 and a flood
exposure light 34, for removing vestiges of prior images.
[0044] In accordance with a preferred embodiment of the invention the liquid toner image
is heated on the intermediate transfer member 52. Heating of the image enhances its
cohesiveness and renders it tacky, so as to enhance its adhesion to the substrate
40.
[0045] Although the invention is not limited in its application to specific materials or
to liquid toner, the following specific example is provided for the purposes of illustration.
There is employed a toner which is prepared in the following manner:
1000g. Elvax II 5550 resin (DuPont) and 500g. Isopar L were mixed in a Ross double
planetary mixer for one hour at 90 degrees C, then for a further hour after addition
of 250g. Mogul L carbon black (Cabot) which had been wetted by 500g. Isopar L, and
finally for another hour after addition of 2000g. Isopar L preheated to 110 degrees
C. Stirring was continued in the absence of heating until the temperature reached
40 degrees C. 3050 grams of the resultant mixture was milled in a Sweco M-18 vibratory
mill (containing 0.5" alumina cylinders) with 4000g. Isopar L for 20 hours at 34 degrees
C; the average particle size of the product was 2.3 microns. The product was diluted
to a 1.5% solids content with Isopar L and between 5 - 20 ml of 10% Lecithin charge
director was added to the diluted dispersion.
[0046] The image 60 located on the intermediate transfer element 52 is heated, by means
which will be described hereinbelow, to a temperature which produces desired tackiness
of the image. Then the heated image establishes contact with the substrate 40 as shown
in Fig. 2A.
[0047] According to a preferred embodiment of the present invention, wherein a toner of
the type described in detail on the preceding page, a toner of the type described
in U.S. Patent 4,794,651, the contents of which are hereby incorporated herein by
reference, or any other liquid toner which solvates at a temperature below its melting
point is used, the image 60 is heated to a temperature below the melting point of
the dry resin but above the temperature at which the resin swells or begins to solvate
with the carrier liquid and becomes tacky, and below the boiling point of the carrier
liquid. Alternatively a liquid toner which does not solvate at a temperature below
the melting point of the pigmented solid particles therein may be employed. In such
a case, heating of the image to a temperature as high as the melting point of the
pigmented solid particles therein is required.
[0048] It is a particular feature of the present invention that while the image 60 is in
contact with both the element 52 and the substrate 40, as shown in Fig. 2B, for a
duration which will be termed the "transfer duration", the heat transfer to the image
from the element 52 and from the image to the substrate 40 is preferably such that
the image is cooled, so as to increase its viscosity, while at least maintaining and
preferably increasing its cohesiveness. In this way, complete or nearly complete transfer
of the image from the intermediate transfer element 52 to the substrate is realized.
Fig. 2C illustrates the complete or nearly complete transfer of the image to the substrate
40.
[0049] If the specific material discussed above is employed as an example, the following
exemplary temperatures may be used. The image 60 and member 52 are initially heated
to a temperature T 1 of 105 degrees C, which is below the melting point of the resin
but above the solvation temperature. During the "transfer duration" the temperature
of the image/paper interface is reduced to a temperature T 2 of 85 degrees C, at which
the viscosity is increased over that at the higher temperature.
[0050] Reference is made in this context to Fig. 3 which is an illustration, not necessarily
to scale, of the dependence of viscosity of an image on temperature. It is seen that
the reduction of temperature from T 1 to T 2 provides a corresponding significant
rise in viscosity.
[0051] It will be appreciated that the image is initially heated to a temperature at which
it solvates, so that it will adhere well to the substrate. The image is then cooled,
increasing its viscosity and thus increasing its cohesiveness. The adhesion of the
image to the substrate is greater than its adhesion to the release coated intermediate
transfer member, and the increased cohesion of the image preserves the integrity of
the transferred image, providing substantially complete transfer of the image to the
substrate.
[0052] Reference is now made to Figs. 4A - 7B which illustrate four alternative embodiments
of intermediate transfer elements constructed and operative in accordance with a preferred
embodiment of the invention.
[0053] According to a preferred embodiment of the invention, the intermediate transfer element
comprises a thin-walled roller 70. Roller 70 preferably is formed of two rigid end
portions 72 and 74 and a thin cylindrical layer 76 typically coated with a release
layer 78. Typical materials and thicknesses are as follows:
Layer 76: |
metalized polyester |
Thickness: |
25 microns |
Release layer 78: |
Teflon (DuPont) |
Thickness: |
5 microns |
[0054] According to an alternative embodiment of the invention, the layer 76 may be a 5
micron thick film of nickel alloy, such as a nickel cobalt or nickel chromium alloy
and the release layer may be a 2 micron thick layer of Teflon.
[0055] According to a further alternative embodiment of the invention, Kapton polyimide
film (DuPont) may be employed instead of polyester.
[0056] According to a further alternative embodiment of the invention the release layer
may be a thin layer of silicone rubber.
[0057] In accordance with a preferred embodiment of the invention, the thin cylindrical
layer 76 is axially tensioned, as by a spring arrangement 80, sufficient to eliminate
most surface irregularities. For the above-described example employing metalized polyester,
for a cylinder of diameter 50 mm, a suitable tension is 10 Kg.
[0058] Further in accordance with a preferred embodiment of the invention, enhanced rigidity
and surface uniformity of the thin-walled cylinder 70 is provided by pneumatically
pressurizing the interior of the cylinder, by any suitable pressurized gas. A valve
82 may be provided for this purpose.
[0059] In accordance with a preferred embodiment of the present invention, the thin-walled
cylinder 70 is heated by the passage of electrical current along layer 76 via conductors
84 and 86, which establish an electrical circuit via end portions 72 and 74. In this
case layer 76 must either be or include a layer which is an electrical conductor of
suitable characteristics.
[0060] In the above stated example, the electrical power required to provide desired heating
of the intermediate transfer element 70 is relatively low.
[0061] Reference is now made to Figs. 5A and 5B which illustrate an alternative embodiment
of a heated intermediate transfer element wherein heating is provided by radiation.
Here a heating lamp 90 is disposed interior of a radiation transmissive tube 92, such
as a quartz tube. Disposed in generally coaxial surrounding relationship with quartz
tube 92 and supported on annular end supports 94 is an intermediate transfer layer
96 having formed thereon a release layer 98.
[0062] According to one embodiment of the invention, layers 96 and 98 may be identical to
layers 76 and 78 in the embodiment of Figs. 4A and 4B. In such a case tensioning apparatus
of the type illustrated in Fig. 4A may be employed. Alternatively layers 96 and 98
which are more massive and thus more rigid than layers 76 and 78 may be employed.
In such a case the release layer 98 is provided with sufficient thermal insulation
capacity to limit the amount of thermal conduction therethrough so that during transfer
of the image to the substrate 40, the image may be cooled as described above in connection
with the thin-walled intermediate transfer element. Suitable materials and thicknesses
for the non-thin-walled intermediate transfer element are as follows:
Layer 96: |
Aluminum |
Thickness: |
5 mm |
Layer 98: |
Silicone rubber |
Thickness: |
2 mm |
[0063] Reference is now made to Figs. 6A and 6B, which illustrate an alternative arrangement
of heated intermediate transfer roller. The roller 100 may be either of the thin-walled
type or of the non-thin-walled type described above. Heating of the roller 100 is
provided externally of the roller by a heating station 102. In the illustrated embodiment,
the heating station 102 employs radiant heaters, which heat the roller by radiation.
Alternatively the heating station 102 may heat the roller 100 by conduction through
direct contact with the roller.
[0064] Reference is now made to Figs. 7A and 7B, which illustrate a further alternative
of heated intermediate roller arrangement. Here, once again, a roller 110 may be either
thin- walled or non-thin-walled. Heating of the roller 110 is provided by an internal
radiant heater assembly 112 which is mounted internally of roller 110. Radiant heater
112 comprises an elongate radiative heat source 114 which is associated with a reflector
116, which prevents direct radiation from source 114 from reaching the area at which
the image is transferred from the roller 110 to substrate 40 (Fig. 1), thus providing
differential heating of roller 110 and permitting cooling of the image during transfer
as described hereinabove.
[0065] A suitable weight 118 may be mounted onto the reflector 116 so that when the reflector
116 and weight 118 are pivotably mounted with respect to the roller, they will retain
the orientation illustrated, notwithstanding rotation of the roller 110.
[0066] It is a particular feature of the present invention that there is provided an intermediate
transfer member including a thin surface which supports the image during transfer,
the thin surface having an effective heat capacity per unit area which is less than
that of the substrate.
[0067] The thin surface may be a cylindrical surface or alternatively an endless belt or
any other configuration. Normally, due to its thinness, the thermal conductivity along
the surface is sufficiently small such that the thermal mass of the supports, such
as end rollers for a cylindrical surface like that shown in the drawings, may be disregarded.
[0068] It is a particular feature of the present invention that the effective thermal mass
of the intermediate transfer element, as sensed by an object coming into contact with
its outer surface is relatively small. This may be achieved either by the use of a
thin-walled roller as described hereinabove, whose inherent thermal mass is limited,
or alternatively by the use of a roller, other than a thin-walled roller, but having
an outer layer which is a sufficiently good thermal insulator such that the heat transfer
characteristics thereof are as required. Such a structure has been described above.
[0069] The advantages of the use of an intermediate transfer element having a low effective
thermal mass are summarized below:
a. enabling the image at the transfer region of the intermediate transfer element
to be cooled during transfer, as has already been described;
b. enabling rapid cooling of the intermediate transfer element and thus eliminating
the need for separating it from the photoconductor when operation is interrupted;
c. limiting the amount of thermal energy passed to the paper and thus reducing energy
consumption and limiting paper deformation;
d. enabling differential heating of the intermediate transfer element such that it
cools down from the onset of transfer to the onset of photoconductor contact to a
temperature at which contact with the photoconductor will not cause photoconductor
damage.
[0070] Reference is made in this context to Fig. 8 which illustrates a variation of the
apparatus of Figs. 7A and 7B, using identical reference numerals where appropriate,
wherein a reflector is oriented so as to prevent direct radiation heating of the roller
from the transfer stage through the photoconductor contact stage. In such a situation
the approximate roller temperature at various locations therealong is as shown in
Fig. 9.
[0071] It can be seen from a consideration of Figs. 8 and 9 that the intermediate transfer
member gives up a measured quantity of heat to the substrate during image transfer
thereto (between locations B and C) and remains at a relatively low temperature, i.e.
below about 85 degrees centigrade, until it contacts the photoconductive surface 12,
at which point it gives up further heat very quickly to the photoconductive surface
12 (between locations D and E). The photoconductive surface does not heat up appreciably
in view of its relatively large thermal mass. The intermediate transfer member remains
at generally the same temperature until it is exposed to radiation heating (at location
0) and is heated gradually until it reaches a steady state temperature (at location
A) just before transfer contact with the substrate (at location B).
[0072] This development takes place at a first temperature T₁; transfer of the image to
the intermediate transfer member takes place at an image temperature T₂, higher than
T₁ and final transfer from the intermediate transfer member to the substrate takes
place at a temperature T₃ higher than temperature T₂.
[0073] It is a particular feature of the present invention that the temperature of the intermediate
transfer member when it is in propinquity to the photoconductive surface 12 is sufficiently
low as to preclude damage to the photoconductive surface 12, even during prolonged
contact or propinquity, as when neither of the surfaces is rotating. Accordingly prior
art mechanisms for separating the intermediate transfer member from the photoconductive
surface 12 when the apparatus is not in operation are not required.
[0074] It will be appreciated by persons skilled in the art that the present invention is
not limited by what has been particularly shown and described hereinabove. Rather
the scope of the present invention is defined only by the claims which follow: