This invention relates generally to a printing process and more particularly
to an ink-jet printing system and process employing mechanisms for controlling
the temperature of phase-change ink during a transfer printing process.
Ink-jet printing systems have been employed utilizing intermediate
transfer surfaces, such as that described in U.S. Pat. No. 4,538,156 of Durkee
et al. in which an intermediate transfer drum is employed with a printhead. A
final receiving surface of paper is brought into contact with the intermediate
transfer drum after the image has been placed thereon by the nozzles in the printhead.
The image is then transferred to the final receiving surface.
U.S. Pat. No. 5,099,256 of Anderson describes an intermediate drum
with a surface that receives ink droplets from a printhead. The intermediate drum
surface is thermally conductive and formed from a suitable film-forming silicone
polymer allegedly having a high surface energy and high degree of surface roughness
to prevent movement of the ink droplets after receipt from the printhead nozzles.
Other imaging patents, such as U.S. Pat. Nos. 4,731,647 and 4,833,530 of Kohsahi,
describe a solvent that is deposited on colorant to dissolve the colorant and
form a transferrable drop to a recording medium. The colorants are deposited directly
onto paper or plastic colorant transfer sheets. The transferrable drops are then
contact transferred to the final receiving surface medium, such as paper.
US Pat. No. 4,673,303 of Sansone et al. describes an offset ink-jet
postage printing method and apparatus in which an inking roll applies ink to the
first region of a dye plate. A lubricating hydrophilic oil can be applied to the
exterior surface of the printing drum or roll to facilitate the accurate transfer
of the images from the drum or roll to the receiving surface.
The above-described processes do not achieve a complete image transfer
from the intermediate transfer surface under normal printing conditions and, therefore,
require a separate cleaning step to remove any residual ink from the intermediate
receiving surface. Prior intermediate transfer surfaces also have not been renewable.
The prior processes are also limited in the degree of image quality
that can be achieve on different types of final receiving surfaces or print media.
Because the inks are fluids, they are subject to uncontrolled bleeding on porous
media, such as paper, and uncontrolled spreading on transparency films or glossy
The above-described problems are addressed by processes and apparatus
described in co-pending US Pat. Application Nos 07/981,646 and 07/981,677 (corresponding
to European Patent Publication Nos 0604025 and 0604023. A transfer printer employing
phase-change ink is described in which a liquid intermediate transfer surface is
provided that received a phase-change ink image on a drum. The image is then transferred
from the drum with at least a portion of the intermediate transfer surface to a
final receiving medium, such as paper.
In particular, the phase-change ink transfer printing process begins
by first applying a thin liquid intermediate transfer surface to the drum. Then
an ink-jet printhead deposits molten ink onto the drum where it solidifies and
cools to about the temperature of the drum. After depositing the image, the print
medium is heated by feeding it through a preheater and into a nip formed between
the drum and an elastomeric transfer roller. As the drum turns, the heated print
medium is pulled through the nip and is pressed against the deposited image, thereby
transferring the ink to the print medium. When in the nip, heat from the print
medium heats the ink, making it sufficiently soft and tacky to adhere to the print
medium. When the print medium leaves the nip, stripper fingers peel it from the
drum and direct it into a media exit path.
In practice, it has been determined that a transfer printing process
should meet at least the following criteria to produce acceptable prints. To optimize
image resolution, the transferred ink drops should spread out to cover a predetermined
area, but not so much that image resolution is lost. The ink drops should not
melt during the transfer process. To optimize printed image durability, the ink
drops should be pressed into the paper with sufficient pressure to prevent their
inadvertent removal by abrasion. Finally, image transfer conditions should be
such that substantially all of the ink drops are transferred from the drum to the
Unfortunately, the proper set of image transfer conditions are dependent
on a complexly interrelated set of pressure, temperature, time, and ink parameters
that have not been well understood, thereby preventing phase-change transfer printing
from meeting its full potential for rapidly producing high-quality prints.
What is needed, therefore, is a phase-change transfer printing process
and apparatus that addresses the problems and challenges of controlling the image
transfer conditions to rapidly produce consistently high-quality prints on a wide
range of print media.
As will be appreciated from the description which follows with reference
to the drawings, the invention provides an improved apparatus and a method for
transfer printing having controlled transfer conditions that provide durable high-resolution
printed images. A will be further so appreciated, the invention facilitates rapid
production of color printed images on a wide range of print media types.
Accordingly, this invention provides a phase-change ink transfer printing
apparatus according to claim 14 and process according to claim 1 that starts by
applying a thin layer of a liquid or other intermediate transfer surface to a heated
receiving surface, such as a drum. Then an ink-jet printhead deposits a molten
ink image onto the heated drum where it cools to the drum temperature and solidifies.
After the image is deposited, a print medium is heated by a preheater to a predetermined
temperature and fed into a nip formed between the heated drum and an elastomeric
transfer roller that is biased toward the drum to form a nip pressure that is about
twice the yield strength of the ink in the deposited image. As the drum turns,
the heated print medium is pulled through the nip at a predetermined rate to transfer
and fuse the ink image to the print medium. When in the nip, heat from the drum
and print medium combine to heat the ink in accordance with a process window, making
the ink sufficiently soft and tacky to adhere to the print medium but not to the
drum. When the print medium leaves the nip, stripper fingers peel it from the drum
and direct it into a media exit path.
The predetermined surface temperature is conveniently in the approximate
range from 30°C to 60°C, for example 40°C to 56°C, preferably 45°C to 55°C.
The predetermined pre-heater temperature is conveniently in the approximate
range from 60°C to 1 50°C, preferably 60°C to 130°C.
A preferred embodiment of the invention will now be described, by
way of example only, reference being made to the accompanying drawings, in which:
Fig. 1 is a pictorial schematic diagram showing a transfer printing
apparatus having a supporting surface adjacent to a liquid layer applicator and
a printhead that deposits the image on the liquid layer.
Fig. 2 is an enlarged pictorial schematic diagram showing the liquid
layer acting as an intermediate transfer surface supporting the ink.
Fig. 3 is an enlarged pictorial schematic diagram showing the transfer
of the ink image from the liquid intermediate transfer surface to a final receiving
Fig. 4 is a graph showing storage modulus as a function of temperature
for a phase-change ink suitable for use with this invention.
Fig. 5 is a graph showing yield stress as a function of temperature
for a phase-change ink suitable for use with this invention.
Fig. 6 is a graph showing fuse grade as a function of media preheater
and drum temperature as determined from a set of fuse grade test prints made to
determine a process window according to this invention.
Fig. 7 is a graph showing pixel picking percentage as a function
of media preheater and drum temperature as determined from a set of pixel picking
test prints made to determine a process window according to this invention.
Fig. 8 is a graph showing dot spread groups as a function of media
preheater and drum temperature as determined from a set of drop spread test prints
made to determine a process window according to this invention.
Fig. 9 is a graph showing high temperature limit as a function of
media preheater and drum temperature as determined from a set of ink cohesive failure
test prints made to determine a process window according to this invention.
Fig. 10 is a graph showing a phase-change transfer printing process
window bounded by the parameter limits shown in Figs. 6-9.
Fig. 11 is an isometric schematic pictorial diagram showing a media
preheater, roller, print medium, drum, drum heater, fan, and temperature controller
of this invention with the drum shown partly cut away to reveal cooling fins positioned
Fig. 1 shows an imaging apparatus 10 utilized in this process to
transfer an inked image from an intermediate transfer surface to a final receiving
substrate. A printhead 11 is supported by an appropriate housing and support elements
(not shown) for either stationary or moving utilization to place an ink in the
liquid or molten state on a supporting intermediate transfer surface 12 that is
applied to a supporting surface 14. Intermediate transfer surface 12 is a liquid
layer that is applied to supporting surface 14, such as a belt, drum, web, platen,
or other suitable design, by contact with an applicator, such as a metering blade,
roller, web, or a wicking pad 15 contained within an applicator assembly 16.
Supporting surface 14 (hereafter "drum 14") may be formed from or
surface coated with any appropriate material, such as metals including but not
limited to aluminum, nickel, or iron phosphate, elastomers including but not limited
to fluoroelastomers, perfluoroelastomers, silicone rubber, and polybutadiene, plastics
including but not limited to polyphenylene sulfide loaded with polytetrafluorethylene,
thermoplastics such as polyethylene, nylon, and FEP, thermosets such as acetals,
and ceramics. The preferred material is anodized aluminum.
Applicator assembly 16 optionally contains a reservoir 18 for the
liquid and most preferably contains a web and web advancing mechanism (both not
shown) to periodically present fresh web for contact with drum 14.
Wicking pad 15 and the web are synthetic textiles. Preferably the
wicking pad 15 is needled felt and the web is any appropriate nonwoven synthetic
textile with a relatively smooth surface. An alternative configuration employs
a smooth wicking pad 15 mounted atop a porous supporting material, such as a polyester
felt. Both materials are available from BMP Corporation as BMP products NR 90
and PE 1100-UL, respectively.
Applicator apparatus 16 is mounted for retractable movement upward
into contact with the surface of drum 14 and downwardly out of contact with the
surface of the drum 14 and its intermediate transfer surface 12 by means of an
appropriate mechanism, such as a cam, an air cylinder or an electrically actuated
A final substrate guide 20, which can also be the lower surface of
the preheater, passes a final receiving substrate 21, such as paper, from a positive
feed device (not shown) and guides it through a nip 22 formed between the opposing
arcuate surfaces of a roller 23 and intermediate transfer surface 12 supported
by drum 14. Stripper fingers 24 (only one of which is shown) may be pivotally
mounted to imaging apparatus 10 to assist in removing final receiving substrate
21 from intermediate transfer surface 12. Roller 23 has a metallic core, preferably
steel, with an elastomeric covering having a Shore D hardness and/or durameter
of 40 to 45. Suitable elastomeric covering materials include silicones, urethanes,
nitriles, EPDM, and other appropriately resilient materials. The elastomeric covering
on roller 23 engages final receiving substrate 21 on a reverse side to which an
ink image 26 is transferred from intermediate transfer surface 12. This fuses or
fixes ink image 26 to final receiving surface 21 so that the transferred ink image
is spread, flattened, and adhered.
The ink utilized in the process and system of this invention is preferably
initially in solid form and is then changed to a molten state by the application
of heat energy to raise its temperature to about 85° C to about 150° C. Elevated
temperatures above this range will cause degradation or chemical breakdown of the
ink. The molten ink is then ejected from the ink jets in printhead 11 to the intermediate
transfer surface 12, where it is cooled to an intermediate temperature and solidifies
to a malleable state in which it is transferred to final receiving surface 21
via a contact transfer by entering nip 22 between roller 23 and intermediate transfer
surface 12 on drum 14. The intermediate temperature wherein the ink is maintained
in the malleable state is between 20° C to 60° C and preferably about 50° C.
Once ink image 26 enters nip 22, it is deformed to its final image
conformation and adheres or is fixed to final receiving substrate 21 by a combination
of nip 22 pressure exerted by roller 23 and heat supplied by a media preheater
27 and a drum heater 28. Drum heater 28 is preferably a lamp and reflector assembly
oriented to radiantly heat the surface of drum 14. Alternatively, a cylindrical
heater may be axially mounted within drum 14 such that heat generated therein is
radiated directly and conducted to drum 14 by radial fins 30.
The pressure exerted in nip 22 by roller 23 on ink image 26 is between
about 68.9476 to about 6894.76 kPa (about 10 to about 1,000 pounds/inch2
"psi"), more preferably about 3447.38 kPa (500 psi), which is approximately twice
the ink yield strength of 1723.69 kPa (250 psi) at 50° C. The nip pressure must
be sufficient to have ink image 26 adhere to final receiving substrate 21 and be
sufficiently flattened to transmit light rectilinearly through the ink image in
those instances when final receiving substrate 21 is a transparency. Once adhered
to final receiving substrate 21, the ink image is cooled to an ambient temperature
of about 20° C to about 25° C.
Figs. 2 and 3 show the sequence involved when ink image 26 is transferred
from intermediate transfer surface 12 to final receiving substrate 21. Ink image
26 transfers to final receiving substrate 21 with a small but measurable quantity
of the liquid forming intermediate transfer surface 12 attached thereto as a transferred
liquid layer 32. A typical thickness of transferred liquid layer 32 is calculated
to be about 1000 angstroms or about 100 nanometers. Alternatively, the quantity
of transferred liquid layer 32 can be expressed in terms of mass as being from
about 0.1 to about 200 milligrams, more preferably from about 0.5 to about 50 milligrams,
and most preferably from about 1 to about 10 milligrams per A-4 sized page of
final receiving substrate 21. This is determined by tracking on a test fixture
the weight loss of the liquid in the applicator assembly 16 at the start of the
imaging process and after a desired number of sheets of final receiving substrate
21 have been imaged.
Some appropriately small and finite quantity of intermediate transfer
surface 12 is also transferred to the final receiving substrate in areas adjacent
to transferred ink image 26. This relatively small transfer of intermediate transfer
surface 12 to ink image 26 and the non-imaged areas on the final receiving substrate
21 can permit as many as 10 pages or more of final receiving substrate 21 to be
printed before it is necessary to replenish sacrificial intermediate transfer surface
12. Replenishment may be necessary after fewer final printed copies, depending
on the quality and nature of final receiving surface 21 that is utilized. Transparencies
and paper are the primary intended media for image receipt. "Plain paper" is the
preferred medium, such as that supplied by Xerox Corporation and many other companies
for use in photocopy machines and laser printers. Many other commonly available
office papers are included in this category of plain papers, including typewriter
grade paper, standard bond papers, and letterhead paper. Xerox® 4024 paper
is assumed to be a representative grade of plain paper for the purposes of this
Suitable liquids that may be employed for intermediate transfer surface
12 include water, fluorinated oils, glycol, surfactants, mineral oil, silicone
oil, functional oils, or combinations thereof. Functional oils can include but
are not limited to mercapto-silicone oils, fluorinated silicone oils, and the
The ink used to form ink image 26 preferably must have suitable specific
properties for viscosity. Initially, the viscosity of the molten ink must be matched
to the requirements of the ink-jet device utilized to apply it to intermediate
transfer surface 12 and optimized relative to other physical and rheological properties
of the ink as a solid, such as yield strength, hardness, elastic modulus, loss
modulus, ratio of the loss modulus to the elastic modulus, and ductility. The viscosity
of the phase-change ink carrier composition has been measured on a Ferranti-Shirley
Cone Plate Viscometer with a large cone. At about 140° C a preferred viscosity
of the phase-change ink carrier composition is from about 0.005 to about 0.030
kg/MS (about 5 to about 30 centipoise), more preferably from about 0.01 to about
0.02 kg/MS (about 10 to about 20 centipoise), and most preferably from about 0.011
to about 0.015 kg/MS (about 11 to about 15 centipoise). The surface tension of
suitable inks is between about 23 and about 50 dynes/cm. An appropriate ink composition
is described in U.S. Pat. No. 4,889,560 issued December 26, 1989, which is assigned
to the assignee of this invention.
The phase-change ink used in this invention is formed from a phase-change
ink carrier composition that exhibits excellent physical properties. For example,
the subject phase-change ink, unlike prior art phase-change inks, exhibits a high
level of lightness, chroma, and transparency when utilized in a thin film of substantially
uniform thickness. This is especially valuable when color images are conveyed
using overhead projection techniques. Furthermore, the preferred phase-change ink
compositions exhibit the preferred mechanical and fluidic properties mentioned
above when measured by dynamic mechanical analyses ("DMA"), compressive yield testing,
and viscometry. More importantly, these work well when used in the printing process
of this invention utilizing a liquid layer as the intermediate transfer surface.
The phase-change ink composition and its physical properties are discussed in
greater detail in co-pending European Pat. publication 0 604 023 A.
The above-defined DMA properties of the phase-change ink compositions
were experimentally determined. These dynamic measurements were done on a Rheometrics
Solids Analyzer (RSA II) manufactured by Rheometrics, Inc. of Piscataway, New Jersey,
using a dual cantilever beam geometry. The dimensions of the sample were about
2.0 ± 1.0 mm thick, about 6.5 ± 0.5 mm wide, and about 54.0 ± 1.0 mm long. A time/cure
sweep was carried out under a desired force oscillation or testing frequency of
about 1 KHz and an auto-strain range of about 1.0 X 10-5 percent to
about 1 percent. The temperature range examined was about -60° C to about 90° C.
The preferred phase-change ink compositions typically are (a) flexible at a temperature
of about -10° C to about 80° C, (b) have a temperature range for the glassy region
from about -100° C to 40° C, the value of E' being from about 1.5 X 109
to 1.5 X 1011 dyne/cm2, (c) have a temperature range for
the transition region from about -30° C to about 60° C, (d) have a temperature
range for the rubbery region of E' from about -10° C to 100° C, the value of E'
being from about 1.0 X 106 to 1.0 X 1011 dyne/cm2,
and (e) have a temperature range for the terminal region of E' from about 30°
C to about 160° C. Furthermore, the glass transition temperature range of the phase-change
ink compositions are from about -40° C to about 40° C, the temperature range for
integrating under the tan δ peak of the phase-change ink composition is from
about -80° C to about 80° C with integration values ranging from about 5 to about
40, and the temperature range for the peak value of tan δ of the phase-change
ink is from about -40° C to about 40° C with a tan δ of about 1.0 X 10-2
to about 1.0 X 10 at peak.
Fig. 4 shows a representative graph of a storage modulus E' as a
function of temperature at 1 Hz for a phase-change ink composition suitable for
use in the printing process of this invention. The graph indicates that storage
modulus E' is divided into a glassy region 40, a transition region 42, a rubbery
region 44, and a terminal region 46.
In glassy region 40 the ink behaves similar to a hard, brittle solid,
i.e., E' is high, about 1 X 1010
dyne/cm2. This is because
in this region there is not enough thermal energy or sufficient time for the molecules
to move. This region needs to be well below room temperature so the ink will not
be brittle and affect its room temperature performance on paper.
In transition region 42 the ink is characterized by a large drop
in the storage modulus of about one order of magnitude because the molecules have
enough thermal energy or time to undergo conformational changes. In this region,
the ink changes from being hard and brittle to being tough and leathery.
In rubbery region 44 the storage modulus change is shown as a slightly
decreasing plateau. In this region, there is a short-term elastic response to the
deformation that gives the ink its flexibility. It is theorized that the impedance
to motion or flow in this region is due to entanglements of molecules or physical
cross-links from crystalline domains. Producing the ink to obtain this plateau
in the appropriate temperature range for good transfer and fixing and room temperature
performance is important when formulating these phase-change ink compositions.
Rubbery region 44 encompasses the ink in both its malleable state during the transfer
and fixing or fusing step and its final ductile state on the final receiving substrate.
Finally, in terminal region 46, there is another drop in the storage
modulus. It is believed that in this region the molecules have sufficient energy
or time to flow and overcome their entanglements.
Several phase-change ink compositions were analyzed by compressive
yield testing to determine their compressive behavior while undergoing temperature
and pressure in nip 22. The compressive yield strength measurements were done
on an MTS SINTECH 2/D mechanical tester manufactured by MTS Sintech, Inc. of Cary,
North Carolina, using small cylindrical sample blocks. The dimensions of a typical
sample are about 19.0 ± 1.0 mm by about 19.0 ± 1.0 mm.
Isothermal yield stress was measured as a function of temperature
(about 25° C to about 80° C) and strain rate. The material was deformed up to about
The preferred yield stresses as a function of temperature for suitable
phase-change ink compositions for use in the indirect printing process of this
invention are described by an equation YS = mT + I, where YS is the yield stress
as a function of temperature, m is the slope, T is the temperature, and I is the
Under nonprocess conditions, i.e., after the final printed product
is formed or conditions under which the ink sticks are stored, and the ink is in
a ductile state or condition at a temperature range of from at least 10° C to
60° C, the preferred yield stress values are described by m as being from about
62.05 ± 13.79 kPa/°C (about -9 ± 2 psi/°C) to about 248.21 ± 13.79 kPa/°C (about
-36 ± 2 psi/ °C) and I as being from about 5515.81 ± 689.48 kPa (about 800 ± 100
psi) to about 15168.47 ± 689.48 kPa (about 2,200 ± 100 psi). More preferably, m
is about 206.48 ± 13.79 kPa/°C (about -30 ± 2 psi/°C), and I is about 11721.09
± 689.48 kPa (about 1,700 ± 100 psi).
Under process conditions, i.e., during the indirect printing of the
ink from an intermediate transfer surface onto a substrate while the ink is in
a malleable solid condition or state, at a temperature of from at least 20°C to
80°C, the preferred stress values are described by m as being from about -41.37
± 13.79 kPa/°C (about -6 ± 2 psi/°C) to about -248.21 ± 13.79 kPa/°C (about -36
± 2 psi/°C) and I as being from about 5515.81 ± 689.47 kPa (about 800 ± 100 psi)
to about 11031.62 ± 689.47 kPa (about 1,600 ± 100 psi). More preferably, m is about
-62.05 ± 13.79 kPa/°C (about -9 ± psi/°C), and I is about 6550.02 ± 689.47 kPa (about
950 ± 100 psi).
Fig. 5 shows the yield stress of a suitable phase-change ink as a
function of temperature. When subjected to a temperature range of from about 35°
C to about 55° C, the ink will begin to yield (compress) when subjected to a corresponding
pressure in a range of from about 1378.95 kPa (about 200 psi) to about 2757.90
kPa (about 400 psi). Optimal nip pressure is about two times the yield stress pressure
of the ink at any particular nip temperature. For example, for a 50° C yield stress
of 1723.69 kPa (250 psi), the nip pressure should be about 3447.38 kPa (about 500
psi). However, as described with reference to Figs. 6-10, print quality depends
more on various temperature-related parameters than on nip pressure.
Referring again to Fig. 1, during printing, drum 14 has a layer of
liquid intermediate transfer surface applied to its surface by the action of applicator
assembly 16. Assembly 16 is raised by an appropriate mechanism (not shown), such
as a cam or an air cylinder, until wicking pad 15 is in contact with the surface
of drum 14. The liquid is retained within reservoir 18 and passes through the
porous supporting material until it saturates wicking pad 15 to permit a uniform
layer of desired thickness of the liquid to be deposited on the surface of drum
14. Drum 14 rotates about a journalled shaft in the direction shown in Fig. 1 while
drum heater 28 heats the liquid layer and the surface of drum 14 to the desired
temperature. Once the entire periphery of drum 14 has been coated, applicator assembly
16 is lowered to a noncontacting position with intermediate transfer surface 12
on drum 14. Alternatively, the drum 14 can be coated with the liquid intermediate
transfer surface 12 by a web through which the liquid is transmitted by contact
with a wick. The wick is wetted from a reservoir containing the liquid.
Ink image 26 is applied to intermediate transfer surface 12 by printhead
11. The ink is applied in molten form, having been melted from its solid state
form by appropriate heating means (not shown). Ink image 26 solidifies on intermediate
transfer surface 12 by cooling to a malleable solid intermediate state as the drum
14 continues to rotate, entering nip 22 formed between roller 23 and the curved
surface of intermediate transfer surface 12 supported by drum 14. In nip 22, ink
image 26 is deformed to its final image conformation and adhered to final receiving
surface 21 by being pressed there against. Ink image 26 is thus transferred and
fixed to the final receiving surface 21 by the nip pressure exerted on it by the
resilient or elastomeric surface of the roller 23. Stripper fingers 24 help to
remove the imaged final receiving surface 21 from intermediate transfer surface
12 as drum 14 rotates. Ink image 26 then cools to ambient temperature where it
possesses sufficient strength and ductility to ensure its durability.
Applicator assembly 16 is actuatable to raise upward into contact
with drum 14 to replenish the liquid forming sacrificial intermediate transfer
surface 12. Applicator assembly 16 can also function as a cleaner if required
to remove lint, paper dust or, for example, ink, should abnormal printing operation
A proper set of image transfer conditions is dependent on a complexly
interrelated set of parameters related to nip pressure, preheater and drum temperature,
media time in nip 22, and ink parameters. Any particular set of transfer conditions
that provide acceptable prints is referred to as a process window.
The process window is determined experimentally by running test prints
under sets of controlled transfer conditions. The test prints were made using some
fixed control parameters. For instance, a diamond-turned unsealed anodized aluminum
drum was used, which is the preferred drum 14. Roller 23 was a typewriter platen
having an elastomeric surface with a Shore D hardness and/or durameter of 40 to
45. Each end of roller 23 was biased toward drum 14 with a 1556.88 N force (350-pound
force) resulting in an average nip pressure of about 3192.27 kPa (about 463 psi).
Final receiving substrate 21 was Hammermill Laser Print paper. Xerox type 4024
paper may also be used but is not preferred for test prints. The liquid forming
intermediate transfer surface 12 was 0.001 m2/s (1000cSt) silicone oil.
Final receiving medium 21 was moved through nip 22 at a velocity of about 13 cm/second.
The importance of velocity, which is determined by drum 14 rotation speed, is not
fully understood. However, the ink temperature in nip 22 substantially reaches
equilibrium in about 2 to about 6 milliseconds.
The process for forming intermediate transfer surface 12 on drum
14 entails manually holding an oil pad against rapidly rotating drum 14 until lines
of oil can be seen on drum 14. The oil is then wiped or buffed off drum 14 by
applying a Kaydry wiping cloth for two seconds against drum 14 and then for five
seconds across the drum. This method of applying intermediate transfer surface
12 is closely duplicated by applicator assembly 16.
Sets of test prints were made for various combinations of the temperature
of media preheater 27 and the temperature of drum 14.
Four primary factors determine the process window: fuse grade, pixel
picking, dot spread, and high temperature limit. Test prints were made as described
below to determine temperature ranges for each factor.
Fuse grade is a number proportional to the amount of ink that is
physically pressed into paper fibers during the transfer printing process. Fuse
grade is quantified by first imaging drum 14 with 4 X 4 cm squares of blue colored
image. The blue colored squares are formed by depositing superimposed layers of
cyan and magenta ink onto intermediate transfer surface 12 of drum 14. The blue
colored squares are then transferred to the paper final receiving medium 21 as
it passes through nip 22. A knife edge is used to scrape the ink from a blue colored
square transferred to each test print. An ACS Spectro-Sensor II spectrophotometer
measures the optical density (reflectance) of the scraped area and compares it
to a blank (white) area of the test print. The reflectance value is the fuse grade,
which is proportional to the amount of ink remaining (fused) in the test print.
The higher the fuse grade, the higher the optical density of the tested area is.
An acceptable minimum fuse grade is 20.
Fuse grade test print data are shown in Fig. 6, which plots iso-fuse
grade lines as a function of drum temperature and media preheater temperature.
The relatively vertical orientation of the iso-fuse grade lines indicates that
fuse grade is more dependent on the temperature of media preheater 27 than on the
temperature of drum 14. An iso-fuse grade line 50 (shown in bold) delimits a left
margin of a temperature region in which the fuse grade equals or exceeds the minimum
acceptable value of 20.
Pixel picking is a factor that relates to the percentage of ink droplets
that are transferred from drum 14 to final receiving media 21 during the transfer
printing process. A pixel picking percentage is determined by first imaging drum
14 with a blue color filled field, formed by overprinting cyan and magenta inks
on the drum 14 and having 475 unprinted squares each measuring a 3 X 3 pixel square
area. A single black ink drop or pixel is deposited in the center of each unprinted
3 X 3 pixel square area. The resulting image is then transferred to final receiving
medium 21 as it passes through nip 22. All of the double-layered blue colored
filled field area transfers, but the single layered 475 black drops within the
field are recessed below the blue filled field and are particularly difficult to
transfer. The percentage of black drops that transfer is the pixel picking percentage
with 80 percent being an acceptable level. Black ink drops not transferred when
the test print passes through nip 22 are easily transferred to a second "chaser
sheet" of final receiving medium 21 where they are counted to determine the pixel
Pixel picking test print and chaser sheet data are shown in Fig.
7, which plots iso-pixel picking percentage lines as a function of drum temperature
and media preheater temperature. Iso-pixel picking percentage lines 60 and 62
(shown in bold) delimit respective left and top margins of a temperature region
in which the pixel picking percentage equals or exceeds 80 percent. The graph
shows that below about 50° C pixel picking depends mostly on media preheater 27
temperature, whereas above about 50° C pixel picking depends mostly on the temperature
of drum 14.
Dot spread is classified into six groups related to the degree to
which adjacent ink drops (pixels) flatten and blend together to cover final receiving
medium 21 during the transfer printing process. Dot spread groups are quantified
by first imaging drum 14 with 4 X 4 cm squares of magenta ink. The magenta squares
are formed by depositing a single layer of magenta ink onto intermediate transfer
surface 12 of drum 14. Each square consists of ink drops deposited on drum 14 at
a uniform spacing defined by the 118 pixel/cm addressability of the test printer.
The deposited ink drops have a smaller diameter than the pixel-to-pixel spacing
before they are compressed in nip 22. The magenta squares are then transferred
to final receiving medium 21 as it passes through nip 22. The process is repeated
under various combinations of media preheater 27 and drum 14 temperatures to yield
a set of test prints that are inspected under a microscope and sorted into three
subjective groups including poor spread, medium spread, and good spread. Poor spread
(groups 1 and 2) is defined as the ability to see individual pixels and/or the
white lines between adjacent rows of pixels. Medium spread (groups 3 and 4) is
defined as the ability to see parts of white lines between adjacent rows of pixels.
Good spread (groups 5 and 6) is defined as viewing a solid sheet of ink with no
white paper showing through the transferred image. Each of the three print groups
was then subdivided into the better and worse prints of each group. Although solid
fill areas appear to have a higher print quality with the higher dot spread group
numbers, text becomes blurry because of reduced printing resolution. Dot spread
groups 4 and 5 strike an acceptable balance between good solid fill and text quality.
Dot spread test print data are shown in Fig. 8, which plots dot spread
group regions as a function of drum temperature and media preheater temperature.
Dot spread groups 4 and 5 are bounded by respective outlines 70 and 72 (shown
in bold), the outer extent of which delimit a temperature region within which the
dot spreading is acceptable. The relatively horizontal orientation of the dot
spread groups indicates that dot spreading is more dependent on the temperature
of drum 14 than on the temperature of media preheater 27. A region 74 (shown cross-hatched)
encompasses the optimized temperature transfer region shared by dot spread groups
4 and 5. The dot spread groups shown in Fig. 8 are outlines of the extreme data
points from each group. Because dot spread groups are determined by a subjective
measurement, some overlap exists among the groups and the extremes are only approximate.
The high temperature limit is defined as the maximum drum temperature
at which ink image 26 can be transferred from drum 14 without some of the ink drops
tearing apart because of cohesive failure, tearing apart from each other because
of adhesive failure, or sticking to drum 14 because of a low yield stress as shown
in Fig. 5. The high temperature limit is dominated by cohesive failure, which
is quantified by first imaging drum 14 with 4 X 4 cm colored squares of cyan, magenta,
yellow, black, green, blue and red ink. The colored squares are formed by depositing
the appropriate number of single or overprinted layers of primary inks (cyan, magenta,
yellow and black) onto intermediate transfer surface 12 of drum 14. The colored
squares are then transferred to final receiving medium 21 as it passes through
nip 22. A set of test prints are transferred with various temperature combinations
of media preheater 27 and drum 14. Cohesive failure is usually observed on edges
of the colored squares and is most easily observed as print remnants left on a
chaser or cleaning sheet. Acceptable prints require substantially no cohesive failure.
High temperature limit test print data are shown in Fig. 9, which
plots the cohesive failure as a function of drum temperature and media preheater
temperature. A high temperature limit line 80 (shown in bold) delimits a top margin
of a temperature region below which the ink will not undergo cohesive failure.
The relatively horizontal orientation of line 80 shows that the high temperature
limit is almost completely dependent on the temperature of drum 14.
However, the high temperature limit is an approximate value because
cohesive failure is dependent on the test image, ink color, ink composition, and
characteristics of intermediate transfer surface 12. In particular, using other
than a solid fill test image has caused cohesive failure at lower temperatures
than those resulting from the yellow squares image. At temperatures approaching
the high temperature limit it is theorized that the intermediate transfer surface
12 becomes a factor in determining cohesive failure if there is an insufficient
amount of the liquid forming the surface on drum 14. Drum surface roughness also
affects cohesive failure.
Fig. 10 shows a process window 90 that is defined by overlaying the
data of Figs. 6-9. Process window 90 has a left margin bounded by iso-fuse grade
20 (line 50 of Fig. 6), an upper margin bounded by 80 percent iso-pixel picking
(line 62 of Fig. 7), a right margin bounded by dot spread groups 4 and 5 (outlines
70 and 72 of Fig. 8), and a lower margin bounded by dot spread group 4 (outline
70 of Fig. 8). The upper margin of process window 90 is a few degrees C below the
high temperature limit (line 80 of Fig. 9).
Knowing process window 90 is useful for deriving the thermal specifications
and tolerances required for obtaining acceptable prints from a phase-change transfer
printer. In particular, media preheater 27, drum heater 28, power requirements,
warm-up times, and cooling requirements can be determined. Process window 90 should
have widely separated temperature boundaries to accommodate thermal mass variations
and temperature nonuniformities associated with drum 14, media preheater 27, and
Referring again to Fig. 1, for the above-described ink and imaging
apparatus 10, a desirable media preheater 27 temperature range is from 60° C to
150° C and a desirable drum 14 temperature range is from about 40° C to about 56°
C. Operation in the window of optimized temperature transfer conditions is preferred
and entails a media preheater 27 temperature range of from about 61° C to about
130°C and a drum 14 temperature range of from about 45° C to about 55° C.
Maintaining drum 14 within the temperature limits defined by process
window 90 requires heating drum 14 during periods of no printing and cooling drum
14 during periods of printing. Cooling is required during printing because heat
is transferred by preheated media contacting drum 14 in nip 22, by printhead 11
depositing molten ink on drum 14, and by radiation from heated printhead 11.
Referring to Fig. 11, heat is added to drum 14 by drum heater 28
that preferably consists of a heater lamp 92 and reflector 94. Heater lamp 92 is
of an infrared heating lamp type such as model No. QIR100-200TN1 manufactured
by Ushio Corporation in Newberg, Oregon.
An alternate embodiment for drum heater 28 consists of a cylindrical
cartridge or radiant lamp heater 96 axially mounted inside or adjacent to a hollow
drum shaft 98. In this embodiment, heat from heater 96 is radiated directly and
conducted to drum 14 by radial fins 30.
Drum 14 is cooled by moving air across radial fins 30 with a fan
100. Of course, fan 100 may blow or draw air in either direction through drum 14
to accomplish cooling. Preferably, fan 100 blows air through drum 14 in a direction
indicated by an arrow 102. Fan 100 is preferably of a type such as model No. 3610ML-05W-B50
manufactured by N.M.B. Minibea, Co., Ltd. in Japan.
Media preheater 27 is set to a predetermined operating temperature
by conventional thermostatic means. Drum temperature, however, is sensed by a thermistor
104 that slidably contacts drum 14 and is electrically connected to a conventional
proportional temperature controller 106. When printing, heat is added to drum 14,
which causes its temperature to exceed a predetermined temperature that is sensed
by thermistor 104. In response, temperature controller decreases electrical drive
power to drum heater 28 and turns on fan 100 to return drum 14 temperature to its
set point. Conversely, when not printing, thermistor 104 senses a decrease in
temperature below the set point. In response, temperature controller 106 turns
off fan 100 and adds power to drum heater 28. Depending on the rate of cooling
or heating required, temperature controller 106 may proportionally control one
or both of drum heater 28 and fan 100. Small temperature changes primarily entail
temperature controller 106 altering the amount of electrical power supplied to
drum heater 28.
Skilled workers will recognize that portions of this invention may
have alternative embodiments. For example, the drum heater 28 may be eliminated
if a process window can be obtained that includes a drum temperature of 30° C.
Monochrome or color printing embodiments of the invention are possible. Other than
a drum type supporting surface may be used, such as a flat platen or a belt. This
invention may be embodied in various media marking applications, such as facsimile
machines, copiers, and computer printers. The process window also may differ depending
on various combinations of nip pressure, ink composition, intermediate transfer
surface composition, drum surface finish and composition, and print medium composition.
The intermediate transfer surface also may be applied to the drum in various ways,
such as by an oil saturated web and metering blade assembly, a wick and reservoir
with a dry cleaning web followed by a metering blade, buffing with an oil-soaked
material, or use of an oil-soaked pad. Also, roller 23 could be heated to facilitate
transfer and fusing of the image 26 to the final receiving substrate 21.
It will be obvious to those having skill in the art that many changes
may be made to the details of the above-described embodiments of this invention
without departing from the underlying principles thereof. Accordingly, it will
be appreciated that this invention is also applicable to ink temperature control
applications other than those found in phase-change ink-jet transfer printers.