U.S. patent application number 13/555414 was filed with the patent office on 2012-12-20 for device for coating a peripheral surface of a sleeve body.
This patent application is currently assigned to AGFA GRAPHICS NV. Invention is credited to Eddie DAEMS, Jackie DUPREZ, Luc LEENDERS, Willem MUES, Hilbrand VANDEN WYNGAERT, Luc VANMAELE, Bart VERHOEST.
Application Number | 20120319013 13/555414 |
Document ID | / |
Family ID | 38801781 |
Filed Date | 2012-12-20 |
United States Patent
Application |
20120319013 |
Kind Code |
A1 |
LEENDERS; Luc ; et
al. |
December 20, 2012 |
DEVICE FOR COATING A PERIPHERAL SURFACE OF A SLEEVE BODY
Abstract
A coating device for coating a peripheral surface of a sleeve
body with a coating formulation includes a vertical support column
for supporting a sleeve body in a vertical position coaxial with a
coating axis, a carriage slideable along the vertical support
column, and an annular coating stage attached to the carriage and
moveable therewith for containing the coating formulation and for
coating a layer of the coating formulation onto the peripheral
surface of the sleeve body during a sliding movement of the
carriage along the vertical support column. The coating device
includes an irradiation stage that is arranged to be moveable with
the annular coating stage and to provide radiation to at least
partially cure the layer of coating formulation onto the peripheral
surface so as to prevent flow off of the coating formulation.
Inventors: |
LEENDERS; Luc; (Herentals,
BE) ; MUES; Willem; (Tremelo, BE) ; DUPREZ;
Jackie; (Gent, BE) ; VERHOEST; Bart; (Niel,
BE) ; VANDEN WYNGAERT; Hilbrand; (Grobbendonk,
BE) ; DAEMS; Eddie; (Herentals, BE) ;
VANMAELE; Luc; (Lochristi, BE) |
Assignee: |
AGFA GRAPHICS NV
Mortsel
BE
|
Family ID: |
38801781 |
Appl. No.: |
13/555414 |
Filed: |
July 23, 2012 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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12438386 |
Feb 23, 2009 |
8286578 |
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PCT/EP2007/059807 |
Sep 18, 2007 |
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13555414 |
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60846924 |
Sep 25, 2006 |
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Current U.S.
Class: |
250/492.1 |
Current CPC
Class: |
B41C 1/182 20130101;
B05C 3/10 20130101; B05C 5/0208 20130101; B05D 3/067 20130101; B05D
2254/02 20130101; B05C 9/14 20130101; B05D 1/18 20130101 |
Class at
Publication: |
250/492.1 |
International
Class: |
G21K 5/04 20060101
G21K005/04 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 18, 2006 |
EP |
06120823.7 |
Claims
1. An apparatus for irradiating a coated layer by UV light, the
apparatus comprising: an array of UV LEDs; a Fresnel lens including
a reflector; and a collimating mirror; wherein the Fresnel lens
including the reflector concentrates UV light received from the
array of UV LEDs into a focal point of the collimating mirror; and
the collimating mirror collimates the UV light received from the
array of UV LEDs into horizontal beams to irradiate the coated
layer.
2. The apparatus according to claim 1, wherein the array of UV LEDs
is annular.
3. The apparatus according to claim 1, further comprising a
radiation lock arranged to stop irradiation of the UV light from
the array of UV LEDs to the coated layer.
4. The apparatus according to claim 3, wherein the radiation lock
is annular.
5. The apparatus according to claim 4, wherein the radiation lock
includes an adjustable iris diaphragm.
6. The apparatus according to claim 1, further comprising a
manifold arranged to add an inert gas to a cure zone at a surface
of the coated layer.
7. The apparatus according to claim 6, wherein the manifold is
annular.
8. The apparatus according to claim 7, further comprising an
annular seal with an adjustable inner diameter.
9. The apparatus according to claim 8, wherein the annular seal is
an adjustable iris diaphragm.
10. The apparatus according to claim 6, further comprising a blow
knife positioned adjacent the cure zone.
11. The apparatus according to claim 10, wherein the blow knife is
annular.
12. The apparatus according to claim 1, wherein the array of UV
LEDs includes a mixture of different spectral output UV LEDs.
13. A method for irradiating a coated layer by UV light, the method
comprising the steps of: emitting UV light from an array of UV
LEDs; concentrating the UV light with a Fresnel lens including a
reflector into a focal point of a collimating mirror; collimating
the UV light with the collimating mirror into horizontal beams; and
irradiating the coated layer with the collimated UV light.
14. The method according to claim 13, further comprising the step
of: adjusting a radiation intensity of the UV light as a function
of a thickness of the coated layer.
15. The method according to claim 13, further comprising the step
of: adjusting a radiation intensity of the UV light as a function
of a chemical formulation of the coated layer.
16. The method according to claim 13, further comprising the step
of: spinning the array of UV LEDs, the Fresnel lens including the
reflector, and the collimating mirror around a cylindrical
surface.
17. The method according to claim 16, further comprising the step
of: moving the array of UV LEDs, the Fresnel lens including the
reflector, and the collimating mirror axially along the cylindrical
surface.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is a 371 National Stage Application of
PCT/EP2007/059807, filed Sep. 18, 2007. This application claims the
benefit of U.S. Provisional Application No. 60/846,924, filed Sep.
25, 2006, which is incorporated by reference herein in its
entirety. In addition, this application claims the benefit of
European Application No. 06120823.7, filed Sep. 18, 2006, which is
also incorporated by reference herein in its entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates to an apparatus and a method
for coating a sleeve body with a single or a multitude of uniform
layers of a coating formulation.
[0004] More specifically the invention is related to a coating
device and method of coating wherein an irradiation stage is
moveable with a coating stage. The irradiation stage at least
partially cures the coated layer applied by the coating stage.
[0005] 2. Description of the Related Art
[0006] Flexography is today one of the most important processes for
printing. It is a method that is commonly used for high-volume
runs. Flexography is employed for printing on a variety of
substrates such as paper, paperboard stock, corrugated board,
films, foils and laminates. Packaging foils and grocery bags are
prominent examples. Coarse surfaces and stretch films can be
economically printed only be means of flexography, which indeed
makes it very appropriate for packaging material printing.
[0007] It uses a rubber printing plate or a flexible photopolymer
plate that carries the printing image in relief. The ink delivery
system for flexography is achieved via an "anilox" engraved
transfer roll.
[0008] Analogue flexographic printing plates are prepared from
printing plate precursors that include a photosensitive layer on a
support or substrate. The photosensitive layer includes
ethylenically unsaturated monomers or oligomers, a photo-initiator
and an elastomeric binder. The support preferably is a polymeric
foil such as PET or a thin metallic plate. Imagewise crosslinking
of the photosensitive layer by exposure to ultraviolet and/or
visible radiation provides a negative working printing plate
precursor which after development with a suitable developer
(aqueous, solvent or heat development) leaves a printing relief,
that can be used for flexographic printing.
[0009] Imaging of the photosensitive layer of the printing plate
precursor with ultraviolet and/or visible radiation is typically
carried out through a mask, which has clear and opaque regions.
Crosslinking takes place in the regions of the photosensitive layer
under the clear regions of the mask but does not occur in the
regions of the photosensitive layer under the opaque regions of the
mask. The mask is usually a photographic negative of the desired
printed image. Flexographic printing plate making according to the
above described process has the disadvantage that the production of
a mask is time consuming and that the dimensional stability of
these masks with changing environmental temperature or humidity is
unsatisfactory for high quality printing and color registration.
Moreover, the use of separate masks in flexographic printing plate
production means additional consumables and chemistry, with a
negative impact on economy and ecology aspects of the production
process, which are far more a concern than the additional time
required to make the masks. As a matter of fact, in most cases
plate exposure and plate development may turn out to be more time
consuming than mask making.
[0010] Direct digital imaging using laser recording of printing
plate precursors, which eliminates the making of a separate film
mask, is becoming increasingly important in the printing industry.
The flexographic plate is made laser-sensitive by providing e.g. a
thin opaque IR-sensitive layer to the photopolymerizable layer of
the flexographic plate. Such a plate is sometimes called a
"digital" flexo plate. The thickness of such IR-ablative layers is
usually just a few .mu.m. The IR-ablative layer is inscribed
imagewise using an IR laser, i.e. the parts in which the laser beam
is incident on are ablated, i.e. removed. The actual printing
relief is produced in the conventional manner: exposure is effected
with actinic light (UV, visible) through the mask produced, being
imagewise opaque to the crosslinking inducing light, and the relief
layer is thus selectively crosslinked. Development can be effected
with an organic solvent, water or heat removing the photosensitive
material from the unexposed parts of the relief-forming layer and
the residues of the IR-ablative layer, either one by one using
different developing steps or simultaneously using one developing
step.
[0011] This method still requires a developing step as in the case
of previous methods and hence the improvement in efficiency for
producing flexo printing plates is limited.
[0012] In the direct laser engraving technique for the production
of flexographic printing plates, a relief suitable for printing is
engraved directly into a layer suitable for this purpose. By the
action of laser radiation, layer components or their degradation
products are removed in the form of hot gases, vapors, fumes,
droplets or small particles and nonprinting indentations are thus
produced. Engraving of rubber printing cylinders by means of lasers
has been known since the late 60s of the last century. However,
this technique has acquired broader commercial interest only in
recent years with the advent of improved laser systems. The
improvements in the laser systems include better focusing ability
of the laser beam, higher power, multiple laser beam or laser
source combinations and computer-controlled beam guidance. The
actual engraving system includes efficient gas- and debris
collecting systems. Direct laser engraving has several advantages
over the conventional production of flexographic printing plates. A
number of time-consuming process steps, such as the creation of a
photographic negative mask or development and drying of the
printing plate, can be dispensed with. Furthermore, the sidewall
shape of the individual relief elements can be individually
designed in the laser engraving technique.
[0013] Although photopolymeric printing elements are typically used
in "flat" sheet form, there are particular applications and
advantages to using printing elements in a continuous cylindrical
form as a rubber or a polymer sleeve. Continuous printing forms
provide improved registration accuracy and lower change-over-time
on press. Furthermore, such continuous printing forms may be
well-suited for mounting on laser exposure equipment, where it can
replace the drum, or be mounted on the drum for exposure by a
laser. Continuous printing forms have applications in the
flexographic printing of continuous designs such as in wallpaper,
decoration, gift wrapping paper and packaging.
[0014] Sleeves are made by coating, mold casting of an elastomeric
layer onto a plastic or metallic cylinder, or winding a rubber
ribbon around a plastic or metallic cylinder followed by a
vulcanizing, grinding and polishing step. The forms preferable are
seamless forms. As an alternative the elastomeric layer may be
first applied on a flat support, which is then bent onto the
carrier and bonded (cfr. NYLOFLEX.RTM. Infinity Technology from
BASF).
[0015] At the print media fair DRUPA held in 2004 in Germany, Asahi
Kasei showed a prototype of the Adless digital engraving technology
for the production of endless photopolymer sleeves for digital
engraving. It allows a liquid photopolymer material to be
continually coated onto a sleeve/cylinder in a short time. The
working principles of the technology are disclosed in published
patent application JP 2003-241397 from Asai Kasei. The Adless
system is based on a horizontal coating stage for applying a
photopolymer coating onto a sleeve core. The gap between the sleeve
core's peripheral surface and the coating stage is gradually
increased, while rotating the sleeve core, to increase the
thickness of the applied photopolymer coating layer. After coating,
the coated material is cured through photo-polymerization or
photo-crosslinking. A post-curing step of grinding and polishing
the cured photopolymer layer is required to provide the necessary
surface characteristics, such as evenness, to the photopolymer
layer in order to make the sleeve suitable as a flexographic
printing sleeve. The post-curing step is required a.o. because of
photopolymer unevenness of the coating process and the presence of
a polymer bulge at the location where the coating stage was
withdrawn from the sleeve when stopping the coating process. The
required grinding and polishing is a disadvantage of the Adless
system. The large floor space required, seen the horizontal
position of the coating system, is also a disadvantage.
[0016] Patent application publication JP 55-106567 from Canon
discloses a vertical coating method and device for uniformly
coating a setting paint onto a drum, fixing the paint onto the drum
by providing a low-hardening energy and further hardening the fixed
paint onto the drum by providing a high-hardening energy. The
coating vessel and the equipment for providing the low- and
high-hardening energy are fixedly mounted. The drum that is to be
coated is attached to a lifting and lowering mechanism for firstly
vertically immersing the drum into the coating vessel and
subsequently lifting the drum out of the vessel and transporting
the drum past an annular low-hardening energy device and then
positioning the drum in front of a vertical high-hardening energy
device. The disclosed coating device is suitable for the coating of
drums limited in size (both length and diameter) because: (1) the
length of the drum is limited to less than half the height of the
equipment and less than the height of the vertical high-hardening
energy device, and (2) the diameter of the drum is limited by the
dimensions of coating vessel and the diameter of the annular
low-hardening energy device.
[0017] U.S. Pat. No. 4,130,084 assigned to Stork Brabant B. V.
discloses a vertical ring coater having an annular receptical
containing a coating liquid and arranged coaxial with a vertically
positioned thin walled perforated sleeve. A layer of coating liquid
is applied on the periphery of the sleeve during vertical movement
of the annular receptical along the vertically positioned sleeve.
The layer of coating liquid is dried via heat energy provided via
mounting flanges into the central part of the sleeve.
[0018] A need exists for a coating device with limited floor space
requirements, suitable for making flexographic printing sleeves for
direct laser engraving, without the need for grinding and
polishing, that offers more flexibility towards types and sizes of
sleeves and further reduces the access time and production cost of
direct laser engraveable sleeves.
SUMMARY OF THE INVENTION
[0019] In order to overcome the problems described above, preferred
embodiments of the present invention provide a coating device that
supports a wide range of sleeve types and sizes, and is capable of
coating a single or a multitude of "uniform" layers of direct laser
engraveable material. The term "uniform" can refer to surface
properties, evenness, smoothness, homogeneity, coating formulation,
etc. of the layer(s). A preferred embodiment provides a coating
device with limited floor space requirements.
[0020] Preferred embodiments of the present invention provide a
coating device having the specific features set out below and a
method for coating as specified below. With this arrangement, large
size sleeves can be coated with acceptable-size equipment and the
uniformity of the coated layer is provided through partial curing
of the layer immediately after coating so as to preserve coating
layer thickness, surface evenness, surface homogeneity and surface
topology.
[0021] Further advantageous embodiments of the invention are set
out in the description below.
[0022] Other features, elements, steps, characteristics and
advantages of the present invention will become more apparent from
the following detailed description of preferred embodiments of the
present invention with reference to the attached drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0023] FIG. 1 shows a vertical ring coater known from the prior
art.
[0024] FIG. 2 shows a preferred embodiment of the present invention
incorporating an annular irradiation stage.
[0025] FIG. 3 shows a cross-sectional view of a preferred
embodiment of an annular irradiation stage.
[0026] FIG. 4 shows a cross-sectional view of another preferred
embodiment of an annular irradiation stage.
[0027] FIG. 5 shows a preferred embodiment of the present invention
incorporating a spinning irradiation stage.
[0028] FIG. 6 shows a preferred embodiment of the present invention
incorporating a spinning laser beam for irradiating the coated
layer.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Definitions & Industrial Applicability
[0029] The present invention related to a coating device and method
for coating one or more uniform layers of photosensitive material
onto a sleeve body, with a coating layer thickness variable between
a few micrometers and some millimeters. Because one application for
the present invention is the coating of an UV-curable material onto
a sleeve core, the discussion below will often refer to sleeves,
UV-curable coating formulations, UV-LED's, etc. to illustrate the
invention. However, it should be understood that the present
invention is not limited to UV light or UV photocuring
technologies. The invention can for example be used for coating of
formulations that solidify through thermal polymerization, an
example of which is provided in WO 2005/084959 to Creo I L. In
general, the invention may be used with any curable formulation
such as an e-beam curable formulation, IR or heat curable or
hardening coating formulation for coating onto for example
xerographic photoconductor drums or other cylindrical objects
needing a protective or functional surface coating, a visible light
curable formulation or a microwave curable formulation.
[0030] The invention may be engrafted on any equipment suitable for
positioning a sleeve core in a vertical position and having a tool
smoothly moveable along the sleeve core in the vertical direction.
Examples of such equipment are vertical ring coaters described in
the prior art or commercially available from Max Daetwyler
Corporation (Switzerland), the Stork Prints Group (The
Netherlands), and others. The description of the present invention
will therefore not elaborate on the basic features of this type of
equipment. Only in summary, a vertical ring coater as shown in FIG.
1 may include a vertical support column 1 that supports the sleeve
core 8 in a vertical position, incorporates a mechanism 4 for
lifting and lowering a coating carriage 5 vertically along the
sleeve core 8, and provides a space envelope for integrating a
number of utilities such as power cabling etc. The coating carriage
5 supports a coating collar 6 that is filled with a coating
formulation for coating onto the sleeve core 8. The sleeve core 8
is mounted in the vertical position by means of flanges or mounting
heads 9 at both ends; the flanges or mounting heads 9 themselves
are supported on the vertical support column 1. The flanges or
mounting heads 9 may be shaped so as to provide a smooth extension
of the sleeve core's peripheral surface, thereby allowing coating
of the sleeve core 8 up to edges, and to provide a sealed home
position for the annular coating collar 6 at one of the flanges or
mounting heads 9. The sleeve core 8 may be coated during an upward
or downward movement of the coating collar 6.
[0031] When the coating collar 6 moves downward during the coating
process, the coating layer is created from the meniscus between the
surface of the coating formulation contained in the coating collar
6, and the peripheral surface of the sleeve core 8. In general, the
thickness of the coating layer applied with this type of immersion
coating technique is determined by the formula
d = 20 * .eta. * v f ( Eq . 1 ) ##EQU00001##
wherein d equals the thickness of the coated layer in .mu.m, .eta.
is the viscosity of the coating formulation in mPas, .nu. is the
coating velocity in mmin.sup.-1, and f is the specific density in
kg/liter.
[0032] In the further description of the invention terms like deep
cure, surface cure, partial cure and full cure of a coated layer
will be often used. In this disclosure "deep cure" refers to the
curing of the bulk/body/mass of curable material in the coated
layer whereas "surface cure" refers to the curing of the surface of
the coated layer. Although a deep cure of a coated layer may also
affect the surface of that coated layer, in that also the surface
is cured to some extent, the focus is not on curing the surface of
the coated layer but the bulk. With a surface cure, the focus is
very much on curing the surface of the coated layer and providing a
skin on the coated layer. The terms "partial cure" and "full cure"
refer to the degree of curing, i.e. the percentage of converted
functional groups, and may be determined by for example RT-FTIR
(Real-Time Fourier Transform Infra-Red Spectroscopy)--a method well
know to the one skilled in the art of curable formulations. A
partial cure is defined as a degree of curing wherein at least 5%,
preferably 10%, of the functional groups in the coated formulation
is converted. A full cure is defined as a degree of curing wherein
the increase in the percentage of converted functional groups, with
increased exposure to radiation (time and/or dose), is negligible.
A full cure corresponds with a conversion percentage that is within
10%, preferably 5%, from the maximum conversion percentage defined
by the horizontal asymptote in the RT-FTIR graph (percentage
conversion versus curing energy or curing time).
Annular Irradiation Stage
[0033] A preferred embodiment of the invention is now described in
detail, with reference to FIG. 2. The coating collar 21 in FIG. 2
includes an annular squeegee 22 providing a slideable seal between
the bottom of the coating collar 21 and the sleeve core 13, in
order to prevent a coating formulation 24 contained in the coating
collar 21 to leak from the coating collar 21. The coating collar 21
may be open at the top. The surface 25 of the coating formulation
24 contained in the coating collar 21 forms an annular meniscus 26
with the peripheral surface of the sleeve 13. The coating collar 21
may be supported by a coating carriage (e.g. coating carriage 5 in
FIG. 1) that is connected to a lifting and lowering mechanism (e.g.
lift mechanism 4 in FIG. 1) incorporated in a vertical support
column (e.g. column 1 in FIG. 1). These features have been omitted
in FIG. 2. The lifting and lowering mechanism can move the entire
coating stage 11, i.e. the assembly of the coating carriage with
the coating collar, up and down along a vertical axis. When a
sleeve 13 is mounted, the lifting and lowering mechanism is capable
of moving the annular coating stage 11 along the peripheral surface
of the sleeve 13, providing a coating meniscus 26 at the top and a
sealing contact at the bottom of the coating collar 21. The coating
axis 10 refers to the vertical axis through the center of the
coating collar 21 and coincides with the axis of the sleeve 13 when
mounted on the coating device. The coating collar 21 moves up and
down, centered round the coating axis 10.
[0034] On top of the annular coating stage 11, an annular
irradiation stage 12 is mounted. The purpose of the irradiation
stage 12 is to sufficiently set the coated layer, just applied by
the annular coating collar 21 during its vertical movement down the
coating axis 10, so as to prevent the coating formulation from
running down. Running down of the coated layer decreases the layer
thickness at upper locations and increases the layer thickness at
lower locations along the sleeve 13, and decreases the topographic
uniformity of the layer and therefore the quality of the applied
coating. It is therefore an advantage to "pin" the coated layer
right after application onto the sleeve 13. The term "pin" does not
necessarily imply a full setting of the coated layer; a partial
setting of the layer to prevent run-down of the coating formulation
from the sleeve 13 is sufficient to provide a uniform layer of
coating material.
[0035] In a preferred embodiment, the irradiation stage 12 may be
360.degree. all round and use UV LEDs combined with concentrating
or collimating optics. UV LED's have several advantages compared to
UV arc lamps, such as their compactness, acceptable wavelength and
beam stability, good dose uniformity and a large linear control
range of the output energy dose. A disadvantage of UV LED's is
their relative low power output. UV LEDs however are relatively
small and can be grouped together in such a way that their combined
power is sufficient to cover the required range of UV curing doses
for different types of coating formulations, different thicknesses
of coating layer, different sleeve diameters and therefore
different distances from the UV LED to the peripheral surface of
the sleeve, etc. A cross-sectional view of a first embodiment of an
annular irradiation stage is shown in FIG. 3. The irradiation stage
is construed around an array of LEDs 31, a Fresnel lens 32 with
reflector 33 and collimating mirror 34. The role of the optics is
twofold: firstly Fresnel lens 32 with reflector 33 concentrates the
light from the array of LEDs 31 into the focal point of the
collimating mirror 34, and secondly the collimating mirror 34
collimates the light from the array of LEDs 31 into parallel
horizontal beams for irradiating the coated layer on the sleeve.
Revolving this optical setup 360.degree. round the coating axis
provides radiation from an annular radiation source, i.e. an
annular LED array, that is substantially collimated in the
horizontal direction and substantially focused onto the coating
axis 10, as illustrated by the arrows in the lower part of FIG. 2.
A cross-sectional view of a second embodiment of an annular
irradiation stage is illustrated in FIG. 4 and shows a LED 41
positioned at the focal point of a parabolic reflecting cavity 44
of collimator base 40. A heat sink 45 for removing heat from the
LED 41 is integrated in the collimator base 40. The small size of
the LED 41 allows it to be positioned in the focal point of the
parabolic reflecting cavity 44 without creating substantial voids
in the collimated output beam. Revolving this optical setup
360.degree. round the coating axis results in an annular radiation
source and annular collimating optics for providing annular
radiation as explained above.
[0036] The radiation energy contained in the collimated beam can be
easily modulated, by adjusting the radiation intensity, so as to
accommodate for the variation in distance or diameter of different
sleeve cores, as well as for variations in the chemistry of the
coating formulation.
[0037] The result is a radiation beam with large beam uniformity,
high beam stability, wide range of beam intensity adjustment (LEDs
can be dimmed to a few % of their maximal output power or can be
time modulated), and precise control of the UV curing process
through ease of calibration. The advantages are: (i) no extra
mechanical adjustments needed when changing sleeve cores or sleeve
core diameters and thus short sleeve change over time, (ii)
adaptable irradiation power and thus no power loss, and (iii)
uniform beam properties and thus accurate and uniform curing for
excellent coating characteristics provided already at the coating
stage (i.e. without post-treatment).
[0038] The annular shape of the UV LED array 41 and associated
collimating optics 44 of the irradiation stage 12 allows a uniform
annular irradiation of the coated layer. Furthermore, its
compactness and low weight allow the annular irradiation stage 12
to be mounted on the annular coating stage 11. In operation, the
annular irradiation stage 12 then moves along with the annular
coating stage 11 and only one drive mechanism for moving the
assembly up and down the sleeve core 13 is required.
[0039] Certain applications may require the use of a multitude of
irradiation stages 12, mounted in cascade, for providing radiation
with different wavelengths, for providing different radiation
power, etc. at different heights from the coating stage 11. The
multitude of irradiation stages may be mounted on top of each other
as one assembly, which itself may be mounted onto the coating stage
11. Mechanically linking the stages together is not mandatory. It
is however preferred that the stages be moveable up and down the
sleeve core in a synchronized way.
[0040] Notwithstanding the movement of the coating stage 11 and
possible disturbances of the surface 25 of the coating formulation
24 in the coating collar 21 during this movement, experiments show
surprisingly that the coated layer, applied with a coating device
as described above, is of a very good homogeneity and surface
evenness.
Rotating Irradiation Stage
[0041] If however the irradiation stage is not all round annular,
but includes one or more distinct circular irradiation sectors, one
or more linear irradiation segments or singular irradiation units,
the invention requires the irradiation stage to spin around the
sleeve in order to achieve a uniform irradiation all round the
coated layer. This is illustrated in FIG. 5. Four singular
irradiation units 50 are shown equably arranged around the coating
axis 10. Each irradiation unit 50 may include a UV LED 51 and
collimating paraboloidal optics 54 to produce a beam of collimated
parallel UV light. A detailed description of one embodiment of a
singular irradiation unit 50 may be found in granted U.S. Pat. No.
6,880,954. The singular irradiation units 50 may be mounted on a
mounting base 59 of the irradiation stage 52. In order to provide
all round uniform irradiation of the coated layer on the periphery
of a sleeve core 13, the mounting base 59 rotates around the
coating axis 10 while coating is performed, i.e. while the coating
stages 11 moves vertically coaxial with the coating axis 10.
Therefore the mounting base 59 is mounted rotatable on the coating
stage 11 by means of e.g. an annular guide 58, and driven by a
motor 56 and gear transmission 57 mounted on the coating stage 11.
The gear transmission 57 may include a gear wheel cooperating with
a crown gear mounted on the mounting base 59, but other
transmission systems may be used as well such as bevel gears. The
mounting base 59 of the irradiation stage 52 further includes
multiple rotational electrical connecting means 55, e.g. slip
rings, for powering the multiple singular irradiation units 50 on
the mounting base 59. Mechanical and electrical drive means and
interconnections between the irradiation stage 52 and the coating
stage 11 (e.g. the motor 56, the annular guide 58, the gear
transmission 57 and the electrical slip connections 55) preferably
refer to (or are mounted on) the coating carriage 29 of the coating
stage 11. This setup allows exchangeability of the annular coating
collar 21, to adapt for different external diameter of the sleeve
cores to be coated, without changing the mechanical and electrical
setup of the coating stage 11 and the rotatable irradiation stage
52. The illustrative embodiment of the rotatable irradiation stage
52 of FIG. 5 uses singular irradiation units 50. As indicated above
the rotatable irradiation stage may include other irradiation units
such as irradiation segments based on an array of LEDs and a
concentrating and collimating mirror, or they may include arc lamp
systems although these are generally more complex and heavier to
mount, connect and rotate. The rotation of the irradiation stage
provides a 360.degree. integration of the radiation from the
different irradiation units and smoothens the radiation intensity
variations between different irradiation units and within each
irradiation unit. An equable distribution of the irradiation units
around the coating axis may be a preferred setup, but it is not
required because the rotation of the irradiation stage will provide
a 360.degree. integration anyhow. A rotatable irradiation stage may
therefore also be realized using only one singular irradiation
unit.
Further Embodiment Details or Alternatives
Alternative Irradiation Stage
[0042] In the embodiments described above the irradiation source,
e.g. an individual LED or an annular LED array, was linked to a
corresponding collimating optics, e.g. a paraboloidal reflector
respectively an annular collimating optics, and was considered as
one assembly. In an alternative embodiment the optics may be
omitted with the LED radiation source directly irradiating the
peripheral surface of the coated sleeve. Rotation of the
irradiation source may provide additional integration and averaging
of the radiation energy. In another alternative embodiment a
non-rotating annular collimating optics may be combined with a
rotating radiation source. In this configuration, the radiation
source orbits between the peripheral surface of the sleeve core and
the annular collimating optics.
Radiation Lock
[0043] From Eq.1 we know that the viscosity of the coating
formulation is an important parameter in controlling the thickness
of the applied layer. It is therefore preferable to shield the
coating formulation in the coating collar from any sources that
directly or indirectly may change the viscosity of the coating
formulation. In radiation curable systems, exposure to radiation
changes the viscosity of the coating formulation, i.e. the
viscosity of a coating formulation increases when exposed to
radiation in order to pin, set or cure the coated formulation. The
coating device according to the invention therefore preferably
includes a radiation lock positioned between the radiation stage
and the coating stage, and moveable therewith, for shutting off
direct and indirect, e.g. scattered, radiation coming from the
radiation source from radiating the coating formulation contained
in the coating collar. The radiation lock is preferably annular
shaped and may for example be realized by providing a cover to the
coating collar reservoir. A more advanced radiation lock would be
an adjustable iris diaphragm as used in optics, the diaphragm
opening being adjusted to be slightly larger than the diameter of
the sleeve to be coated. The annular radiation lock may be
mechanically integrated in the coating stage, in the irradiation
stage or as a separate unit in between both stages.
[0044] Another process parameter influencing the viscosity of the
coating formulation contained in the coating collar may be the
temperature of the coating formulation. In a preferred embodiment,
the coating formulation contained in the coating collar or the
coating collar itself may therefore be thermostatically
controlled.
Inert Environment
[0045] In applications using free radical UV curable formulations,
it is known that the curing, in some cases, may be retarded or even
not initiated due to the presence of oxygen in the cure zone. In
this case, an inert atmosphere may be used to enhance the cure
capabilities. In relation to UV curing, the term `inert` simply
means the elimination (in ideal situations) or the minimizing (in
more realistic situations) of the amount of inhibiting oxygen at
the surface of the coated layer within the UV cure zone. In a
vertical coating device according to the invention, the cure zone
refers to the space between the surface of the coated layer and the
irradiation stage. An inert environment may be created by (i)
adding a gas such as nitrogen, argon or carbon dioxide to the
atmosphere in the cure zone and especially close to or at the
surface of the coated layer, and (ii) minimizing the ingress of
air, as a result of the drag effect resulting from the relative
movement between the non-moving coated layer and the moving
irradiation stage, in the cure zone.
[0046] Adding an inert gas, such as nitrogen, argon or carbon
dioxide, to the atmosphere in the cure zone may be realized by use
of an annular manifold, moving together or integrated with the
irradiation stage and connected with flexible tubing to a source of
inert gas housed in the vertical support column of the coating
device. Annular clearance seals at both ends of the cure zone, i.e.
at the upper and lower end of the irradiation stage, having a small
clearance to the peripheral surface of the coated sleeve may be
used to prevent the inert gas from flowing out of the cure zone.
These seals preferably have an adjustable inner diameter to fit
with a small clearance to the various sleeve diameters. Iris
diaphragms may be suitable seals for this purpose. A controlled
flow of inert gas within the cure zone may be realized with two
manifolds, i.e. an inlet and outlet manifold.
[0047] The ingress of air in the cure zone is likely to occur at
the lower end of the cure zone, when the coating stage moves
downward during the coating process. That is, the air is likely to
enter from between the coating stage and the irradiation stage.
Counteracting this air intake may be accomplished by means of an
annular blow knife positioned at the lower entrance of the curing
zone, i.e. between the irradiation stage and the coating stage. The
annular blow knife, moving with and between the coating stage and
the irradiation stage, may be connected with flexible tubing to a
source of inert gas housed in the vertical support column of the
coating device, for blowing inert gas and blocking the air
intake.
[0048] The above "closed" inert environment has been described in
relation to oxygen inhibition in free radical UV curing systems.
Depending on the coating formulation and the way the coated
formulation is cured, other embodiments of an inert environment may
be thought of.
[0049] Instead of providing and integrating a series of
supplementary tools in and around the moveable irradiation stage to
create an local inert environment in the cure zone, the entire
coating device may be capped and closed off from ambient
environment, in which case the task of creating an inert
environment in the cure zone is much simpler, i.e. the inert
environment is created within the closed cap. Alternatively, the
entire coating device may be installed in an inert environment or
room provided by the end user.
Full-Cure Irradiation
[0050] As described before, the radiation from the irradiation
stage is targeted at at least partially curing the coated layer so
as to prevent run-down of coating formulation from the sleeve and
thus fixing the coating layer thickness. The initial curing dose,
applied immediately after coating, often provides insufficient
energy to fully cure the coated layer. A full cure of the coated
layer may be provided off-line using existing sleeve processing
devices or may be provided in-line using an additional radiation
system as disclosed in Japanese patent application JP
54-014630.
[0051] An alternative to the irradiation system disclosed in JP
54-014630 is a robotic arm including two shells of a vertical full
cure irradiation tunnel, much like the two halves of a sun tube
solarium. The irradiation tunnel may be mounted and supported by
the existing vertical support column of the coating device or may
have its one mounting column. A dedicated mounting column for the
vertical irradiation tunnel may for example be positioned opposite
the vertical support column of the coating device, with respect to
the coating axis, and therefore interfere as little as possible
with the operation of the coating process. The two shells of the
full-cure irradiation tunnel, when closed, preferably are designed
to substantially completely surround the sleeve along the full
length (height) and provide an all round radiation to fully cure
the coated layer on the sleeve. A full cure is typically performed
when the coating stage and the irradiation stage are positioned at
one of the flanges or mounting heads so as to leave the surface of
the coated sleeve completely accessible to the vertical irradiation
tunnel. After full curing the coated layer on the sleeve, the
vertical full-cure irradiation tunnel may be opened to release the
sleeve for either removal from the vertical coating device or for
application of a next layer of coating formulation (see further).
Opening and closing the vertical tunnel may be performed by an
operator or may be automated via actuators (e.g. controllable
spring hinges) in the vertical support column in the way two
co-operating robotic arms would be controlled. The vertical
irradiation tunnel may be equipped with radiation sources different
from the type used in the irradiation stage that moves along with
the coating stage to partially cure to applied layer of coating
formulation. As the target of a partial curing is to prevent
run-down of coating material from the sleeve, the irradiation stage
preferably includes a radiation source for deep curing the coated
layer. In the case of UV-curable formulations, this preferably is a
UVA radiation source irradiating wavelengths between about 320 nm
and 400 nm. As a full cure is targeted at further curing the
formulation coated on the sleeve, up to about the maximum
conversion percentage of functional groups in the formulation, the
vertical irradiation tunnel preferably includes radiation sources
for deep curing and optionally surface curing of the coated layer.
In the case of UV-curable formulations, this preferably is a UVA
radiation source (for deep cure) and optionally a UVC source
irradiating wavelengths below 280 nm (for surface cure). If UV arc
lamps are used instead of UV-LEDs, a single lamp may irradiate at a
range of wavelengths, possibly including UVC (below about 280 nm),
UVB (between about 280 nm and 320 nm), UVA (about 320 nm and 400
nm) and UV-Visible light (above about 400 nm). Multiple irradiation
sources, each with a different or overlapping narrow irradiation
spectrum, may also be combined in the vertical irradiation tunnel
to provide a wide spectrum of UV radiation for full curing the
coated layer.
[0052] A full cure of the coated layer may also be realized in-line
by adding radiation capacity to the existing irradiation stage. The
additional capacity may be realized by increasing the available
radiation power (e.g. additional UV LED arrays), adding a different
type radiation (e.g. adding surface cure UVC wavelength radiation
to existing deep cure UVA radiation), using specially adapted
collimating optics for delivering a variable irradiation intensity
as a function of the vertical distance from the coating meniscus
(e.g. collimating optics for providing high irradiation intensity
close to the coating meniscus, to realize an short but intense deep
cure of the coated layer, and a vertically spread out lower
irradiation intensity further away from the coating meniscus, to
realize further deep and surface curing of the coated layer), or by
straightforward duplicating existing irradiations stages.
[0053] As described before, mechanically linking the irradiation
stage with the coating stage is not mandatory but movement of both
stages in a synchronized way so as to maintain a constant time
delay between coating of the layer of coating formulation and
partial curing of the coated layer is preferable. In a further
embodiment of the invention, it may be preferable, after the layer
of coating formulation has been applied and partially cured, to
physically and/or control-wise disconnect the irradiation stage
from the coating stage and provide for the irradiation stage to
move up and down the coated sleeve as an independent unit, as often
as required, to further cure (possibly full cure) the applied layer
of coating formulation. Before a next layer of coating formulation
is applied, the irradiation stage may again physically or
control-wise be connected to the coating stage. Note that in the
setup of an independent moving irradiation stage, the irradiation
stage must be connected to an independent carriage for moving the
irradiation stage up and down the sleeve independently from the
coating stage. The coating stage may then be hooked onto the
carriage of the irradiation stage and move as a slave unit with the
irradiation stage (instead of vice versa as described in previous
embodiments) or the coating stage may keep its independent carriage
and move in a synchronized way with the irradiation stage.
Pre- or Post-Treatment
[0054] A vertical tunnel as described above may also be designed to
apply a special surface treatment of the sleeve or the coated
layer. A vertical tunnel may for example incorporate corona devices
to enhance the adhesion of the coating formulation onto the surface
of the sleeve body, or it may include UV irradiation sources with a
radiation wavelength below 280 nm to reduce the tackiness of the
final surface of the coated layer.
LED Technology
[0055] An advantage of using LED technology for irradiating the
coated layer is that the radiation intensity, and therefore the
amount of radiation energy received by the coated layer, is easily
adjustable. In one example the radiation intensity may be adjusted
as a function of the coated layer thickness or a corresponding
process variable (see Eq.1 above), e.g. the viscosity of the
applied coating formulation or the coating speed. In another
example the radiation intensity may be adjusted as a function of
the coating formulation or a component in the coating formulation,
e.g. the amount of photo-initiators or sensitizers included in the
coating formulation. In still another example the radiation
intensity may be adjusted as a function of the optical distance
between the radiation source and the peripheral surface of the
coated sleeve. For example, in FIG. 2 the received radiation energy
per unit area of the coated layer decreases with increasing sleeve
diameter. This variation may be calculated and compensated for by
adjusting the radiation intensity or power of the LEDs so as to
keep the radiation energy per unit area of the coated layer
constant.
[0056] Compared to alternative radiation technologies such as for
example arc lamp sources, LED technology provides the advantage of
a small footprint and good beam and wavelength stability.
[0057] A further advantage of LED technology is their narrow
bandwidth and singular spectral output, and the possible choice of
a mixture of different spectral output UV LEDs. The choice of
single wavelength UV output or a combination of spectral outputs
allows for the further tuning of the UV curing process and the
coating chemistry. A combination of spectral outputs can easily be
realized by providing multiple banks of LEDs, each bank including
LEDS with a different spectral output, and switching one or more
banks ON or OFF in order to realize the spectral combination
sought. Furthermore, the nearly complete absence of any IR
radiation from these UV LEDs eliminates the need for IR-absorption
filters, such as water-filled reservoirs, and is a bonus in
reducing local and uneven subject heating.
[0058] Still further advantages of LED technology are its
compactness, low weight and the ongoing technological trend towards
higher power LEDs.
Laser Curing
[0059] An alternative embodiment of a coating device according to
the invention is shown in FIG. 6 and may include a rotating
irradiation stage with a laser beam 64 as a rotating single
irradiation unit. The laser beam 64 may be provided from a fixed
laser source 60 above the sleeve core 13, possibly mounted onto the
coating device's vertical support column (see FIG. 1). For that
purpose, the sleeve is single-ended mounted via the bottom flange
or mounting head of the coating device. The laser source 60 may be
mounted coaxial with the coating axis 10 for creating a laser beam
64 starting off along the coating axis 10. A spinning optical path
is provided for guiding the fixed laser beam starting off at the
laser source 60 to a spinning mirror 63 used for directing the
laser beam onto the peripheral surface of the sleeve 13. In the
embodiment shown in FIG. 6, the spinning optical path is created
via a rotating central mirror 61 deflecting the starting laser beam
64.sub.1 in a direction perpendicular to the coating axis 10 and
spinning the laser beam around the coating axis 10. A first
spinning mirror 62, co-operating with the rotating central mirror
61, deflects the spinning laser beam 64.sub.2 parallel with the
coating axis but at the outside of the sleeve. Finally, a second
spinning mirror 63 that is part of the rotating irradiation stage
52 co-operates with the first spinning mirror 62 and deflects the
spinning laser beam 64.sub.3 towards the coating axis 10 thereby
projecting laser beam 64.sub.4 in a spinning way onto the coated
layer on the peripheral surface of the sleeve 13. The
synchronization of the multiple co-operating mirrors 61-62-63 may
be realized by fixing their angular position via a mechanical
framework 65-66 attached to the mounting base 59 of the rotating
irradiation stage 52. The framework 65-66 therefore spins along
with the irradiation stage 52. The spinning of the laser beam 64 is
therefore completely controlled by and synchronized with the
rotation of the irradiation stage 52.
[0060] In a preferred embodiment, as shown in FIG. 6, a vertical
guiding system 67 may be installed to keep the rotating framework
element 66, and mounted thereon central mirror 61 and first
spinning mirror 62, at a fixed vertical position independent of the
vertical position of the coating stage 11 and irradiation stage 52
and the movement of these stages during a coating operation. The
main advantage is to reduce the height for the coating device. This
may be realized in two ways. If the vertical position, i.e. the
height, of the rotating framework element 66 is mechanically fixed
for example via a link to the coating device's vertical support
column, a vertical guiding means 67 may include simple bearings to
allow relative movement between the coating/irradiation stage and
the framework element 66. Alternatively, if the vertical position,
i.e. the height, of the rotating framework element 66 is not
mechanically fixed, the vertical guiding system 67 preferably
includes an active linear motion system (not shown) for controlling
a relative movement between the coating/irradiation stage and the
framework element 66 so as to keep the framework element 66 at a
fixed vertical position independent of the vertical position of the
coating/irradiation stage.
[0061] In another embodiment the rotating framework element 66 may
be mounted completely independent from the coating/irradiation
stage. The spinning framework elements 65 and vertical guiding
system 67 may then be omitted and the rotation of the framework
element 66 is then synchronized with the rotation of the
irradiation stage and the framework element 66 in order to preserve
the continuous spinning optical path that guides the laser beam 64
onto the peripheral surface of the sleeve 13. The coating device
may then include two synchronized independent spinning
entities.
[0062] In order to avoid collision of the spinning framework
elements 65 with the mechanism for lifting and lowering the coating
carriage, the lifting and lowering mechanism as illustrated in FIG.
1, integrated in the peripheral vertical support column, preferably
is replaced by a linear motion system operating completely within
the space envelope of the spinning framework element 65. Telescopic
lift systems operating within this space envelope may for example
be used.
[0063] The above disclosed embodiment of the invention is described
with reference to a laser system. The inventive concept however is
not limited thereto and in general includes the use of a fixed
mounted radiation source linked to a spinning optical path to guide
the radiation beam from the fixed radiation source round the
peripheral surface of the sleeve, and in synchronism with the
vertical movement of a coating stage. Any radiation source that
provides the required type of radiation, with enough power to at
least partially cure the coated layer on the peripheral surface of
a sleeve, may be used.
Different Sleeve Sizes
[0064] It has been mentioned in a previous section that the
radiation power may be adjusted as a function of the optical
distance from the irradiation source to the peripheral surface of
the sleeve, such that adequate curing (at least partial) or
"freezing" of the coated layer onto the peripheral surface of the
sleeve is achieved. This improves the compatibility of the
irradiation stage with different sleeve diameters. Three
alternative configurations are provided: (1) the irradiation stage
configuration is fixed and designed to accommodate the largest
sleeve diameter within a range of different sleeve diameters and
the radiation power of the irradiation units is adjusted as a
function of sleeve diameter used, (2) the irradiation stage
configuration is adjustable and designed to adjust the radial
position of the irradiation units to the diameter of the sleeve
used and (3) the irradiation stage is adjustable and designed to
adjust the spinning velocity of the irradiation units around the
coating axis in order to keep the radiation energy received per
unit area on the peripheral surface of sleeves of different sleeve
diameters constant.
[0065] With respect to the coating stage, the coating meniscus and
the annular seal are important issues when changing sleeve
diameter. The annular seal around the peripheral surface of the
sleeve prevents leakage and run down of coating formulation from
the coating collar. When changing sleeve diameter, either the
entire coating collar (including the annular seal) may be replaced
by another coating collar suited for the new sleeve diameter or
only the annular seal may be replaced or adjusted to fit with the
new sleeve diameter. If the annular seal is realized as an iris
diaphragm of which the aperture is adjustable within a range, no
replacement parts are required when changing sleeve diameter,
provided that the sleeve diameter falls within the range of the
adjustable aperture. If the annular seal is removably attached to
the coating collar, a seal with a different fixed internal diameter
may be used.
[0066] Preferably the coating stage and the irradiation stage are
designed to support the same range of sleeve diameters so that both
modules can be pre-assembled as a tandem and treated as one
assembly that can be easily replaced for operating with different
ranges of sleeve diameters.
Drive Systems & Process Control
[0067] In the embodiments described above, the irradiation stage or
multitude of irradiation stages are mounted on top of the coating
stage and move together with the coating stage as a single "coating
assembly". From a mechanical point of view, this provides the
advantage that only one lifting and lowering mechanism is required
to operate the vertical coating device, whereas from an electrical
point of view, all electrical connections to the "coating assembly"
may be provided through a single cable carrier between the
stationary vertical support column and the moving "coating
assembly".
[0068] An energy dose controlling system may be added to the
"coating assembly" for measuring the effective curing rate of the
applied layer and adjusting the applied energy dose, spinning
velocity (if applicable) and/or coating speed in a closed loop
system in order to obtained the desired coating layer thickness and
uniformity. An infrared spectrometer, such as an FTIR, may for
example be used to measure the degree of UV or EB curing, i.e. the
curing rate, of the coated layer.
[0069] However, if the purpose of the irradiation stage is to only
partially set the coated layer to prevent run-down of the coating
formulation from the sleeve, the irradiation dose is less critical
and monitoring of the irradiation dose in a closed loop system may
not be required. A calibration of the irradiation stage combined
with open loop control may be sufficient.
Operation
Preparation
[0070] The coating device according to the invention may be set up
and prepared for coating operations without the presence of a
sleeve core. Thereto, one of the flanges or mounting heads, for
mounting the sleeve onto the coating device, may be used to provide
a home position to the coating assembly (i.e. coating
stage+irradiation stage). The flange or mounting head providing
this home position has a similar or slightly smaller external
diameter than the diameter of the sleeve cores intended to be used
with the flange or mounting head. When the coating assembly is in
its home position, the annular seal of the coating collar may be
adjusted to fit with the sleeve core diameter, even prior to
mounting the sleeve core in the coating device, and the coating
collar may be filled with a coating formulation, without leakage.
The coating stage is then ready for coating operations.
[0071] If flanges or mounting heads are used with substantially
different external diameter than the diameter of the sleeve cores
to be coated, the preparation of the coating assembly can not be
performed without the presence of a sleeve core mounted on the
coating device. A home position for the coating assembly should
then be provided by the sleeve itself. This is however not a
preferred situation as it requires additional care and setup of the
coating collar with each change of sleeve core.
Immersion Coating
[0072] After preparing the coating assembly and mounting the sleeve
core on the coating device, the lifting and lowering mechanism
moves the coating assembly to a start position with the coating
meniscus close to or just past an end of the sleeve core, depending
on the type of flange or mounting head used. The coating process
preferably starts at the upper end of the sleeve core and continues
in a downward direction to the lower end of the sleeve core while
the lifting and lowering mechanism moves the coating assembly
downwards. In this setup, the coating process is equivalent to an
immersion type coating process. As the coating assembly moves
downward, the irradiation stage follows immediately after and
irradiates the just coated layer to at least partially cure the
coated layer, which prevents run down of the applied coating
formulation. If a spinning irradiation stage is used, the
irradiation stage not only follows the coating meniscus at a fixed
distance, but in addition spins around the sleeve core to generate
a uniform 360.degree. irradiation of the coated layer. At the lower
end of the sleeve core, the lifting and lowering mechanism halts
the coating assembly with the coating meniscus close to or just
past the lower end of the sleeve core, depending on the type of
flange or mounting head used. If the flanges or mounting heads
allow end-to-end coating of the sleeve core, the coating assembly
will be moved that far downward to allow the irradiation stage to
irradiate the coated layer up to the lower end of the sleeve
core.
[0073] The thickness of the coated layer may be controlled via the
velocity of the coating assembly moving downward, the viscosity of
the coating formulation or the number of successive coating
operations applied (see hereinafter). After the coating process,
the irradiation stage may be physically or control-wise
disconnected form the coating stage (if the setup of the coating
device so allows) and be moved up and down the coated sleeve
independent from the coating stage to further cure the coated
layer. Depending on the thickness of the coated layer, the level of
curing preferred and the type/amount of curing energy available
from the irradiation stage, one or more cycles up and down the
sleeve may be required. Alternatively, after the coating and
irradiation stage have reached the lower flange or mounting head,
the vertical irradiation tunnel may be used to further cure the
coated layer while the coating and irradiation stage remain
stationary at the lower flange or mounting head. After the coating
process, the coating assembly is left at its position against the
lower or upper flange or mounting head and the coated sleeve may be
removed, without special care for the coating collar.
Squeegee Coating
[0074] Alternatively, a coating layer may be applied while the
coating assembly moves upward, in which case the coating mechanism
is a squeegee type coating mechanism, instead of the immersion type
coating during the downward movement of the coating assembly as
described above. Application of a coating layer during upward
movement of the coating assembly, may require an irradiation stage
positioned below the coating stage and moving together with the
coating stage to at least partially cure the squeegee coated layer.
Squeegee type coated layers, associated with an upward movement of
the coating collar, may be substantially thinner than immersion
type coated layers, associated with a downward movement of the
coating collar. Unfortunately there is no formula, analogous to
Eq.1, known to the inventors to predict the thickness of the
squeegee type coated layer. Each of the alternatives may therefore
have advantages in specific applications.
Multiple Pass Coating
[0075] The coating device may also operate in a multiple pass mode
with "intermediate" curing of each of the applied layers. The
purpose of intermediate curing is to sufficiently set the coated
layer in order to avoid deformation of this layer during a next
coating step or during a sliding contact with the annular squeegee
of the coating collar when the coating stage is moved. The
intermediate cure preferably does not generate a full cure of the
coated layer. More specifically it preferably generates enough
curing in the bulk of the coated layer (to avoid deformation) but
leaves the surface of the coated layer not fully cured (to maintain
good adhesion properties of a next layer of the coating formulation
onto the intermediate cured layer of the coating formulation). An
intermediate cure step may be provided by the irradiation stage via
additional up/down movements thereof (disconnected from the coating
stage) or by an additional vertical irradiation tunnel, as
described above.
[0076] A multiple pass operating mode may include the steps of
applying a first immersion coated layer while moving a coating
assembly downward and at least partially curing the first immersion
coated layer with an upper irradiation stage positioned above and
moving with the coating stage; optionally providing an intermediate
curing step to further cure the coated layer and then moving the
coating assembly upward again in sliding contact with the just
previously coated layer; then applying a second immersion coated
layer while moving the coating assembly downward and at least
partially curing the second immersion coated layer with the upper
irradiation stage moving with the coating stage and optionally
providing an intermediate cure step, etc. As the annular squeegee
of the coating collar is designed to prevent leakage of coating
formulation from the coating collar, at the sliding contact between
the coating collar and the sleeve, the thickness of the layer
applied via a squeegee type coating during the upward movement of
the coating assembly typically is significantly less than the
thickness of the layer applied via immersion coating during the
downward movement of the coating assembly. In the description of
the multiple pass coating method above, the squeegee type coated
layer is therefore disregarded because it only has a marginal
contribution to the thickness of coated multilayer. Indeed, there
is a main (immersion) coating action during the downward movement
of the coating assembly and only a fractional (squeegee) coating
action during the upward movement thereof. The coating is thus
primarily unidirectional. Intermediate curing of the fractional
(squeegee) coating layer may therefore not be necessary as it will
merge with the significantly thicker subsequent main (immersion)
coating layer from a subsequent coating action. The squeegee coated
layer, merged with the immersion coated layer, is of course
irradiated using the upper irradiation stage when the coating
assembly is moved downward. So, a multiple pass coating device
according to the invention not necessarily includes an upper and a
lower irradiation stage to at least partially cure the coated layer
in both coating directions; a single irradiation stage linked with
a main coating direction may serve.
[0077] Multiple pass operation of the coating device as described
may be used for applying uniform thick layers of a coating
formulation onto sleeve cores. It may for example be used in cases
where physico-chemical parameters of the coating formulation, e.g.
viscosity, or limitations of the coating device, e.g. coating
velocity, would limit the thickness of a coated layer as predicted
from Eq.1 to a value below what is functionally required for the
application. Especially for flexographic sleeves or printing
masters, the relief-forming layer may require a thickness of
several millimeters, which is hard to achieve in a single pass
coating process.
[0078] Multiple pass operation of the coating device may also be
used for applying a multitude of layers of different coating
formulations. The coating formulations may have different
physicochemical properties, e.g. viscosity, or the corresponding
coated layers may have different physicochemical or mechanical
properties such as compressibility, hardness, wear-resistance,
wettability, etc. E.g. for the production of flexographic sleeves,
it may be desirable to have a compressible base (suitable for
absorbing for example the unevenness in corrugated board printing
material) and a hard surface or top layer (for increased durability
and suitable for longer print runs). If desired a complete
physicochemical thickness profile may be created for the coated
multilayer. If multiple pass operation is used to apply multiple
layers of different coating formulations, the coating collar may
need to be drained and replenished with a different coating
formulation. These actions may be performed when the coating stage
is located at one of the flanges or mounting heads; the flanges or
mounting heads provide a sealing home position or service position
for the coating collar. These positions also allow for altering the
squeegee internal diameter (e.g. by changing to a larger internal
diameter as the total thickness of the coated multilayer increases)
while the sleeve remains mounted in the coating device or even
replace a complete coating collar if required. The home or service
position of the coating collar can be either one of the upper or
lower flange or mounting head.
Post-Processing
[0079] Single pass and multiple pass operation of the coating
device may be concluded with a post-processing of the coated
sleeve, e.g. a post-baking of the applied coating or a tackiness
reducing treatment of the top layer, e.g. with UVC light. These
steps may be provided on-line using for example the vertical
irradiation tunnel described above, having the appropriate
irradiation sources, or off-line using other equipment.
[0080] The ability to apply multiple layers of coated material and
provide pre- and post-processing of the sleeve with a single piece
of equipment significantly reduces handling of unfinished sleeves
and therefore improves quality of the coated sleeves.
Flexibility
[0081] The flanges or mounting heads may require regular cleaning
to remove coating formulation residues from end-to-end coating
processes or from replenishing and draining the coating collar at
the flange or mounting head (i.e. home position). A coating
repelling layer on the flanges or mounting heads may facilitate
this cleaning.
[0082] Only if a different size of sleeve is to be coated,
different flanges or mounting heads may be installed and the
annular seal of the coating collar may be changed or adjusted to
match with the new sleeve diameter. An example of an adjustable
annular seal is an adjustable iris diaphragm including overlapping
sealing leaves wherein the diaphragm opening, i.e. the aperture, is
adjustable through adjustment of the position of the leaves
relative to each other, as known in photography. The higher the
number of leaves in the iris diaphragm, the better the sealing
property of the iris diaphragm around the peripheral surface of the
sleeve. Another embodiment of an adjustable annular seal is an
inflatable tube allowing the internal diameter of the tube to be
adjusted by inflating or deflating the tube.
[0083] Both embodiments also allow a stepwise increase of the
internal diameter of the annular sleeve as the thickness of the
coated layer stepwise increases with additional layers of coated
material in a multiple pass operation, i.e. the internal diameter
of the annular sleeve may be adjusted after each coating step (when
the coating stage is at the lower flange or mounting head) and
before moving the coating assembly upward again in a sliding
contact with the just coated and intermediately cured layer of
coating material.
[0084] Adjustable annular seals may also allow a mode of operation
wherein there is no sliding contact between the coating collar and
the sleeve during the upward movement of the coating assembly. For
this mode of operation to be used, the coating collar is filled
with the coating formulation when the coating stage is positioned
at the upper flange or mounting head and after adjusting the
internal diameter of the annular squeegee to fit the external
diameter of the sleeve. After a first downward coating operation,
when the coating stage is at the lower flange or mounting head, the
coating collar is drained and the internal diameter of the annular
seal is increased so as to create enough play between the annular
seal and the just coated and intermediate cured layer of coating
material during an upward movement of the coating stage. Once the
coating stage is again at the upper flange or mounting head, the
internal diameter of the annular sleeve is decreased to fit with
the increased external diameter of the already partially coated
sleeve and the coating collar is replenished with the same or a
different coating formulation. The previous steps are repeated
until the full coating profile is applied.
On-Demand Production and Sleeve Tagging
[0085] The flexibility offered by a coating device according to the
invention allows for a personalized design of flexographic sleeve
blanks, as far as parameters as sleeve size, coating layer
thickness, functional layer setup, post-treatment etc. concern.
Flexographic sleeve blanks can therefore be produced on-demand. In
on-demand production, it is of major importance that specifications
and/or production parameters of each coated sleeve be traceable.
One way of implementing this requirement is by tagging each sleeve.
Particular embodiments of tags on printing blanks and printing
masters have been disclosed in EP-A-1 679 549 to Du Pont de Nemours
and EP 0 962 824 to Agfa Corporation. The first reference discloses
a photoluminescent tag incorporated in the printing blank, whereas
the latter reference uses a slug line written or engraved on the
printing master outside of the image area.
[0086] If rewritable tags are used, e.g. RFID tags, the tag may be
attached onto the sleeve core before coating. The coating may then
be applied on top of the tag such that the tag is encapsulated in
the coating layer. The tag is then (re)written after coating.
[0087] Alternatively, a 2D/3D bar code or label may be written into
the coated layer through laser marking, provided this marking is
outside the image area.
[0088] The data contained in the tag may include items related to
the printing blank (not engraved coated sleeve) as well as the
printing master (engraved sleeve). Printing blank data may be:
manufacturer identification, client, layer thickness, sleeve
dimensions, shore hardness (if applicable per identifiable layer),
coating solution identification, production date, type of sleeve
core, etc. Printing master data may include: job identification,
time stamp of job completion, engraving engine identification,
operator identification, user-defined graphics as for example a
client's logo. If the tag is rewritable, data may be added as the
production process from sleeve core to printing master
proceeds.
[0089] While preferred embodiments of the present invention have
been described above, it is to be understood that variations and
modifications will be apparent to those skilled in the art without
departing the scope and spirit of the present invention. The scope
of the present invention, therefore, is to be determined solely by
the following claims.
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