Note: Descriptions are shown in the official language in which they were submitted.
The present invention relates to flexographic printing plates,
methods of their production and laminates for use in their production.
There exist two common methods for the preparation of photopolymer
flexographic printing plates. In the first a manufactured plate comprising
a solid layer of photopolymerisable elastomer on a backing is image-wise
exposed to crosslink desired portions of the solid layer. Uncrosslinked
portions of the layer are removed by washing with a solvent which dissolves
the uncrosslinked material but not the crosslinked material. This leaves a
plate having areas in relief. The plate is treated in conventional manner
to provide a smooth surface on the areas in relief.
Such manufactured plates are sold having a thin layer of non-tacky
material on the face of the solid layer away from the backing. This in turn
may be covered by a protective sheet to facilitate storage. The backing may
comprise more than one layer, and in many cases comprises a relatively thin
support layer in contact with the solid layer and a softer flexible layer on
the face of the support layer remote from the solid layer. Manufactured
plates of this type are described, for instance, in British Patent Specifica-
tion Nos. 1,489,193 and 1,525,965. Such manufactured plates are prepared for
use merely by removing the protective layer.
As such plates are manufactured in a factory, their production can
be closely controlled. The plates are therefore made to high tolerances,
with each layer being of uniform thickness. Thus it can be ensured that
the exposed and developed plates have uniform thickness over all of its areas
in relief. However, manufactured plates are expensive as their cost
includes the cost oE the many layers of materials and the cost of production.
Moreover, the material comprising the solid layer of photopolymerisable matter
tends to be expensive and it must contain various components to prevent it
from crosslinking during storage.
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In the second method of producing photopolymer flexographic printing
plates, a liquid photopolymerisable material is used to produce the exposed
plate in _itu. In a typical process, a negative is placed on a glass platen
and covered with a thin layer of material. A quantity of a liquid photo-
polymerisable elastomeric material is spread evenly over the thin layer and
is covered by a backing. Generally, the backing and the liquid material are
applied simultaneously. The laminate thus formed is image-wise exposed to
crosslink and therefore solidify desired areas of the liquid material and
to bond it to the backing. Uncrosslinked liquid material is washed off with
a suitable solvent and the laminate is treated in conventional manner to
produce the final flexographic printing plate.
It is possible to carry out the above process because the liquid
material is formulated to have a high viscosity. It therefore remains in
a discrete layer for a time long enough to allow image-wise exposure and
crosslinking.
Unless plant conditions and machine maintenance are carefully con-
trolled, the flexographic printing plates produced by this second method are
not always of good quality. The thickness of the layer of photopolymerised
material is not uniform which leads to wleven inking of the plate and
therefore badly printed products.
The variable thickness may be caused by one or a combination of the
following factors. The platen on which the negative is placed may be uneven,
either because of bad maintenance or because it is unevenly heated by the
lights used to expose the plate. This causes it to warp. The equipment
used to apply the backing and layer of liquid material to the thin layer
may not be sophisticated enough to provide a layer of uniform thickness.
Even if it is, if it is badly maintained, it may lose its ability to
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produce a uniform layer. The viscosity oE the liquid material may vary,
either because of variations in atmospheric conditions or because of the
uneven heating of the platen.
It has therefore proved difficult -to produce high quality flexo-
graphic printing plates by this method.
According to the present invention there is provided a flexographic
printing plate comprising,
b~se
a~layer of a grindable elastomer, having a thickness of up to 1000
microns, firmly bonded to layer of photopolymerised elastomeric material,
which has been produced by exposure of a photopolymerisable liquid to
actinic radiation.
Optionally a support layer, having a thickness typically from 75
to 150 microns, is interposed between and firmly bonded to the layers of
grindable elastomer and photopolymerised elastomeric material.
The grindable elastomer may be any of the commonly available
rubbers, such as polybutadiene, butadiene-acrylonitrile, butadiene-styrene,
isoprene-styrene, silicone, or polysulphide rubber. Preferably, the
elastomer is a natural rubber, a polychloroprene rubber, or a polyurethane
rubber. The elastomer may contain conventional fillers in order to enable
it to be ground more easily. The elastomer should have a Shore A hardness
of at least 30 but not more than 80. Preferably its hardness is between
40 and 60 Shore A.
The grindable elastomer may contain a reinforcing and stabilising
matrix or web, such as textile or non-woven fabric, and this will be
especially desirable when the support layer is absent.
If the liquid material is not rapidly polymerisable, the elastomer
should be transparent to enable the liquid material to be exposed from
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both sides. However, if the liquid material polymerises rapidly on
exposure, the elastomer may be opaque.
Preferably, the elastomer layer has a thickness from 100 to 500
microns, and most preferably is about 400 microns thick.
When present, the support layer may be any one of those commonly used
in the art. It may comprise for instance a sheet of aluminium or other
metal foil, a sheet of a plastics material or a layer of a crosslinked
surface coating. The support layer may be reinforced, for instance by
glass or textile fibres.
Preferably, the support layer comprises a film of polyester plastics
material having a thickness of about 100 microns.
The support layer should provide a sufficiently hard surface
without impairing the flexibility of the finished plate. The thickness
of the support layer should be determined with these criteria in mind, and
for certain materials may fall outside the typical values indicated above.
Preferably, the liquid material from which the layer of photopoly-
merised elastomeric material is produced has a viscosity of about 500 to
500,000, more preferably 5,000 to 50,00Q,cP at 23C.
A suitable material for use in producing this layer is a methacry-
late capped polyurethane polyether. This may have an average molecular
weight of about 5000 to 50,000 typically about 25,000.
Other suitable liquids which are photopolymerisable to form solid
elastomers and which have appropriate viscosities are known in the art.
Generally, the liquid material will contain a photo-initiator, such
as benzoin or ethers formed from benzoin and methanol or isopropanol,
or a photosensitiser such as benzophenone. The liquid may also contain
chain transfer agents and molecular weight controlling agents. In the
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instance of the methacryla-te capped polymer mentioned above, a proportion
of acrylate or methacrylate monomer may be included.
The liquid material may include other conventional additives, such
as thermal polymerisation inhibitors, ~for instance p-methoxyphenol,
hydroquinone or salts of N-nitrosocyclohexyl-hydroxylamine), antioxidants,
and plasticisers.
It should be appreciated that the liquid material, once crosslinked
by image-wise exposure to actinic radiation will be a solid having relief
surfaces and having uncrosslinked material surrounding these surfaces.
The uncrosslinked material will be removed in conventional manner to leave
the relief surfaces above the rest of the plate. References to the thick-
ness of the elastomeric material layer are to be construed as references
to the thickness of the relief areas, unless otherwise indicated.
In some cases it may be necessary ~o provide an anchor layer
between the grindable elastomer and the photopolymerised material. Where
a support layer is present, an anchor layer may be provided between it
and either or both of the other layers surrounding it~ Such anchor layers
are provided to promote bonding between adjacent layers.
The anchor layer may comprsie a coating on the support layer and/or
on the grindable elastomer. Alternati~ely, it may comprise a layer of
adhesive applied to the support layer and/or the grindable elastomer.
Clearly the nature of or necessity for an anchor layer will depend on
the natures of the layers to be bonded. However, a person skilled in
the art will find no ~'ifficulty in selecting suitable adhesive or coating
for any particular application.
Preferably where a support layer is present, the anchor layer
comprises a coating on both sides of the support layer. Where the support
layer is a polyester, the anchor layer may be a coating of a mixture o-f
polyvinylchloride and polyvinylacetate.
The flexographic printing plate of the present invention may be
made by adhering a layer of grindable elastomer to a plate made in con-
ventional manner using a liquid photopolymerisable material. However, it
is preferred that the plate is made in a conventional apparatus using a
laminate comprising the grindable elastomer and the support layer as the
backing.
Therefore according to a second aspect of the present invention,
there is provided a laminate for use in producing a flexographic printing
plate, comprising a layer of a grindable elastomer having a thickness up
to 1000 microns firmly bonded to a support layer having a thickness
typically from 75 to 150 microns.
The flexographic printing plate will have been subjected to conven-
tional processing after exposure to provide a flat surface on the relief
areas of the plate. However, such a plate will still suffer from the dis-
advantage of having non-uniform thickness in the layer of elastomeric
material. This disadvantage can be overcome by grinding off parts of the
layer of grindahle elastomer, the ground plate having a uniform thickness
over the relief areas.
Therefore according to another aspect of the present invention, a
process for producing a flexographic printing plate having uniform thickness
comprises grinding a flexographic printing plate according to the first
aspect of the invention to uniform thickness by removal of por~ions of the
grindable elastomer.
The present invention also includes processes for preparing the
intact flexographic printing plate, either by use of conventional
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processing using a laminate as previously described or by adhering a layer
of grhldable elastomer to a conventionally produced plate. The processes
may also include the grinding step as set out above.
The conventional processing may include contact or off-contact
exposure. Both types of processing are commonly used in the printing
industry for the production of relief printing plates. Off-contact exposure
is often used in newspaper printing.
The grinding may be carried out on conventional grinding machines,
and preferably should give a tolerance of not exceeding ~ 40 microns.
By use of the flexographic printing plate of the present invention,
it is possible to use the liquid method to produce a printing plate of
uniform thickness, which will enable the production of high quality printed
products.
The invention will now be described, by way of example only, with
reference to the accompanying drawing which shows a diagrammatic cross-
sectional side view of a flexographic printing plate according to the
invention.
Referring now to the drawing, the flexographic printing plate
comprises a layer 1 of a filled natural rubber having a Shore A hardness
of 50. This is firmly bonded by a layer of rubber adhesive 2 to a layer
3 comprising a polyester sheet which has been surface coated 4 to increase
its adhesion to a layer 5. The surface-coated polyester sheet 3 is sold
under the trade name~Bexford LP 40 ~Bexford Ltd is a subsidiary of ICI~.
5~
As can be seen from the Figure, layer 5 has areas 6 in relief
surrounded by areas 7 of lesser thickness. The layer 5 comprises a photo-
polymerised elastomeric material produced by photopolymerising a meth-
acrylate capped polyurethane polyether. In the present case a commercially
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available liquid material sold by W. R. Grace Ltd. of Park Royal, London,
under the designation Flexopolymer Type 40 was used. The areas 6 in relief
were those which were photopolymerised during image-wise exposure, whereas
the other areas 7 are those from which uncrosslinked material has been
washed away. The portion of the layer 5 nearest to the support layer 3 was
photopolymerised by back exposure to actinic radiation so that the areas 7
have some thickness.
Layer 1 is 400 microns thick, layer 3 is 100 microns thick and layer
5 is about 6 mm thick, although its thickness is not uniform.
The flexographic printing plate was produced using a conventional
machine, such as a Model ALF machine manufactured by Asahi Chemical Industry
Co. Ltd. in the following manner.
A negative was placed on the glass platen of the machine and covered
with a thin layer of a non-tacky, tear resistant transparent polypropylene
film. A quantity of Flexopolymer Type ~0 was applied to the polypropylene
film and a~ the same time a sheet of Bexford LP ~0 coated polyester film
was applied to the top of the Flexopolymer Type 40. The laminate was then
exposed on both sides to actinic radiation which caused at least some of
~ the Flexopolymer Type ~0 to crosslink and therefore solidify. It also
caused the solidified material to bond to the polyester film.
The exposed plate was then treated in conventional fashion to remove
uncrosslinked material and to prepare the surface of the relief areas for
printing. After conventional processing, the plate was laminated to a
sheet of filled natural rubber using a conventional rubber adhesive.
In another experiment, the same procedure was carried out except that
the filled natural rubber was laminated to the polyester support before it
was used in the AI,Fmodel machine.
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Flexographic printing plates as shown in the Figure and made by
either of the methods described above were ground to uniform thickness in
a Vandercook rubber grinder. These ground plates were used in a flexo-
graphic printing process, and produced printed products with even ink
distribution and of good quality. In contrast, if the -flexographic
printing plates were used without grinding, the ink was spread unevenly
over the relief surfaces, resulting in uneven printing and therefore
products of lesser quality.
It can thus be seen that the present invention enables the use of
liquid based flexographic printing plates which can be used to produce
high quality printed products. Hitherto this has only been generally
possible using expensive solid based manufactured plates.
