Note: Descriptions are shown in the official language in which they were submitted.
CA 02248~99 1998-09-23
SCREEN PRINTING STENCIL PRODUCTION
BACKGROUND TO THE INVENTION
5 Field of the Invention
The present invention relates to the production of
stencils for screen printing.
10 Related Background Art
The production of screen printing stencils is generally
well known to those skilled in the art
One method, referred to as the "direct method" of
15 producing screen printing stencils involves the coating of a
liquid light-sensitive emulsion directly onto a screen mesh.
After drying, the entire screen is exposed to actinic light
through a film positive held in contact with the coated mesh
in a vacuum frame. The black portions of the positive do not
20 allow light to penetrate to the emulsion which remains soft
in those areas. In the areas which are exposed to light, the
emulsion hardens and becomes insoluble, so that, after
washing out with a suitable solvent, the unexposed areas allow
ink to pass through onto a substrate surface during a
25 subsequent printing process.
Another method, referred to as the "direct/indirect
method" involves contacting a film, consisting of a pre-coated
unsensitised emulsion on a base support, with the screen mesh
by placing the screen on top of the flat film. A sensitised
30 emulsion is then forced across the mesh from the opposite
side, thus laminating the film to the screen and at the same
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time sensitising its emulsion. After drying, the base support
is peeled off and the screen is then processed and used in the
same way as in the direct method.
In the "indirect method" a film base is pre-coated with
5 a pre-sensitised emulsion. The film is exposed to actinic
light through a positive held in contact with the coated film.
After chemical hardening of the exposed emulsion, the
unexposed emulsion is washed away. The stencil produced is
then mounted on the screen mesh and used for printing as
10 described above for the direct method.
In the "capillary direct method" a pre-coated and pre-
sensitised film base is adhered to one surface of the mesh by
the capillary action of water applied to the opposite surface
of the mesh. After drying, the film is peeled off and the
15 screen then processed and used as described for the direct
method.
In addition to the above methods, hand-cut stencils can
be used. These are produced by cutting the required stencil
design into an emulsion coating on a film base support. The
20 cut areas are removed from the base before the film is applied
to the mesh. The emulsion is then softened to cause it to
adhere to the mesh. After drying, the base is peeled off.
The screen is then ready for printing. This method is
suitable only for simple work.
One problem generally associated with all the prior art
methods is that many steps are necessary to produce the
screen, thus making screen production time-consuming and
labour-intensive.
Another problem is that normal lighting cannot be used
30 throughout the screen production process in any of the methods
except hand cutting. This is because the stencil materials are
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light-sensitive. In addition, it is necessary to provide a
source of actinic (usually W) light for exposing the stencil.
This usually incurs a penalty of initial cost, space
utilisation and ongoing maintenance costs.
Other methods of preparing printing screens are
available. CA-A-2088400 (Gerber Scientific Products, Inc.)
describes a method and apparatus in which a blocking
composition is ejected directly onto the screen mesh surface
in a pre-programmed manner in accordance with data
10 representative of the desired image. The blocking composition
directly occludes areas of the screen mesh to define the
desired stencil pattern.
EP-A-0492351 (Gerber Scientific Products, Inc.) describes
a method and apparatus where an unexposed light-sensitive
15 emulsion layer is applied to a screen mesh surface and a
graphic is directly ink-jet printed on the emulsion layer by
means of a printing mechanism, using a printing ink, to
provide a mask through which the emulsion is exposed before
the screen is further processed. The apparatus described has
20 been commercialised under the name "Gerber ScreenJet", which
is marketed as a screen imaging system for textile screen
printing. The apparatus comprises a mechanism for mounting
variously sized screen frames in a horizontal position and
orientated with respect to the X, Y and Z printing axes of the
25 apparatus, a control computer providing data defining the
graphic, an ink-jet facility with one or more print heads
which are movable in the X and Y directions and an ultraviolet
lamp assembly for exposure of the light sensitive emulsion
layer on the screen mesh surface. Several other similar ink-
30 jet screen imaging systems are also available, including the"Luscher JetScreen", utilising a hot-melt ink-jet process, and
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a horizontal flat-bed ink-jet machine from Swiss company
Mantel.
There is thus commercially available from several sources
apparatus in which a screen printing screen can be received
5 and onto which a liquid can be applied dropwise in preselected
areas under control of data encoding information determining
the areas of application and non-application. Such apparatus
is referred to herein as being "of the kind described".
Dropwise liquid application apparatus such as ink-jet
10 printers operate by ejecting a liquid onto a receiving
substrate in controlled patterns of closely spaced liquid
droplets. By selectively regulating the pattern of droplets,
ink-jet printers can be used to produce a wide variety of
printed materials, including text, graphics and images on a
15 wide range of substrates. In many ink-jet printing systems,
ink is printed directly onto the surface of the final
receiving substrate. An ink-jet printing system where an image
is printed on an intermediate image transfer surface and
subsequently transferred to the final receiving substrate is
20 disclosed in US-A-4538156 (AT&T Teletype Corp.). Furthermore,
US-A-5380769 (Tektronix Inc.) describes reactive ink
compositions containing at least two reactive components, a
base ink component and a curing component, that are applied
to a receiving substrate separately. The base ink component
25 is preferably applied to the receiving substrate using ink-jet
printing techniques and, upon exposure of the base ink
component to the curing component, a durable, crosslinked ink
is produced.
EP-A-0635362 (Riso Kagaku Corp.) Describes in example 5
30 a process which involves direct selective dissolution by a
water-based solvent, which can be applied dropwise, of a resin
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layer coated on a polyester fibre cloth.
SUMMARY OF THE INVENTION
According to the present invention there is provided a
method of producing a screen-printing stencil having open
areas and blocked areas for respectively passage and blocking
of a printing medium, the method comprising:
providing a screen printing screen having a stencil-
10 forming layer coated thereon;
positioning the coated screen for application thereto
dropwise of a chemical agent capable of reacting with the
stencil-forming layer to produce areas thereof having lower
and higher solubilities corresponding respectively to the
15 blocked and the open stencil areas;
applying dropwise to the coated screen the chemical agent
under control of data encoding the respective lower and higher
solubility areas; and
washing away the stencil-forming layer in the areas of
20 higher solubility, thereby to produce the screen-printing
stencil.
In the method of the invention, the stencil is formed by
chemical means without the need to use either special lighting
conditions or actinic radiation.
Also, it is possible to carry out the method at reduced
expenditure of time and labour, compared with the known
processes.
The stencil-forming layer can be applied to the screen
by any convenient means. One way is by means of a coated film
30 comprising a support base on which the stencil-forming agent
is coated. The coated film can be adhered to the screen by
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application of a liquid, for example water. This can be done
by placing the coated film with its coated surface uppermost
on a solid flat surface and placing the screen mesh on top
such that there is close contact between the mesh and the
5 coated film. In another variant, the coated film can be rolled
down a screen mesh which has been thoroughly wetted with a
gentle spray of a liquid, for example water. After drying,
the support base can be peeled away from the mesh to leave the
stencil-forming layer. Alternatively, the stencil-forming
10 layer may be coated directly onto the screen mesh, for example
in the form of an emulsion applied by use of a coating trough
or squeegee, preferably to both sides of the screen
simultaneously.
The dropwise application of the chemical agent is
15 conveniently carried out by means of an apparatus of the kind
described, in which the coated screen is first positioned.
If desired, the chemical agent may be produced in situ
by reaction between two or more precursor materials,
separately applied to the stencil-forming layer, at least one
20 of which is applied in the said areas corresponding to the
blocked areas of the stencil to be produced. This may
conveniently be achieved by use of a plurality of drop-
ejection heads, in a suitable apparatus of the kind described.
The dropwise application is controlled according to data
25 encoding the desired pattern of blocked and open areas of the
stencil to be produced. This control may be by a computer,
for example a personal computer. Thus, data representative
of the desired output pattern is input to a controller as pre-
recorded digital signals which are used by the ejection head
30 to deposit or not deposit the liquid containing the chemical
agent as it scans the surface of the stencil forming layer.
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The material of the stencil-forming layer is selected to
react with the chemical agent to produce lower solubility
areas corresponding to the said blocked areas.
5 DETAILED DESCRIPTION OF THE INVENTION
The invention will be described further by way of example
with reference to the drawings of this specification, in which
Figures 1 to 4 show schematically the successive steps
10 in the production of a printing screen in accordance with one
method according to the invention, and
Figures 5 to 7 show schematically the successive steps
in the production of a screen in accordance with a second
method according to the invention.
Referring to Figures 1 to 4, these show the formation of
a screen printing stencil shown in Figure 4, starting with a
coated film shown in Figure 1.
Figure 1 shows the coated film which consists of a
stencil-forming layer 1 coated on a flexible support base 2,
20 the film having been applied to a screen mesh 3, mounted in
a suitable screen frame 4.
Figure 2 of the drawings shows the coated screen after the
support base 2 has been peeled away.
Figure 3 shows a chemical agent 5 being applied to the
25 stencil-forming layer 1 in droplets 5 which are ejected from
an ejection head (not shown) of an apparatus of the kind
described controlled by a computer. The chemical agent 5 is
absorbed into the stencil-forming layer 1 to form areas 6
corresponding to the areas that have reacted with the chemical
30 agent to produce areas of insoluble material.
Figure 4 of the drawings shows the final screen after the
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screen has been washed out so that the areas 6 of the stencil-
forming layer to which the chemical agent was applied remains
and the higher solubility areas have been washed away.
Figures 5 to 7 of the drawings correspond to figures 1
5 to 4 but show the production of a stencil by a method in which
the stencil-forming layer is coated directly onto the screen
without the use of a coated film. Here the stencil-forming
layer is formed from a stencil-forming agent composed of a
liquid unsensitised emulsion which is directly coated onto
10 both sides of a screen mesh mounted in a frame.
Reference numerals increased by "10" are used in figures
5 to 7 to identify integers corresponding to integers of
figures 1 to 4.
Figures 5 and 6 show operations corresponding to the
15 operations of figures 1 and 3 and figure 7 corresponds to
figure 4.
The key criterium in selecting a suitable combination of
stencil-forming agent and chemical agent is that the chemical
agent should form a good image on the layer formed from the
20 stencil-forming agent; for example, a drop of the chemical
agent should neither be so repelled by the layer as to produce
a defective image nor it should not spread so far as to reduce
the resolution of the image. Moreover, it should not spread
so anisotropically (because of irregularities in the layer)
25 as to deform the image.
The stencil-forming agent may comprise a polymer which
reacts with the chemical agent and thus forms a part of the
final screen stencil.
A typical example of such a polymer is polyvinyl alcohol
30 which is preferably present in an amount of 5 to 100 wt.~ of
the stencil-forming layer with the balance comprising, for
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example, other suitable polymers and/or suitable fillers,
binders and plasticisers. The polyvinyl alcohol preferably
has a degree of hydrolysis of 20 to 99.9 mole ~ and,
independently thereof, a degree of polymerisation of 100 to
5 3500.
Numerous other reactive polymers could alternatively be
utilised in the present invention. Suitable polymers include
those that change their solubility characteristics on
treatment with a suitable chemical agent. Examples of such
10 polymers are:
gelatin and its derivatives;
cellulose derivatives that are water soluble, including starch
and hydroxypropyl cellulose;
epoxy resins; and
15 amino resins, including urea-formaldehyde and melamine-
formaldehyde.
In methods according to the invention, the polymers and
other components are chosen so that the chemical agent forms
a good image when applied. Layers that are not compatible with
20 any solvent used in the chemical agent (typically, water) will
produce insufficient spread of the liquid and a poor-quality
image will result. If the layer has too great an affinity with
the chemical agent, the liquid will spread too far, giving a
blurred, low resolution image.
When the stencil-forming layer is applied by use of a
coated film, the stencil layer thickness is typically from 5
to 100 ~m.
The support base may comprise a non-reactive polymer,
preferably an organic resin support, e.g. polyethylene
30 terephthalate, polyethylene, polycarbonate, polyvinyl chloride
or polystyrene. The organic resin support can optionally be
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coated with a subbing layer to give desired adhesion
properties with the stencil-forming layer. The support base
is removed as a coherent film in the screen production method,
preferably prior to the application of the chemical agent to
5 the stencil-forming layer.
The chemical agent is preferably applied dropwise to the
stencil-forming layer by an apparatus of the kind described,
for example a Gerber ScreenJet apparatus. The liquid applied
should exhibit desirable stability, surface tension and
10 viscosity characteristics and may therefore contain
surfactants, viscosity modifiers, light stabilisers and/or
anti-oxidants. When the active component(s) of the chemical
agent is/are not liquids, the chemical agent may include a
suitable carrier, for example a suitable solvent or dispersant
15 for the active component(s).
Examples of suitable active components include
boron salts e.g. boric acid, Group I and Group II metal
borates;
aldehydes, e.g. formaldehyde;
20 dialdehydes, e.g. glyoxal and glutaraldehyde, optionally
activated by treatment with mineral acid;
isocyanates and their derivatives, e.g. toluenediisocyanate;
carbodiimides and their derivatives, e.g. 1,3-
dicyclohexylcarbodiimide;
25 transition metal compounds and complexes, e.g.
pentahydroxy(tetradecanoate)dichromium and its derivatives;
aziridine and its derivatives;
amlne s;
multifunctional silane compounds, e.g. silicon tetraacetate;
30 N-methylol compounds, e.g. dimethylolurea and
methyloldimethylhydantoin; and
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active vinyl compounds, e.g. 1,3,5-triacryloyl-hexahydro-s-
triazine.
After application of the chemical agent to the stencil-
5 forming layer, the imaged screen is dried if necessary, then
the resulting screen stencil can be developed by washing away
the portion of higher solubility with a suitable solvent, for
example water, thereby leaving behind areas of reduced
solubility to occlude areas of the mesh.
Optionally, the stencil can be further toughened by a
post-treatment, for example using extra chemicals, actinic
radiation or heat. The extra chemicals (or precursors
thereof) may be resident in the original stencil-forming
agent, or may be supplied externally. Examples of chemical
15 toughening agents are ones operating at pH 7 or higher and
include dialdehydes particularly glyoxal, and aqueous bases,
for example aqueous potassium carbonate. It is presently
believed that these toughening agents will only work when a
boron salt is used as the chemical agent.
The screen produced is then ready for use as a printing
medium using techniques familiar to those skilled in the art.
Where the chemicals used are those cited in the Examples 1 to
4 which follow, the broad physical properties, chemical
resistances, washout solvent (water) and reclaim chemicals
25 (typically periodate systems) will in many cases be those used
routinely by screen printers. So, although the method of
producing the stencil is new, the resulting product will often
be familiar and highly acceptable to screen printers.
Surprisingly, we have found that when the active
30 component of the chemical agent is a boron-containing salt,
the stencil can be reclaimed with dilute acid without the use
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of the industry-standard periodate system. This low cost and
environmentally-friendlier reclaim system is a distinct added
advantage.
The advantages of the method of the present invention
5 include: a screen stencil can be produced directly from
digital information sources; it is not necessary to use safe-
lights during the stencil making process; there is no
requirement for an exposure step utilising an actinic
radiation source; and a finished stencil can be produced in
10 a shorter time than by conventional screen printing
techniques.
The present invention is illustrated by the following
examples without however being limited thereto. In these
examples, various commercially-available materials are listed
15 by their trade names; the following letters identifying the
following companies:
(a) 3M, UK
(b) Autotype International, UK
20 EXAMPTE 1
A liquid containing a chemical agent was prepared
according to the formula:
water - 87 wt.~;
potassium tetraborate - 10 wt.~; and
25 "Fluorad FC-93" (a) (lwt. ~ aqueous solution) - anionic
fluorinated surfactant - 1 wt.~.
The stencil-forming layer comprised a typical (but
unsensitized) commercially available capillary screen film -
"Autoline HD" (b) - consisting of a pigmented polyvinyl
30 alcohol/polyvinyl acetate emulsion on a support base, with an
approximate emulsion (stencil-forming agent) thickness of 15
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~m.
A degreased screen mesh of mesh count 120 threads per cm
was mounted in a suitable frame, then the mesh was wetted
thoroughly with a gentle spray of water and the receptor
5 element was rolled down the mesh using light pressure. Slight
pressure was applied to the print side of the receptor element
by squeegee, then excess water was removed from the opposite
side of the screen, followed by drying the screen using a fan
with a maximum temperature of 40 ~C. The support base was
10 removed from the mesh.
The resulting screen was positioned in a Gerber ScreenJet
apparatus to the manufacturer's instructions, then the
prepared chemical agent was applied to the stencil-forming
layer of the screen from a cartridge in a preprogrammed manner
15 to form the desired image. The imaged screen was removed from
the "ScreenJet" apparatus and dried by warm air fan (maximum
40 ~C) to produce on the screen a stencil-forming layer having
areas of lower and higher solubility. Any remaining unreacted
part of the receptor element was removed and the screen was
20 washed out using cold running water, until the portion of the
assembly of higher solubility was washed away to waste.
The stencil was then placed in a standard screen printing
machine and prints of an acceptable quality were obtained
using standard solvent-based screen printing inks.
EXAMPLE 2
A liquid containing a chemical agent was prepared
according to the formula:
water - 87 wt.~;
30 potassium tetraborate - 10 wt.~; and
"Fluorad FC-93" (a) (lwt. ~ aqueous solution) - anionic
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fluorinated surfactant - 1 wt.~.
The stencil-forming agent comprised a typical (but
unsensitized) polyvinyl alcohol/polyvinyl acetate screen
emulsion - "2000" (b). This was directly coated onto a
5 suitably degreased screen mesh of 120 threads per cm., mounted
in a suitable frame. Using a coating trough, two coats of this
emulsion were applied, wet on wet, to the print side of the
screen, followed by two coats, wet on wet, to the other side
of the screen. The resulting screen was then dried
10 horizontally, print side down using a warm air fan at a
maximum temperature of 30 ~C.
The resulting screen was positioned in the "ScreenJet"
apparatus according to the manufacturer's instructions, then
the prepared chemical agent was applied to the stencil-forming
15 layer of the screen from a cartridge in a preprogrammed manner
to form the desired image. The imaged screen was removed from
the "ScreenJet" apparatus and dried by warm air fan (maximum
40 ~C) to produce on the screen a stencil-forming layer having
areas of lower and higher solubility. Any remaining unreacted
20 part of the receptor element was removed and the screen was
washed out using cold running water, until the portion of the
assembly of higher solubility was washed away to waste.
The stencil was then placed in a standard screen printing
machine and prints of an acceptable quality were obtained
25 using standard solvent-based screen printing inks.
EXAMPLE 3
The procedure of Example 1 above was repeated exactly to
produce a screen stencil.
This stencil was then treated with a 10 wt.~ aqueous
solution of potassium carbonate, which was applied by brush
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so as to cover the entire stencil area, then finally allowed
to dry. This produced a toughened stencil.
The stencil was then placed in a standard screen printing
machine and prints of an acceptable quality were obtained
5 using standard solvent-based screen printing inks.
EXAMPLE 4
The procedure of Example 1 above was repeated exactly to
produce a screen stencil.
This stencil was then treated with a 2 wt.~ solution of
35 wt.~ hydrochloric acid, which was applied by brush so as
to cover the entire stencil area. This treatment disrupted the
screen stencil and allowed the resulting residue to be washed
away to waste using a cold water spray, giving a reclaimed
15 screen with no observable stain present.
