Photographically produced stencils

There is provided a method for producing a stencil for use in screen printing which method comprises coating a screen mesh with liquid photopolymerizable composition, imagewise exposing the coated mesh to radiation to polymerize liquid composition on the mesh in the exposed areas, and removing unexposed liquid composition from the mesh to develop the stencil. Suitably the liquid composition is applied with the screen mesh in contact with a protective film and, after imagewise exposure, the stencil is developed by stripping of the film such that unexposed liquid remains on the film. This provides a simple method of producing a stencil which does not require highly skilled personnel.

BACKGROUND OF THE INVENTION 
This invention relates to photographically produced stencils for use in 
screen printing (herein, for the sake of brevity, referred to as 
photostencils). 
In screen printing the image to be printed is defined by masking off whole 
or part apertures of a screen printing mesh with stencil material. Thus, 
during printing, this stencil material prevents the passage of screen 
printing ink through the masked areas. 
Although such stencils may be produced manually it is normal practice to 
produce them photographically. The production of such a photostencil 
requires the preparation of a dry water-soluble photosensitive polymer 
layer which, on imagewise exposure to suitable illumination, is made 
water-insoluble in the exposed areas such that the layer material may be 
washed away with cold or warm water in the unexposed areas to produce a 
photostencil. Such photosensitive layers used in screen printing have in 
practice been generally based on polyvinyl alcohol, polyvinyl 
alcohol--polyvinyl acetate mixtures, or gelatin, sensitised by a 
water-soluble dichromate salt, a diazonium salt or a ferric salt. 
Two different methods have been used for applying the photostencil to the 
screen mesh; these are termed the direct and indirect methods. In the 
direct method, an aqueous solution of polyvinyl alcohol, usually mixed 
with polyvinyl acetate aqueous emulsion and containing the sensitiser, to 
give about 10-30% by volume of nonvolatile material, is coated as a 
viscous liquid onto the tensioned screen mesh and dried by evaporation. 
Since substantial contraction of the layer thickness occurs on drying, 
several coats are generally applied, each with intermediate drying, to 
build up sufficient dry layer thickness to provide a photostencil with 
adequate mechanical strength. The dry sensitized mesh is then exposed by 
contact to a high contrast photographic positive in a vacuum frame to 
obtain close contact with the positive. The exposing radiation may be 
mercury metal halide, mercury vapour, pulsed xenon or carbon arc 
radiation, all of which have high intensity in the range 350-450 nm to 
which the sensitised polymer is most sensitive. The image is developed by 
carefully washing out the unexposed polymer with cold or warm water, and 
the remaining polymer is then dried again to form the photostencil. 
In the indirect method, an aqueous solution of sensitised polymer, usually 
at 10-25% concentration by volume, is coated as a thick layer onto a 
colourless transparent film base, such as polyester film, which may be 
coated to facilitate subsequent dry stripping. After a long drying stage 
in a complex drying tunnel, the dry polymer layer is exposed to a high 
contrast photographic positive in a vacuum frame using the same 
illumination as in the direct method. The image is developed usually by 
washing out the soluble, unexposed polymer with cold or warm water and, 
while wet, the remaining polymer, still on the film base, is applied under 
light pressure to the screen mesh and dried in situ. Finally the film base 
is stripped off to leave the printing stencil on the mesh. 
The direct and indirect methods for producing photostencils both therefore 
require at least two lengthy drying stages under carefully controlled 
conditions to avoid drying defects and dust contamination. Also the final 
stencil is not resistant to water-based screen inks nor to many inks 
containing organic solvents with hydroxyl groups. Water-based screen inks 
are widely used in textile printing on both flat and cylindrical screens 
and then the stencil must be hardened chemically or protected by baking on 
a protective lacquer. 
Moreover, in the direct method, the evaporation of water during the first 
drying stage may cause shrinkage and loss of image quality; particularly 
the formation of saw-tooth edges due to defective bridging of mesh 
apertures. Multiple coats with intermediate drying are generally used to 
reduce this effect, but limited print detail resolution is still obtained 
in the direct method. The indirect method also suffers from some print 
quality loss due to shrinkage but the main problem is one of poor adhesion 
since the stencil is applied to the mesh in a swollen but not liquid 
condition and thus does not encapsulate the mesh. Both polyvinyl alcohol 
and gelatin have poor specific adhesion to polyester monofilament mesh 
which is the most widely used mesh in screen printing. 
DETAILED DESCRIPTION OF THE INVENTION 
According to the present invention, there is provided a method for 
producing a stencil for use in screen printing which method comprises 
coating a screen mesh with liquid photopolymerisable composition, 
imagewise exposing the coated mesh to radiation to polymerise liquid 
composition on the mesh in the exposed areas, and removing unexposed 
liquid composition from the mesh to develop the stencil. 
With the method of the invention, photostencils of high quality can be 
prepared quickly with simple equipment requiring minimal skill, and 
without producing liquid effluent or requiring possible hazardous 
chemicals, such as dichromate and diazonium salts to be used. Another 
important advantage of the present method is that there is no need to 
carry out a drying stage in the preparation of the photo-sensitive layer 
since the preferred photopolymerisable liquids are substantially 
non-volatile. Adequate layer thickness can therefore be obtained in a 
single coating operation. This contrasts with prior art methods in which a 
screen mesh coated with e.g. a sensitised polyvinyl alcohol solution 
cannot be used to produce a photostencil by exposure before drying (i.e. 
while in the wet, liquid state) because a water-insoluble exposed layer 
which adheres to the mesh is not obtained. Exposure and water-development 
is only possible after drying the coated mesh. 
There are a number of methods by which the stencil may be developed 
according to the invention, i.e. the unexposed liquid composition removed, 
including dissolution by applying a solvent to wash away the unexposed 
liquid, or dispersion by spraying with water and a surface active agent to 
disperse the unexposed liquid. Further the liquid composition may adhere 
by surface tension to the screen mesh without the need for support. 
However, according to a preferred embodiment of the invention, one side of 
the screen mesh (that is the printing side) is in contact with a radiation 
transmitting sheet material, liquid photopolymerisable composition is 
applied to the other side of the mesh, imagewise exposure is made through 
the sheet material and the stencil is developed by separating the sheet 
material and mesh so that unexposed liquid composition remains on the 
sheet material. The radiation transmitting sheet, termed a protective 
film, may be a plastics film for example, a clear polyester or 
polypropylene film, or glass. 
The coated screen mesh may be exposed to projected radiation or by contact 
between a photographic positive and the protective film in a printing 
frame. In the latter case, the protective film prevents contact of the 
positive with the liquid. After exposure, the protective film is simply 
stripped away to develop the stencil. Most of the unexposed liquid will 
adhere to the film and be removed with it. The coated screen mesh may be 
exposed by direct contact with the photographic positive, which is then 
stripped off to develop the stencil if the positive is of suitable 
material. 
After stripping, the mesh apertures in the printing (unexposed) areas are 
substantially open and the screen may therefore be used for immediate 
printing. A thin film of liquid composition is usually left on the surface 
of the mesh filaments, as in normal screen printing when the screen is 
opened, but this thin film will dissolve or disperse into the screen ink 
during the first few prints. Alternatively, the thin liquid film on the 
mesh filaments can be removed by applying a solvent or dispersant for the 
liquid prior to printing. Another alternative is to re-expose the mesh to 
uniform radiation to polymerise the liquid and therefore fix the residual 
liquid onto the mesh filaments. This fixing technique is of value when it 
is required slightly to reduce the mesh apertures and so reduce the 
quantity of ink applied by the screen. The fixing technique also increases 
the strength of the photostencil by reinforcing the edges of the 
photostencil image. 
A further important advantage of using a protective film and development by 
stripping is that a proof of the stencil is produced on the film. The 
photopolymerisable liquid may be coloured with pigments or soluble dyes to 
aid inspection of the mesh coating operation and inspection of the derived 
photostencil. Consequently after stripping, the liquid print on the film 
is similarly coloured and clearly visible. This liquid print may be 
polymerised by exposure to the uniform radiation to give a true proof of 
the photostencil which is a facsimile of the print that will be obtained 
by printing the stencil. 
By using appropriately coloured, namely yellow, magenta, cyan and black, 
photopolymerisable liquids, a set of colour proofs may be obtained from 
four such photostencils as used for four colour half-tone printing, which 
proofs can be superimposed on each other to show colour progressives and 
the final full colour result. These are of great value to the printer to 
control colour printing densities. 
When exposing by contact with a positive in a vacuum frame a second similar 
protective film can be placed on the upper mesh surface to prevent contact 
of the rubber blanket of the frame with the photopolymerisable liquid in 
the mesh apertures. Both protective films may be removed by stripping for 
development. Alternatively, or additionally, a fine powder such as silica 
aerogel or talc can be applied to the upper mesh surface to avoid 
contamination of the rubber blanket. 
The present method may be used in conjunction with all types of screen mesh 
including mono- and multi-filament polyester, polyamide, stainless steel, 
phosphor bronze, nickel, silk and organdle (cotton) meshes. The mesh may 
be a woven, knitted or non-woven mesh, or may be of a perforated metal or 
plastics material, or may be a mesh electroformed or formed by etching. 
Photostencils on both flat screens and cylindrical screens can be produced 
by the present method and the conventional screen printing methods of the 
prior art may also be used. 
The photopolymerisable liquid compositions are polymerised on the mesh to 
the solid state by exposure to suitable radiation e.g. ultraviolet or 
visible light or a mixture thereof. Generally, sensitivity to visible 
light and near ultraviolet is preferred since this light is readily 
generated by mercury vapour lamps, mercury metal halide, pulsed xenon and 
carbon arcs, and is relatively safe to use. 
By suitable selection of components of the liquid photopolymerisable 
composition and its layer thickness, high adhesion to the screen mesh to 
give photostencils suitable for long run printing are readily achieved. 
Photostencil thickness can be readily controlled over wide limits by 
controlling the thickness of the liquid photopolymerisable layer applied 
to the mesh. When the mesh is encapsulated by photopolymerised material a 
very high strength photostencil is obtained assisted by mechanical 
adhesion. Since there need be no drying of the liquid photopolymerisable 
layer according to the present method, there is negligible contraction of 
the original layer. The liquid layer can be coated flush with both mesh 
surfaces, lower than the mesh surfaces or coated proud of one or both mesh 
surfaces depending on the coating technique used. High adhesion of the 
photostencil to the mesh can be obtained by the use of particular 
photopolymerisable compositions particularly acrylated urethanes and 
compositions giving a flexible photostencil of high tensile strength by 
control of cross-link density. 
Photopolymerisable liquid compositions used according to the present 
invention preferably comprise ethylenically unsaturated monomers or 
prepolymers or mixtures thereof. An ethylenically unsaturated prepolymer 
contains a polymeric component in the molecule. It is very desirable to 
use fast photopolymerising materials to reduce the exposure time to the 
radiation. 
Fast photopolymerisation may be obtained by photoinitiated vinyl addition 
polymerisation of monomers and prepolymers containing terminal or pendant 
acryloyl or methacryloyl groups: CH.sub.2 =CR--CO-- where R is H or 
CH.sub.3 -- respectively. The acryloyl group is faster polymerising than 
the methacryloyl group and reference below to acryloyl groups includes 
methacryloyl groups. 
To obtain good coating properties, the liquid photopolymerisable 
composition must possess a suitable viscosity which is generally in the 
range of 2 to 50, more usually 10 to 30, poises and this can be readily 
controlled by the molecular weight and composition of the 
photopolymerisable materials. Conveniently a prepolymer of high viscosity 
is used in admixture with a monomer of low viscosity to give precise 
control of viscosity. 
Low viscosity and liquid photopolymerisable materials are monomers, that is 
materials which do not contain polymeric groups in the molecule and 
suitable materials are esters of acrylic or methacrylic acid and a mono- 
or poly-hydric alcohol, particularly acrylate esters of mono, di, tri and 
tetra-hydric alcohols. Monomers are preferred which have very low 
volatility and low skin and eye irritancy and these properties are 
generally obtained with monomers of higher molecular weight. Acrylate 
esters of the following alcohols are suitable and are given by way of 
example: 
Monohydric alcohols: 2 phenoxyethanol, 2-phenoxyethoxyethanol and 
hydrogenated derivatives thereof. 
Dihydric alcohols: tripropylene glycol, bisphenol A, hydrogenated bisphenol 
A and hydroxyethyl ethers and hydroxypolyethoxy and propoxy ethers of 
bisphenol A and hydrogenated bisphenol A. 
Trihydric alcohols: trimethylolpropane 
Tetrahydric alcohols: pentaerythritol 
Polyhydric alcohols: dipentaerythritol 
All hydroxyl groups may be esterfied or one or more groups may be left 
unesterfied and free hydroxyl groups may be further reacted or partially 
reacted with mono, di or tri- isocyanates to produce urethanes. 
High viscosity is readily obtained by photopolymerisable prepolymers which 
range from highly viscous liquids to solids and have molecular weight 
range of about 250-5000. The terminal or pendant acryloyl groups can be 
incorporated in polymeric components such as polyurethane, polyepoxide, 
polyether, polyester and polyaminoformaldehyde polymers. 
Generally 2-9, and preferably 2-4 acryloyl groups are incorporated in the 
polymer molecule and this can be carried out for example by reacting 
acrylic acid or acryloyl chloride with a polymer or polymerisable material 
containing free hydroxyl groups. Alternatively such groups can be 
incorporated by reaction of a hydroxyalkyl acrylate with a polymer or 
polymerisable material containing isocyanate, epoxide, carboxylic acid, 
anhydride or aminoformaldehyde groups. 
For example an acrylated epoxy prepolymer is prepared by reacting bisphenol 
A polyglycidyl ether with terminal epoxide groups with acrylic acid which 
open the oxirane ring and the hydroxyl groups so produced can be further 
reacted with acryloyl chloride to introduce additional acryloyl groups. 
Acrylated urethane prepolymers are prepared for example by reacting 
hydroxyethyl or hydroxypropyl acrylate with hexamethylene di-isocyanate, 
other di- or polyisocyanates and the terminal isocyanate groups are then 
reacted further with hydroxyl containing polyethers or polyesters. 
Alternatively acryloyl polyether urethanes and acryloyl polyester urethanes 
may be prepared by reacting an excess of a di- or polyisocyanate with a 
polyether or polyester having free hydroxyl groups and then reacting this 
polymer with a hydroxyalkyl acrylate. 
To obtain the correct balance of properties more than one monomer and more 
than one prepolymer may be used in the liquid photopolymerisable 
composition. Also one or more photoinitiators are dissolved or dispersed 
in the composition at a concentration of 0.01 to 30% and more usually 1 to 
10% by weight based on the weight of unsaturated material, to 
photoinitiate polymerisation. The following are examples of 
photoinitiators: Ketones and derivatives such as benzophenone, 4, 
4'-dimethyl-aminobenzophenone, acetophenone, 2,2-diethoxyacetophone, 
halogenated benzophenone, benzil, benzil dimethyl acetal. Acryloins and 
derivatives such as benzoin, benzil dimethylacetate and benzoin isopropyl 
ether. Thio compounds such as thioxanthone, 2-chlorothioxanthone, benzoyl 
diphenyl sulphide, polynuclear quinones and derivatives such as 
benzoquinone, chloroanthraquinone. Chlorinated hydrocarbons such as 
hexachlorethane and diazo compounds including fluoroborate salts of 
diazonium compounds. 
The effect of photoinitiators may be accelerated by inclusion of a tertiary 
amine such as ethyl dimethylaminobenzoate or an amino acrylate polymer. 
Other types of unsaturated monomers and polymers can be added to the main 
photopolymerisable materials listed above to participate in the 
photopolymerisation such as N-vinylpyrrolidone, vinyl acetate, allyl and 
cinnamyl esters, acrylamide derivatives such as (N-isobutoxymethyl) 
acrylamide, triallylcyanurate. Unsaturated polyesters include maleate, 
fumarate, itaconate and citraconate esters of glycols. 
Non-reactive polymers can also be dissolved or dispersed in the main 
photopolymerisable materials such as a high acid value polyester to give 
alkali solubility to the photopolymerised photostencil or dispersed finely 
powdered polyvinylchloride or vinyl chloride-acetate copolymer which 
solvate during photopolymerisation to increase film strength and 
flexibility. 
Finally various other additives may be added to the photopolymerisable 
composition such as pigments, fillers, flow agents and waxes. 
Photopolymerisation can be subject to inhibition by atmospheric oxygen 
which effects mainly the outer surface of the layer. This can lead to a 
reduction in film strength and oxygen inhibition may be prevented in the 
invention by the use of the protective film over the liquid composition 
during exposure, which reduces access by atmospheric oxygen. 
A number of photostencil properties are affected by the crosslink density 
including solvent resistance, flexibility, tensile strength and Young's 
modulus. 
Crosslink density is mainly determined by the number of photopolymerisable 
ethylenically unsaturated groups per molecule of the materials used in the 
liquid compositions, termed the "functionality". One ethylenic group per 
molecule cannot crosslink and gives a soft and very extensible 
photostencil. Two ethylenic groups per molecule generally give a flexible 
and strong photostencil and three ethylenic groups give high Young's 
modulus values which may lead to an inflexible stencil. A mixture of 
materials with one, two and three ethylenic groups is a useful means of 
achieving crosslink density which will then be an average value. 
Flexibility is also achieved by using flexible chemical groups in the 
photopolymerisable materials such as long alkyl chains, polyether and 
polyester groups, combined with control of the crosslink density. 
Generally di-functional materials, that is materials containing two 
acryloyl groups per molecule, provide the best balance of properties. A 
mixture of mono-acrylate and tri-acrylate or higher acrylate materials is 
an alternative means of controlling crosslink density. In such a mixture 
the unsaturated monomer, prepolymer or mixture thereof should preferably 
contain an average of 2 to 3 acryloyl groups per molecule. Generally a 
flexible stencil provides the highest adhesion and is used in general 
screen printing and higher modulus values are useful for improving 
dimensional stability in cylindrical printing screens. 
It is desirable to some screen printing work to reclaim the screen after 
printing has been completed by removal of the photostencil from the mesh. 
This can be achieved in the present invention by several methods. A 
non-photopolymerisable material can be incorporated in the liquid 
composition which has specific solubility properties. For example, a low 
molecular weight polymer having a high acid value due to a high content of 
carboxylic acid groups, such as polyvinyl phthalate or mellitic anhydride 
glycerol polyester, may be dissolved in the photopolymerisable liquid 
composition to render the photopolymer formed on the mesh soluble or 
dispersible in aqueous alkali. Alternatively, or in addition, a 
photopolymerisable high acid value material such as acrylic acid may be 
incorporated in the photopolymerisable liquid to give alkali solubility. 
The use of predominantly di-functional materials such as di-acrylate 
monomers and di-acrylate prepolymers to control cross-link density 
produces a photopolymerised stencil which is soluble or softenable in 
specific organic solvents such as methyl pyrrolidone and ketones. Removal 
of the softened photostencil from the mesh can then be carried out with 
high pressure water spray of 500 or 1000 psi. 
The present invention also provides stencils for use in screen printing 
produced by the present method. 
The invention further provides a process of screen printing which process 
comprises passing screen printing ink through such a stencil on to a 
substrate. 
The invention is schematically illustrated by way of example in the 
accompanying drawings.

DETAILED DESCRIPTION OF THE INVENTION 
FIG. 1 shows an enlarged section through a monofilament mesh 1 which has 
been coated with the photopolymerisable liquid 2 against a transparent 
plastics film 3. A photographic positive 4 bearing a high density image 5 
has been placed in contact with the coated mesh for exposure by radiation 
6 which causes photopolymerisation of the liquid. 
FIG. 2 shows the development of the exposed mesh of FIG. 1 in which the 
exposed material has been converted to the solid state and retained in the 
mesh to form the photostencil 7 while most of the unexposed liquid 2 has 
been removed with the plastics film 3. A thin layer of unexposed liquid 8 
may remain on the mesh surface and may be removed by cleaning with a 
solvent or dispersant or screen removed in the first few prints by the 
passage of ink through the mesh. Alternatively liquid 8 may be 
photopolymerised on the mesh to reduce the open area of the mesh by 
exposure of the mesh to uniform radiation. 
The invention is further illustrated in the following Examples. 
EXAMPLE 1 
A foam rubber sheet of 3 mm thickness is placed on a flat table support and 
a clear transparent polyester film of 50 .mu.m thickness is placed on top 
as protective film. A printing screen having a tensioned polyester mesh of 
120 meshes per centimeter with filament diameter 40 .mu.m and plain weave, 
is placed on the film with the underside (printing side) of the screen in 
contact with the polyester film. 
A trichromatic blue photopolymerisable liquid of the following composition 
(parts by weight) 
______________________________________ 
1. Acrylated urethane 65.08 
2. Trimethylolpropane triacrylate 
12.6 
3. Phenoxyethyl acrylate 
18.0 
4. Dimethoxyphenylacetophenone 
3.6 
5. 4,4'-Dimethylaminobenzophenone 
0.27 
6. Phthalocyanine blue pigment 
0.45 
100.00 
______________________________________ 
(ingredient 1 is obtained by reaction of hydroxyethyl acrylate with 
hexamethylene di-isocyanate and reaction of the terminal isocyanate groups 
obtained with a hydroxyl-containing polyester, and ingredients 4 and 5 are 
photoinitiators) is coated on the exposed mesh surface by drawing a bead 
of liquid slowly across the screen with a sharp hard rubber squeegee. The 
mesh apertures are completely filled with liquid and the liquid is cleanly 
sheared level with the mesh surface to give an exact and uniform coating 
weight controlled by the physical measurements of the mesh. If entrapped 
air should occur in the coating, the coating operation is carried out with 
the flat table surface inclined upwards or even vertically and coating 
commenced from the bottom. Several squeegee strokes can also be carried 
out to expel air. 
The screen is exposed for 6 minutes to a projected photographic image of 
high contrast using as a light source a 2 kw mercury metal halide lamp 
having peak radiation in the 400-450 nm band. 
Alternatively, the high contrast photographic halftone positive is placed 
in contact with the protective film and the assembly placed in a vacuum 
exposure frame. A second protective polyester film is placed on the top 
mesh surface. The screen is exposed under vacuum to a mercury halide lamp 
of 2 kw at a distance of 1 meter for 4.5 minutes and the screen removed 
from the vacuum frame. 
Both polyester films are then stripped off the mesh. In the black opaque 
regions of the photographic positive, where no light falls on the 
sensitised screen, the composition remains liquid and is removed with the 
polyester films giving a positive working process. The polyester film thus 
carries a finely detailed blue image. The image on the polyester film from 
the lower mesh surface is re-exposed to the same light source, without the 
photo photographic positive, for 4 minutes which converts the liquid to a 
hard layer with excellent adhesion to the polyester film and which forms 
an excellent proof of the photostencil. 
The use of the cyan pigment provides a colour proof suitable for the cyan 
printer of a trichromatic or four colour half-tone set and additionally 
forms a very clearly visible photostencil on the screen mesh. Yellow, 
magenta and black pigments or soluble dyes may be used in the 
photopolymerisable liquid instead of the cyan pigment when the 
photostencils for the other colours of the four colour half-tone set are 
produced. 
Any blue liquid remaining on the surface of the mesh filaments after 
stripping can be removed by washing the screen with isopropanol but it is 
preferred to use the screen directly for printing. The first print will 
clean the filaments of the liquid film. 
Instead of using the polyester protective film on the underside of the 
mesh, the photographic positive can be used instead and after stripping 
the liquid composition removed with solvent such as 2-ethoxyethanol which 
does not damage the positive. 
EXAMPLE 2 
The screen printing mesh of Example 1 dyed to an intense yellow colour by a 
yellow disperse dye, is tensioned and fixed to the screen frame. A 
photostencil is prepared by the method described in Example 1, except that 
exposure is increased by 50% to compensate for light absorption by the 
yellow mesh. 
The proof on the polyester film is found to have improved detail in the 
highlight dots and in fine line detail due to a reduction in light scatter 
by the mesh, the yellow colour strongly absorbing actinic light scattered 
by the mesh. 
EXAMPLE 3 
A monofilament polyester mesh having 90 mesh/cm of 40 .mu.m filament 
diameter and dyed yellow, is tensioned and fixed to a screen frame and 
coated with the photopolymerisable liquid of the following composition 
(parts by weight) 
1. Mellitic anhydride-glycerol polymer: 19.00 
2. Trimethylolpropane triacrylate: 12.6 
3. Acrylated urethane (as in Example 1): 33.4 
4. Tetraethylene glycol diacrylate: 5.4 
5. Talc: 19.0 
6. Modaflow (Trade name of Monsanto): 0.8 
7. Phenoxyethyl acrylate: 5.4 
8. 4,4'-Dimethylaminobenzophenone: 0.3 
9. Di-methoxyphenylacetophenone: 3.0 
10. Benzophenone: 0.5 
(Ingredient 1 is a very high acid value polyester which confers solubility 
in aqueous alkali. Ingredient 5 is an extender for giving good rheology 
during coating and stripping. Ingredient 6 prevents foaming during 
stripping. Ingredients 7, 8, 9 and 10 are photoinitiators). 
During coating, the liquid is contained in a coating tray and the screen 
coated while supported in a nearly vertical position. The coating try has 
a doctor blade with a 45.degree. trailing angle to the mesh and a coating 
edge radius of 2.0 mm. A single coating is applied to the print side of 
the mesh. The viscosity of the liquid is approximately 25 poise with a 
high yield value so that the liquid does not run or sag after coating when 
the screen is maintained in a vertical position during light exposure. 
Coating is carried out on the printing side (underside) of the screen and 
after coating a completely smooth, uniform, layer of photosensitive liquid 
approximately 5 .mu.m thick, remains above the printing surface of the 
mesh as well as completely filling the mesh apertures. This layer of 
liquid provides a thicker photostencil of increased strength and gives 
particularly good print quality for line photostencils. 
The screen is exposed for 3 minutes to a high contrast projected 
photographic image using a 5 kw metal mercury halide lamp and condenser 
lens to concentrate the liquid from the lamp onto the small 15.times.10 cm 
photographic positive which is projected with a high aperture process 
lens. 
After exposure the stencil is developed by pressing a sheet of smooth 
absorbent cast-coated paper in contact with the printing side of the mesh 
using a roller and then stripping off the paper. Alternatively, the image 
is developed by applying .beta.-butoxyethanol solvent with a soft sponge 
to wash away the unexposed liquid or by simply printing the screen with 
solvent-based ink. 
A good photostencil is produced which is resistant to many types of screen 
inks including water-based screen inks, solvent-based inks based on 
aromatic and aliphatic hydrocarbon solvents, and ultra-violet curing 
screen inks. 
The photostencil is removed after use by cleaning with solvent to remove 
residual ink and then application of 2% aqueous caustic soda followed by 
high pressure water spray. 
EXAMPLE 4 
A screen frame with the tensioned monofilament polyester mesh of Example 1 
was placed on the clear polyester protective film of Example 1 or the 
glass sheet of a vacuum exposure frame, and the following composition is 
coated over the top mesh surface using a sharp, hard rubber squeegee 
(parts by weight): 
______________________________________ 
1. Di-acrylate ester of the bis- 
hydroxyethylether of bisphenol A 
90 
2. Reaction product of Pentaerythritol di- 
acrylate and hexamethylene di-isocyante 
5 
3. Benzil 2 
4. 4,4'-Dimethylaminobenzophenone 
3 
100 
______________________________________ 
[Item 1 is a di-functional acrylated liquid monomer and Item 2 is an 
acrylated urethane prepolymer having four acrylate groups per molecule. 
Items 3 and 4 are photoinitiators.] The composition has a viscosity of 
15-20 poises. 
A second clear polyester protective film was placed on the top mesh 
surface. 
The high contrast photographic positive is placed with its emulsion side in 
contact with the protective film on the lower mesh surface and exposed in 
a vacuum printing frame to a 2 kw mercury metal halide lamp peaking at 
400-450 nm, for 3 minutes. The upper protective film was stripped off and 
discarded and the lower protective film was stripped off to develop the 
photostencil. 
When printed with a solvent-based screen ink containing 2-ethoxyethanol as 
solvent, residual liquid photopolymerisable material on the mesh threads 
was removed with the first few prints.