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These instructions are written specifically for the
Selectasine Diazo Photo Emulsion but can also be applied
to the Speedball Diazo Photo Emulsion.
 A screen is coated with photo sensitive emulsion and
allowed to dry. The stencil is made by placing a "film
positive" (i.e.. your design printed onto a transparency,
or hand drawn onto draftfilm/polydraw using plumtree
opaque) on the treated screen, which is then exposed to UV
light. Where the light is blocked, which will be your
design, the emulsion remains water-soluble and will wash
away. The remaining emulsion will have been hardened by
the light and act as the stencil (or resist) and the image
is printed through the area left clear.
Instructions for Use
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Thoroughly degrease the mesh using Mesh Prep Gel.
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Dry the mesh completely and apply the photo emulsion using a
coating trough. Coating should be carried out under
low-level tungsten or yellow light. Exclude all sources
of ultraviolet light and ensure that the coating and
drying area is as dust free as possible.
-
Apply one coat of Diazo Photo Emulsion to the underside of the mesh and turn
the frame through 180º and apply two coats to the
squeegee side of the mesh. Remove surplus emulsion from
the screen by running the coating trough up each side of
the screen with a dry blade edge. You should aim to
achieve a thin, even coating.
-
Dry the screen in the horizontal print position with
the squeegee side uppermost, supported on blocks. Drying
must be carried out in a U.V. light proof area, low
level tungsten or yellow light is suitable. Dark
conditions are better, but in all cases the area should
be as dust free as possible. Gentle heat which must not
exceed 35ºC is recommended.
-
The air flow is more important than the velocity of
air. When the screen is completely dry it may be dated
and stored in a cool, dark, dry cabinet or used
immediately.
Exposing (see also
Ruth Browns' notes on
Exposure)
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Place the film positive to the underside of the
screen and tape it into position with clear cellulose
tape. Place the screen into the "exposure unit" with
your positive to the glass surface and apply U.V. light
to the screen. Due to the various combinations of
coating, mesh grade, mesh colour, output of lamp,
distance etc it is not possible to give the optimum
exposure times, therefore a trial step-wedge test should
be made.
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After
exposure, remove positive and wash, with a spray of cold
water. Firstly spray the squeegee side of the mesh and
immediately spray the underside of the mesh. The screen
should be washed out in the vertical position starting
on the squeegee side and turned to wash out the
underside.
-
When the
stencil is fully developed it should immediately be
blotted with either unprinted newspaper (Newsprint), or
a well-wrung out chamois leather to remove excess water,
and then fully dried preferably in the print position,
again using heat not exceeding 35ºC. This final drying
can be carried out under normal lighting.
N.B. The
shelf-life of the Emulsion, once it is sensitised, is 3
months. It should be stored in a cool dark place. The
shelf-life can be increased by storing the sensitised
Emulsion in a refrigerator, but in this case, it must be
removed from the refrigerator some hours in advance of
using, to allow the Emulsion to warm to room temperature
and become less viscous. Frequently Asked
Questions Is it necessary to degrease the screen
mesh?
The monofilament screen meshes we supply are very
accurately woven, using a single, smooth, polyester fibre.
This offers very little surface area for the emulsion to
adhere to (this also applies to films and fillers, if you
are using them). If there is any trace of grease, and this
can be from your hands, then the stencil is likely to
suffer premature breakdown. We would always recommend that
you degrease your screen, prior to coating, as a regular
practice.
Can you use washing up liquid as a degreasing agent
instead of Mesh Prep Gel?
No! Washing up liquid contains softeners, which will leave traces on
the mesh.
I have heard that you can use abrasive detergents to
“rough-up” and degrease the surface of the mesh. Would you
recommend this?
Definitely not!
Do you have to have a dark room to carry out
this procedure?
No, but do not carry it out in bright sun light. When
the emulsion is in a liquid state it is not that
sensitive. Therefore, coating a screen can be carried out
under low-level light. As it dries it becomes more
sensitive, so it should be put away in a cupboard or dark
doom, to dry and be shielded from bright light and sources
of heat, before processing further.
Can I assist the drying process?
Yes, but please note, heat can have the same
detrimental effect as a bright light (i.e. it can harden the
emulsion so that it renders it unusable). A fan heater can
be used on a low heat setting, blowing air across the
screen. Do not blow directly onto it, as this can cause
“heat spots”. Make sure that dust is not blow up onto the
emulsion as this will cause “pinholes” in the stencil.
How many coats of emulsion should I apply?
What you should be aiming for, is a thin, even coating.
One, might be quite sufficient, especially if you are
applying it with a squeegee. If you have a coating trough,
then you can apply as many coats as you like, but you must
remove the excess from both sides of the screen, by
running the dry blade, (i.e. uncharged with emulsion) up
each side of the screen.
How do you coat a screen with a
squeegee?
Lay the screen upside down. Using temporary
tape (packing tape is good for this) put one or
two strips across each end and another down each side,
within the width of the squeegee. Pour a bead of emulsion
across the tape at the top end of the screen and pull the
emulsion down to the bottom end using your squeegee.
Remove surplus emulsion from the tape and put it back into
its tub and leave to dry. Strip away your tapes, and you
will have a fine, clean coating.
How long is the
exposure?
This is a difficult question to answer. There
are so many ponderables. It is very important that you
have a very good “film positive” to work with when you
calibrate your light source. Your design must be perfectly
opaque to the light. The emulsion will harden where it is
exposed to sufficient UV light, but will remain soft and
wash away in water where it has been shielded. Film
positives produced from photocopiers and computer printers
are not truly opaque and should be avoided for this
procedure. Here are the points you have to consider:
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The light source you are using.
Ideally this should be an appropriate Ultra Violet
Reprographic Light. It is the emission of Ultra Violet,
mainly in the invisible part of the spectrum, which
hardens the emulsion.
-
The area you are exposing.
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The thickness and colour of the mesh. A course mesh (eg;
43) will have a thicker stencil than a finer mesh (eg;
61), but a coloured mesh (eg; 77 Yellow) will require an
extra 50% exposure time.
-
The distance of your lamp is from your work.
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The age of your lamp. These lamps gradually wear out
and although they might appear bright, they gradually
loose some of the effective radiation. It will mean that
the exposure will have to be gradually increased and
eventually the bulb replaced.
In the end, you will have to a make bold decision, chose a
time and see what happens. If the stencil is stubborn to wash out, but you can see
the image, then you have overexposed it. If it washes out
very quickly, with more besides and the working side of
the emulsion is tacky, then it is underexposed. In
the first case halve the time, in the latter, double the
time and follow the same procedure if you are still
getting the similar results.
Do I use hot or cold water for washing out the stencil?
Use cold water. Initially the unexposed areas are
absorbing the water, swelling and then breaking away from
the hardened areas. So to start with, the screen need only
to be bathed in water and once your design has started to
clear, it can be sprayed more rigorously. We would not
recommend using a high pressure washer.
Why do I have to “swab-off” the excess water after washing
out?
If water is left on the screen, then “scumming” can occur,
leaving a translucent veil in your design, especially the
most intricate parts, causing blockages.
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