How to Build Thick Stencils

Emulsion

The emulsion companies have made this job easy with the introduction of stencil
material coated to a polyester sheet.  The sheets are available in various
thicknesses from 10 to 1000 microns thick.  The sheets can also be easily
laminated to create thicker films.

A capillary film is applied with plain water to the screen.  Some films
are not capillary and require coating the screen with liquid emulsion to adhere
the film.  A film package will be marked with the thickness.  So
capillary film produces a stencil thickness as marked on the package whereas
a liquid and film combination can produce a thicker stencil.  If screens
are low tension producing a thicker coating of liquid emulsion at the center
of the screen than closer to the frame, the stencil thickness across the image
area may be inconsistent.

A stencil film can be purchased clear or dyed.  Clear allows light to
pass through easily, but also is associated with undercutting of the image
and light halation.  Image resolution may not be as sharp as dyed stencil
material.  The dyed material will restrict the thickness of film that
can be used.  The exposure time required for 200 microns is more than
double the time required for 100 microns.

A simple solution is to use a thinner stencil like 50 microns, expose, wash
out, and apply a second sheet of film over the  first.  The positive
will have to be mounted exactly where it was previously to expose the 2nd sheet
of film.  A pin system or home made registration guide will make this
easy.  Such a laminating technique is particularly recommended for screen
printers with exposure units less than 1000 watts.

Laminating is an important technique, because thick films only wash out after
the stencil has absorbed water.  A 50 micron absorbs water quickly compared
to 200.  If 50 is exposed and washed out, and then 200 applied, the 200
will wash out easier without the mesh supporting the stencil in the image area.

The key to film stencils is water, lots of water.  Capillary film should
be rolled down an upright screen while holding the water hose at the bottom,
and touching, the screen.  The water is directed up at the stencil being
rolled down in a heavy sheet of water.  The capillary film can be wrapped
around ½” I.D. PVC pipe so the capillary film is applied with
even pressure.  The PVC pipe is recommended with film wider than 4”.  Capillary
film should not be squeegeed into the mesh unless the stencil is 200 microns
or thicker.

When laminating, the screen is upside down so water can be sprayed on the
capillary film and a second sheet of capillary film applied.  Once water
dries, the plastic sheet can be removed.  A small fan with a heating coil
as sold in the winter as a space heater will accelerate screen preparation
time.

The unexposed emulsion in the image area will come out faster by laying the
screen flat, stencil side up, and laying a paper towel over the image.  Spray
with water.  After a minute or two, remove the towel, spray  that
portion of the stencil that has absorbed the water, and repeat the process,
if the stencil did not wash out easily.

Thick stencils are an easy solution to high resolution images on textured
material.