Once the art has been created, the screen stretched and coated with emulsion
it is time to expose it. The stencil material on the mesh is UV light sensitive.
Specialized exposure units are used to 'burn' the image into the stencil.
The emulsions function by hardening when exposed to the bright UV light source
and remain water soluble in the areas not exposed. The film positive art
blocks the areas that need to print. Once the exposure is complete the screen
is hosed down. The unexposed areas of emulsion will simply wash off of the
mesh creating openings for the ink to flow through. Exposure units differ
in the type and arrangement of the light source, and higher quality units
a better suited for complex work. Some use a bank UV specific fluorescent
tubes while other uses a single halide bulb. 'Undercutting' is an undesirable
effect that is caused by the light leaking around the black areas of the
film and appears as a loss of dots or a 'choking' or proportional inward
reduction of an area to be printed. This problem can make it difficult to
print detailed or half toned art and will cause registration gaps in simpler
designs. It can be alleviated by using a good vacuum system to secure the
film to the emulsion during exposure and a properly placed 'point' source
light. Certain bulb types also offer more of the specific wavelengths needed
to harden the emulsion and create better stencils.
After the screen has been exposed and rinsed it can be set aside or put in
a heated closet to dry.
After the screen has been exposed and rinsed it can be set aside or put in a heated closet to dry.