Exposure

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.