The most popular photographic stencil method used in the screen printing industry is the "direct" stencil. It's popularity is due to its low cost and ability to create extremely fine detail. There are some drawbacks, however, since it is somewhat difficult for beginners to achieve even coating of the screen.
The direct photostencil gets its name from its direct application to the fabric before it is exposed. The direct stencil is referred to as an "emulsion". An emulsion is a liquid with solids suspended in it. This makes the consistency much thicker than water or paint. The liquid emulsion can be purchased as a "pre-sensitized" solution or must be "sensitized" according to the type and manufacturer. "Sensitizing" the emulsion requires adding a special chemical that makes the material light sensitive. Once the material has been sensitized, it is usually handled only under subdued light or safelights in a darkroom.
Applying Direct Emulsion
The direct emulsion is often applied with a "scoop coater". The scoop coater is a special "trough-like" tool that holds a small amount of emulsion. The emulsion is coated onto the surface of the fabric in thin coats. Sometimes coats are repeated, and the screen rotated 180 degrees before each coat is applied. For some coarse fabrics with large mesh openings, the screen is allowed to dry, and then it is recoated. Refer to the manufacturer's instructions for recommended coating procedures. For general applications, see the illustration below.
Exposing the Stencil
Once the screen fabric has been coated and dried, it is ready to be exposed. There are many methods of exposing screens, but the best method for direct stencils is to use a vacuum frame. The vacuum frame holds the screen and positive in place while the exposure takes place.
Regardless of the method used, the principle is the same. The light passes through the clear areas of the film positive, striking the emulsion on the screen. These areas are hardened by the light. The black image areas block the light, protecting the emulsion from the light. These areas are not struck by the light, therefore remaining soft. These areas are then washed away. The exposure is illustrated below.
Washout of the Stencil
After the exposure, the stencil is removed from the exposure unit and placed in the washout area or sink. The screen is wet on both sides with a gentle spray of cold water. Then it is washed with a spray of cold water from the printing side of the screen until the image areas are completely opened. To be sure that the screen is clear of any unexposed material, rinse the screen from the squeegee side. After the screen has dried, it is ready to be masked in preparation for printing.
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