1. At the top of the stack we see the Film. This is a film positive, which is clear with the exception of the image to be printed which is opaque. (black) There are other methods, like painting directly onto the silk with a “hold-out” solution, but typically we use a film, or vellum printed from a computer.
2. Next, a photo-sensitive material called “photo mask” or “photo stencil”, which behaves much like photo film, is exposed to strong light using the film positive as a mask. In this process the light-exposed areas of the photo-sensitive “gel” emulsion becomes hardened. The emulsion areas of the stencil which are protected by the opaque areas of the film remain unexposed and thus soft. In the “development” stage of the process, the exposed areas of the stencil remain hardened, while the unexposed areas of the stencil soften and get washed away, forming the image areas where the inks will pass through.
3. The “photo stencil” is embedded (either before or after exposure) into fabric stretched very tightly across the printing frame. After the stencil dries, the “Screen Frame” is pressed against the substrate (a shirt in this scenario) and ink poured onto the surface of screen is dragged across the design using a squeegee. This results in ink deposited on the shirt. And that’s how screen printing works.