Making the Separations
The first step in screen-printing a T-shirt is to print out the separations of the design. The design needs to be separated into color layers, with each layer printed separately on special translucent film in black ink.
Preparing the Screens
The screens are spread with a thin layer of emulsion, a light-sensitive substance, and must be kept away from UV light. A printed film is then placed on top of an emulsioned screen and moved to the exposing unit, which subjects the emulsion to high levels of UV light for a short period of time. Any emulsion that is not covered by the printed design hardens, and the remainder is washed away with water, leaving a stencil of whatever design was on the film. A separate screen is made for every color in the design.
Registering the Design
The completed screens are then fitted into the press. A system of clamps and screws allows for very minute adjustments to the position of the screen. If the design features only one color, then the screen must simply be straight; if there are two or more colors, the design must be registered, or lined up perfectly. Screens are generally made with cross-hair registration marks to aid in this process.
Printing the Shirt
The next step is to fit the shirts onto the press. Ink is poured into the screen, which is "flooded"---completely spread with ink. The ink is scraped across the screen with a squeegee with enough pressure to push it through the mesh in the stencil area. For a multicolor design, the shirt is dried briefly before a second color is printed to avoid smudging.
Drying the Shirt
When the shirt is printed it is taken off the machine and put through a conveyor-belt dryer to set the ink. In between colors of a multicolor design, a flash dryer is used to allow the shirt to be dried while it is still on the press.