Wednesday, March 27, 2013

Screen Press Printing: Looking At The Process

Screen printing also called silk screen print also called serigraph was invented in the Far East around 2000 years ago. No supplemented data on when or where exactly it originates, but originally used to make the screen printer's screen. A paper stencil was stiched onto the screen to control the print area.

Around a hundred years ago screen printing was reinvented in the west and it has become a common method of printing on textiles. Most common, the custom t-shirts. The screens used for screen printing custom t shirts, are made with a mono filament mesh. Photopolymer emulsion is used to make the stencil. Thus, silk screen is coated with the liquid emulsion, and allowed to dry and harden. The emulsion is light sensitive. The stencil is made in the "silk screen" by placing a positive of the print over the coated silk screen then exposing the screen to light. The areas of the screen shielded from the light remain water soluble and can be washed out of the screen. The light affects the emulsion in that once the dried emulsion has been exposed to light, water will not dissolve it. Take a look at the photograph to the left. In this case the positive for the screen print is hand drawn.

The availability of the screen printing products can be purchased locally from screen printing process supply stores. An "emulsion cup" is used to coat the silk screens with emulsion. The emulsion cup is a trough like cup that holds the liquid emulsion and allows it to be spread evenly across the screen. The screen is held vertically and the emulsion is spread from bottom to top with one smooth motion. After the silkscreen has been coated with emulsion it will be left to dry. Ambient or soft light can expose the screen but it would take hours or days. Even so care should be taken that the silk screen is kept in a dark place.

To "burn" a screen, the positive must make tight contact with the screen. A four inch thick foam rubber pad is placed under the screen with the glass placed on top. Sunlight can now expose the silk screen. It can take anywhere from around two to fifteen minutes. After the screen has been exposed it is taken to a wash out area usually a metal bathtub like trough or large sink. The screen is set in the trough and sprayed with a garden hose. Rinsed with water, after a second the design will be visibly lighter in the emulsion. After a minute or so of rinsing the design area should be free of emulsion. Rinsing is continued for another minute or so to wash away emulsion scum that if allowed staying in the screen and dry will clog printing areas. The washed out screen is then allowed to dry.

After it has dried the screen must be taped off. This is usually done with masking tape. The areas around the edges not coated with emulsion are taped front and back. The screen printer applies tape liberally over the frame and screen areas. The screen is then "pin holed". Sometimes imperfections in the screen printing process cause tiny "pin holes" in the emulsion that's why these must be taped over or blocked with emulsion or block out fluid. This is done on the bottom of the screen.

The emulsion can be removed from the screens so that the screens can be reused. This is called "reclaiming" the screen. After the print run the ink is scraped out of the screen and mineral spirits or ink wash is used to clean the ink from the screen. All the masking tape is pulled off the screen and the screen is sprayed with "emulsion remover." This chemical is left on the screen for 2 or 3 minutes and a high pressure sprayer or even a garden hose can be used to wash away the emulsion. After the emulsion has been removed the screen must be degreased. There are degreasing liquids available also ivory soap works well. The screen is scrubbed thoroughly on both sides and then rinsed. It can now be dried and if kept clean reused. The screens must always be clean and degreased and dry before they can be coated with emulsion.

These procedures may sound very difficult but the application is totally easy. See and try the screen printing yourself!

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