Saturday, 19 March 2011

How does screen printing work?

The equivalent of the printing plate for the screen printer is the screen - a wooden or aluminium frame with a fine nylon mesh stretched over it. The mesh is coated with a light sensitive emulsion or film, which - when dry - will block the holes in the mesh. The image that needs to be printed is output to film either by camera or image-setter (in our case by inverted images on acetate. This film positive and the mesh on the screen are sandwiched together and exposed to ultra-violet light in a device called a print-down frame or exposure unit. The screen is then washed with a jet of water which washes away all the light sensitive emulsion that has not been hardened by the ultra-violet light. This leaves you with an open stencil which corresponds exactly to the image that was supplied on the film. Once washed the screen must be dried (at college this is quite quick as we have dedicated heated draws for this). The substrate to be printed (i.e. paper, fabrics, card, wood pretty much anything with a flat surface) is placed in position under the screen and ink is placed on the top side of the screen, (the frame acts also as wall to contain the ink). A rubber blade gripped in a wooden or metal handle called a squeegee is pulled across the top of the screen; it pushes the ink through the mesh onto the surface of the substrate you are printing. After the initial stroke the squeegee should be used to pick up the excess ink and return it to the start position ready for another stroke or a fresh peace of substrate. In this way screen printing can be used to create multiple high fidelity copies of a single colour (or multiple colour but this requires multiple screen exposed for each separate colour) image quickly.

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