Greenprint Guide to Screen printing
Screen printing, also known as serigraphy, uses a fine mesh stencil wrapped tightly over a frame. The mesh allows ink to pass through unless it is filled in or blocked. A simple stencil can be made by applying a non-permeable material to the areas which are not to be printed (non-image areas). Most industrial stencils are made by coating the stencil with a photo-sensitive emulsion, then shining a bright light source through a film containing a positive image of the area to be printed onto the stencil. The non-image area which is exposed to the light turns solid (fixed) and the stencil is then washed to remove the un-fixed emulsion, leaving the image area open which allows ink to pass through.
The stencil is loaded into the screen press, where ink is applied to the surface and forced through the image areas using a hard rubber squeegee blade. The ink is deposited directly onto the material under the stencil. For short runs of a few hundred items, simple ‘handbench' machines are used. The material is inserted under the screen manually and the operator squeegees the ink through by hand, one impression at a time. For longer runs and multi-colour prints, semi or fully automatic screen presses feed the material, squeegee the ink and can even dry the ink inline using ovens or UV light.
Screen printing is used for printing on fabrics, metals, plastics - almost any material can be screen printed. Even the printed circuit boards used in computers and other electronic equipment are printed using the screen printing technique. One of the limitations of screen printing is that the size of the holes in the mesh prevents fine dots from printing cleanly and very small type fonts can "break up" even when using the finest mesh stencil.