In screen printing a stencil acts as a mask. It is supported by a fine material mesh that is stretched over a metal or wooden frame. The print is made by pressing colour through open areas of the stencil.
Preparing the screen: A positive of the desired picture that is opaque to UV light is required, e.g., a high quality photocopy. The material mesh of the screen is coated with a light-sensitive layer. Positive and screen are placed on the glass surface of a special vacuum table, positive under the screen ready for illumination by a short intense burst of UV light. This follows once the cover has been closed and the pressure reduced. After the photoreaction, the screen is extensively washed with water. Regions of the photosensitive layer that were exposed to the UV light remain, others wash away, i.e., the stencil, a negative of the desired picture, if formed.
Objects that will not damage the fine mesh of the screen can also be used as ‘positives’, e.g., leaves, flowers, pieces of string etc.
It is also possible to paint a negative of the desired picture directly on the screen using a liquid filler. A stencil can also be modified in this way.
Screen printing is an ‘open’ or ‘shut’, ‘colour’ or ‘no colour’ technique, intermediate tones are not possible. These can be achieved by using a raster of dots when preparing the ‘positives’, cf. newspaper pictures.
A screen has to be made for each colour used. A third colour will be obtained if regions of two screens overlap and the second colour printed is partially transparent.
Printing: The frame is positioned slightly above the paper. Paint is poured onto a closed region at one end of the mesh and dragged across the stencil using a squeegee, which is at the same time pressed down onto the paper. The method is simple but requires practice. The angle of the squeegee and the speed with which it is moved are both important.
Further information can be found in Wikipedia under Screen printing