Most screen printing emulsion is similar to gum bichromate. It's a PVA type of adhesive sensitised with potassium dichromate. It is applied either by hand from a coating trough or by an automatic machine. It is also possible to buy screen emulsion in sheet form which is applied to the mesh before exposure. Autotype Cappilex is one which we use.

http://www.macdermidautotype.com/products

The image you use needs to be either clear film base or solid black. It is applied to the screen emulsion and exposed to ultra violet light. The UV light hardens the emulsion and areas not exposed are washed out. What was black on the photographic image gets printed.

When looking at the artwork the right way round, the emulsion should be on the top. This is then placed emulsion to emulsion on the underside of the frame to make a contact exposure. It is normal to coat the underside but at work, we have a machine which coats both sides at once.

A halftone screen will turn a full tone image into a series of varying size dots just as is used for newspaper pictures. You might get suitable results by just exposing line film without a halftone screen. It has a high enough contrast that the output will be clear or black and the amount of exposure will determine the changeover point.


Steve.