Photo-Etching or Chemical Milling as it is sometimes called, is a method of producing metal components from flat sheet by removing unwanted material via acid erosion. In traditional etching, an acid resistant coating such as wax or shellac was applied to a metal, a design scribed through the coating, and the sheet then immersed in acid. The acid ate in to the exposed metal to the required depth, the sheet was then rinsed and the resist removed.
Modern Photo-Etching still uses the same basic technique. A chemically cleaned and de-greased sheet of metal is coated both sides with a light sensitive photo-resist material. Upon exposure to ultra-violet light the resist is fixed. Any area of resist not exposed to light can be removed in a chemical developer, leaving areas of uncoated metal. The developed sheet is then passed through a sprayed hot acid etchant bath. The uncoated areas of metal are etched by the acid whilst the coated areas are unaffected. The metal is repeatedly passed through the etchant until the desired depth of etch is achieved.
The photo-tool is the device for preventing exposure of the light sensitive resist. In it’s simplest form a photo-tool could be a sheet of clear film with a black circle on it. If the film was placed in contact with the resist coated sheet of metal, exposed to light and then developed, the sheet would have a circle of exposed metal surrounded by a coated area of protective resist. This could be etched to the required depth.
However the material is coated both sides. An image could be printed and developed on both sides of the material. If the metal was etched half way through from the front and the back, any area where the two images coincided would produce a hole right through.
A photo-tool is a front image film attached to and in register with a back image film. This means that an etched image can be produced on the front of the metal at the same time as an image on the back, and where the two coincide, the metal is etched all the way through.