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Photochemical Machining

Photochemical machining is a process in which a metal, like aluminum, titanium, copper, and more, is coated in a UV-sensitive material and sandwiched between two sheets called a phototool in order to imprint an image on the metal. The metal is then etched with this image as a guideline. Photochemical machining is a relatively small but growing industry, with only around 100 shops around the United States, including shops such as Photofabrication Engineering Inc. These shops have to adhere to strict government, internal, and environmental standards.

Photochemical machining is an extremely fast and inexpensive process. Phototools only cost between 300 and 400 dollars, and the work can be completed within a day (often, within an hour). It is an extremely precise process that can produce complex metal parts with a very small margin of error. There are many applications for photochemical machining, including those in aerospace engineering, manufacturing, microengineering, and more.

More specifically, in photochemical machining, a strip of metal is cut to size and chemically cleaned so that foreign contaminants do not prevent the photoresist from sticking to the metal sheet. This used to be done with solvents, but many companies today now just use soaps for the health of the environment. Sheets may go through one or many rinses and scrubbings before they are deemed truly clean for the process.

Once the sheets are clean, the photoresist is applied to them. There are many different kinds of photoresist, including dry-film and wet-film photoresists. A dry-film photoresist is different from other wet chemical photoresists because it is already in a uniform-thickness sheet that can be applied to a surface via lamination techniques. Dry-film photoresists have the advantages of being easy to apply in bulk, easier to control during application, and easy to strip from the material. Typically, before application, dry-film photoresists are protected with sheets of plastic.

Both sides of a sheet of metal are generally coated with the photoresist, though thinner gauges of metal may not need this. Once applied, the photoresist is cut to the size of the sheet metal. The coated sheet of metal is then sandwiched between a phototool (which has two sheets) that has the design needed for the part. The phototool and metal are then subjected to a vacuum to remove air bubbles between the two pieces, and the whole thing is exposed to UV rays. Once exposed to UV, any places that the phototool does not cover will cure, while areas that it does cover will not cure.

The photoresist that was covered by the phototool is rinsed off in solution so that the bare metal is all that is left. Wherever the photoresist has cured will be protected from the etchant, which is usually some form of acid. Most shops will use different types of etchant depending on the material being used, but the most common acid is ferric chloride. The bare metal is then dissolved in the etchant, leaving a precisely cut metal piece. The final step is to strip the cured photoresist from the remaining metal.
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