Saturday, October 20, 2012

The Photochemical Machining and Etching Processes

Photochemical machining, or photochemical etching, is a manufacturing process ideal for crafting precise sheet metal components. Chemical etching is achieved by using a photoresist stencil as the method of removal over selected areas. The first photoresist was developed in 1826, however the photochemical machining industry did not take off until the 1950s.

The photochemical machining process begins with the manufacture of a photo-tool. There are a variety of methods for the production of photo-tools. The most common method is to produce a photo-tool of the correct and precise size by using a laser photo plotter to selectively expose a photographic film according to computer-aided design data.

The next step is to select and prepare the metal for photochemical etching. The ease with which a material can be etched depends largely on its chemical composition, since etching is a controlled chemical corrosion reaction. Before it can be laminated with the photo-resist, the sheet metal cleaned with electrolytic alkaline to remove all dirt, rust, greases and oils. This pretreatment ensures that good adhesion to the photo-resist. Cleaning the surface with electrolytic alkaline is rapid and reliable.

The laminated metal substrate, covered with photo artwork, is exposed under an UV source. Exposure time depends on the thickness of the metal. During development, the unexposed negative working photo-resist will be washed away, and the exposed photo-resist would remain on the substrate to form the protective layer for etching.

This chemical machining process is commonly compared with metal stamping because both methods can be used to produce precision sheet metal parts. Photochemical is more economical for the manufacture of complex parts. Precision metal stamping has a low unit production cost, but a very high tooling cost, especially for complicated parts.

Photochemical machining also takes less time for production. Typically, it takes 2 or 3 days to produce sample quantities. In contrast, with metal stamping on average it takes several weeks to make the tooling. If there is any design change required, the photo tool can easily be modified at low cost within a short period of time with photochemical machining. Metal stamping also requires further abrasive treatments to eliminate the burrs and to finish the components.

Additionally, metal stamping cannot process pre-hardened sheet metal. On the other hand, photochemical machining manufacturing can process sheet metal of any hardness. The process of photochemical machining also does not affect the inherent physical chemical properties of the sheet metal. This is beneficial when working in the fabrication of magnetic materials. Metal stamping greatly reduces the magnetic permeability and also causes stresses that require a costly and time consuming annealing process to restore permeability.


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