Acid etching, otherwise known as chemical milling or chemical etching, is a chemical process used by many industrial metal parts manufacturers to etch, cut or engrave very delicate or precise lines into metal. This process involves very little mechanical work, for it is an almost entirely chemical procedure. Almost every type of metal can be chemically etched, including most types of steel, titanium, brass, nickel, copper and so forth.
Etched metals are usually quite thin, such as sheet metals or foils, although thicker metals such as coins and plaques are often etched as well. Parts made from chemical milling include stencils, plaques, printing plates, printed circuit boards, foil-stamping dies, and embossing dies for electronics, aerospace and automotive industries among others. The acid etching process has six main steps. First, the metal sheet to be etched must be stripped of all oils and chemicals. Cleansers are typically an alkaline cleaner to strip organic materials followed by an acid cleaner to remove chemical residue; neither of these cleansers can be too strong, or the polished surface of the metal will be scratched. Next, a masking is applied to the entire surface. Masking types are often tapes or paints, elastomers (rubber) or plastics. A pattern is cut into the masking in the same shape the metal is to be cut, then the cut masking is removed from the areas to be etched, and the chemical, or "reagent", is applied.
After the acid has achieved its desired etch, both the reagent and the remaining photoresist are stripped. The metal is polished, any irregularities in the cut are burnished, and the etching is finished. Various types of metal require different etchants, or reagents. For example, steel typically takes hydrogen chloride or nitric acid; aluminum uses sodium hydroxide; stainless steel etching and copper use iron chloride or nitric acid, and so on. When the reagent is applied, many factors are considered to determine how fast the etch or cut will be made, how much of a side-cut will be made, etc.; temperature, agitation, and often the concentration of hydrochloric acid all determine what type of cut will be made. Acid etching and engraving has become increasingly popular as chemical technology has improved, since chemical milling is much faster and more economic than mechanical milling. Mechanical milling usually involves large and costly machinery, and each part must be milled individually. Parts can be completed by chemical milling within hours of design. This is likely the reason why larger OEMs and industrial machinists are turning to chemical and photo milling.