Photoengraving

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Photoengraving also known as photo-chemical milling is a process of engraving using photographic techniques. The most common type of photoengraving involves using a material that is photosensitive and resistant to acids or other etching compounds. This material, called a photoresist, is applied to a metal to be engraved. It is then exposed to light (usually strong ultraviolet) through a photographic negative causing it to harden where the negative allows light to pass. The photoresist is then developed by washing in a solvent that removes the unhardened parts. Finally, the metal to be engraved is exposed to an acid or other etching compound which dissolves the exposed parts of the metal.

Photoengraving is used to make printed circuit boards, printing plates, foil-stamping dies and embossing dies. It is also used to make nameplates, presentation plaques and other decorative engravings. A similar process called photolithography is used to make integrated circuits.

One method of photoengraving produces a shallow depression in the metal. This is used for intaglio printing plates or for decorative purposes. It is also the same method used for printed circuit boards. The engraving is usually made in copper or brass. The process can be done in open trays but is much more effective if the etchant (often ferric chloride) is sprayed onto the metal. When ferric chloride is used as the etchant, no metal parts other than titanium can be used in the etching equipment. Decorative engraving is often filled by spray-painting then sanding to remove the paint from the raised parts of the engraving.

Another method produces a deep engraving with sloped shoulders. In this method the metal (usually zinc or magnesium) is held face-down and a mixture of nitric acid and a soap-like oil is splashed onto it. As the acid etches the surface the oil adheres to the edges of the exposed area. This progressively reduces the area being etched resulting in a sloped edge; a single dot will end up as a cone-shaped mound protruding from the etched area. This method is used for printing plates (the shoulder supports the printing surface), foil stamping dyes and embossing dyes. Decorative engravings made by this method may go through a second process to produce a decorative background. The raised parts and their shoulders are painted with an etchant-resistive material and a pattern of etchant-resistive material is applied to the deep parts of the engraving. The resist for the background may be another photoengraving or may be randomly splashed-on. The engraving is etched again for a short time to produce a raised pattern in the background. Decorative engravings of this type may also be spray-painted and sanded as in the previous method.