Photetching or PE as it is commonly known is an intaglio process in which the etching plate is coated with a light-sensitive acid-resistant ground and exposed, through a dot screen, to a photographic image. A "negative" resist dissolves in the areas that are exposed to light, while hardening in areas not exposed to it. Thus pits are etched and ink is held in the areas that appear light in the photographic image. A positive resist dissolves where it is not exposed to light.
Typically this is in modeling used to create detailed parts in scale thickness that are beyond injected styrene technology.
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