It is still a chemical process and utilizes the same masking and etching technique, with one significant difference: instead of using masking material and cutting out the area to be etched, a photoresist is applied and developed through a patterned exposure to light.
The result is the same; the undeveloped areas of the photoresist protect the metal’s surface from being etched, while the developed areas are washed away and exposed to etching agents.
Photo etching and engraving is often used not only on metals, but on glass as well; this technique allows photos or complex images to be engraved rather inexpensively into decorative items.
Like chemical milling and etching, the first step to photo engraving is to clean the metal surface. Next a thin layer of photoresist is laid on the area to be etched. This photoresist protects the metal surface not being engraved but also breaks down under targeted ultraviolet light exposure over the metal surfaces which are to be engraved. The developing process, as with photography, can be reversed; positive or negative working photoresists may be used.
After the photoresist has been applied, the metal is exposed to a precise light pattern. Negative and positive working photoresists achieve different types of images and desired results.
If the photoresist is negative working, the exposed portion will protect the metal while the unexposed portion will dissolve away when chemicals are applied. If the photoresist is positive working, the metal portion exposed to light will dissolve under chemicals while the rest remains.
After the metal has been exposed to a light pattern, a specific chemical will be applied in varying strength and force, again, depending on the desired depth of cut, sideways etch, etc. After the acid has achieved its desired etch, both the ferric chloride and the remaining photoresist are stripped.
The metal is polished, any irregularities in the cut are burnished, and the etching is finished. The benefits of using photo engraving as opposed to regular chemical milling or manual engraving are substantial.
Chemical milling requires parts of the masking to be cut and removed, whereas with photo engraving, the surface merely needs to be exposed to a pattern of light and the photoresist rinsed.
Photo etching allows for much more complex patterns and images to be laid and this is often the only way to etch photos or intricate designs. Photo etching is achieved more quickly and cost-effectively than regular chemical milling.