Photo Engraving
Photo engraving, or photo
etching, often called "photochemical milling", is a process very
similar to regular chemical
etching or chemical
milling. 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 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 serves two purposes: to protect the metal surface
not to be engraved and to break down under targeted ultraviolet light exposure
over the metal surfaces which are to be engraved. It is essentially the same
technology used for photo development, where a negative is produced by light
exposure then chemicals are used through the negative pattern to create a positive
image. 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. 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. Negative
and positive working photoresists achieve different types of images and desired
results. 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, et cetera. The most common chemical used for photochemical
milling is ferric chloride dissolved in hydrochloric acid (the amount of HCl
determines the speed and depth of the cut). 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
need be exposed to a pattern of light and then the developed photoresist rinsed.
Photo etching allows for much more complex patterns and images to be laid;
this is often the only way to etch photos or intricate designs, and the etching
is achieved much quicker and more cost effectively than even regular chemical
milling.