(2) Classification based on etching process configurations

Immersion etching is used mainly for chemical milling. In cases for small parts, spray etching method is also used. The spray etching has limitations on large format products due to facility requirements and process uniformity problems. Etchants used are the same as the chemical blanking (mentioned previously).

They are: (a) Entire perimeter milling; (b) Partial milling; (c) Stepped milling; and (d) Tapered milling. Dotted line areas show before milling, and solid line area show after milling areas.

The entire perimeter milling is equivalent with etching used for reducing the overall dimensions or weight, and is often used for forged or cast products. The process does not require any masking and therefore is simple.

Partial milling is the equivalent of normal machine milling. However, the chemical milling can simultaneously perform multiple stepped milling by having independently exposed steps on one surface. This is called Group Milling.

Step milling is performed in repeated steps where the work is raised out of the etchant after the first area etching has reached its desired depth, then the following area is unmasked and immersed in the etchant again. The process of stepped milling is shown in [Fig.2].

Tapered milling is performed by slowly lowing and raising the work piece in and out of the etchant, at controlled rates. By varying the speed of immersion and extraction, not only linear tapered features but curvatures can also be created. In practices, complex shapes are created by combining all of these techniques.