from kemactech.com
Photochemical Etching Process:
When starting photo chemical maching, thin metal sheets are cleaned with chemical solutions and laminated with a very thin layer of photo-resist film. Using a CAD created Photo-Tool, images are printed on both sides of the sheet using calibrated Ultra Violet lights.
Printed sheets are developed in chemical solutions to expose the metal surface in the printed areas. The sheets are then sprayed with proprietary solutions so that the exposed metal dissolves away. The photo-resist film used in the photochemical etching process protects the material in the areas that were not washed away in the developing process. After etching, the residual photo-resist film is stripped in alkaline solutions. Process parameters at each stage are controlled to obtain the desired dimensions and finish. Upon completing a dimensional and visual inspection, components are ready for any necessary secondary operations such as forming, plating, heat-set, passivation, anodizing, etc.
I don't know if this is how they do it with pipes....but it sure made my head hurt reading it.