Photochemical machining (PCM) involves creating engraved images or designed cutouts using a computerized process that exposes thin sheets of metal to light and various chemicals. Industries can use PC milling on virtually any type of sheet metal including aluminum, brass, or copper, along with nickel and silver. Photochemical machining techniques may be used to produce delicate electronic components, medical implants, or extremely intricate etchings. Rudimentary chemical etching using citric acid dates back thousands of years.
Engineers typically create the desired image using computer aided design software, commonly called CAD drawing software. When used for cutting particular parts, technicians replicate this pattern in columns and rows forming multiple images on one screen. The computer transfers the image to laminate film that has a Mylar® base and silver emulsion coating. Before photochemical machining, the metal of choice undergoes a thorough cleaning process, which ensures adhesion to the photographic film. After being cleaned with a diluted solution, the sheet undergoes a water rinse and a heated drying process.
When machining a complete metal cutout, technicians laminate, or sandwich, the piece of metal between two pieces of phototool film. Only one side of the metal needs to be covered with film during engraving or etching. Technicians laminate the metal using a dry roller or wet dip method. They use cameras during this process to ensure proper metal and film alignment. The laminate and phototool match each other identically in dimensions.
The roller method involves passing the metal through a roller where the machine inserts the sheet between two pieces of laminate. Lamination requires contamination free environments and elimination of possible air bubbles. The wet dip method entails dipping the metal into a liquid film and baking the sheet in an oven to harden the film. The photochemical process continues by exposing the laminated metal to high intensity ultraviolet light, hardening the image on the phototool.
After exposure, technicians expose the laminated metal to developing solution, which removes any undeveloped laminate. Via a conveyor belt, the laminated metal enters a chamber lined with spray nozzles positioned above and below the conveyor. Hot etching acid sprays the metal from one or both sides, depending on the desired design. The acid dissolves the metal not covered with laminate without leaving rough edges or altering the metal quality. This step in the photochemical machining process creates the finished image formed on the CAD drawing.
The part now undergoes a water rinse and exposure to a stripping solution that removes any remaining laminate. The sheet goes through one final water rinse followed by hot air drying. Technicians may use microscopes for a final inspection as a means of quality control.