Basic Principles of Photochemical Machining
By Richard (Dick) Beaupre
Lincoln, Rhode Island
The process of photochemical machining (PCM) is recognized by the metalworking industry as one of several effective methods for metal parts fabrication. The technique, also called photo etching, chemical etching, and chemical blanking, competes with stamping, laser cutting, and electrical discharge machining (EDM). It uses chemicals, rather than mechanical or electrical power or heat, to cut and blank metal.
Photochemical machining has several distinct advantages over these other processes. Low tooling costs associated with the photographic process, quick turnaround times, and the intricacy of the designs that can be achieved by the process are some of the advantages, as are high productivity and the ability to manufacture burr-free and stress-free parts. Of paramount importance in using this process are the cost savings associated with generating prototypes. For as little as $250, a test prototype can be made. Because it is chemical rather than mechanical, the process does not alter the physical characteristics of the metal.
However, many engineers do not completely appreciate the capabilities of the process. It has been the mission of the Photo Chemical Machining Institute (Littleton, Massachusetts) to promote the process, to educate engineers throughout industry as to the advantages of PCM, and to develop and enhance the technology via its technical publications and biannual technical conferences. For more from the Institute, go to www.pcmi.org
It is also true that many designers, manufacturing engineers, tool and die people, purchasing groups, and model shops recognize the advantages that PCM has to offer. Some of the benefits of this process are the ability to make the "impossible" or complex part that is beyond hard tooling capability, and to fabricate it of "dead soft" or "full hard" metals, without burrs or metal distortion. Another significant advantage is response time: an order may be filled in days. Hence, PCM can fill a need in pre-production runs or assembly line setup while hard tooling is being prepared.
Three Distinct Operations
Photochemical machining may be broken down into three distinct operations: artwork generation, photographic tooling, and blanking or etching of the metal. The artwork generation step is usually done in a CAD file. This is downloaded to a laser imager that then exposes the desired image onto photographic litho film. Blueprints may be submitted in lieu of a CAD file.
Most PCM job shops provide in-house artwork and photographic tooling departments to prepare the desired CAD file. The exposed silver halide film is then developed. Areas of the film exposed by the laser imager become opaque to light, whereas unexposed areas become transparent. It is through this process that the desired image is formed. This becomes the master photo "tool" that is used to apply an image to the metal blank.
The etching or blanking step is the process by which chemicals are sprayed onto the surface of the metal and, through chemical oxidation and dissolution, the part is formed. Before this step, the metal must be prepared so that an image can be applied to it using the previously prepared photo "tool."
One of the more confusing aspects of the PCM process is that there are two photographic or imaging processes. First is the making of the permanent photo tool, and second, the application of a photographic film, called a photoresist, to the metal blank by hot lamination. The photoresist is purchased in 1000-foot rolls and is applied to both sides of the metal blank using a hot rubber roll laminator. Once the metal blank has a photographic film applied, it is imaged or contact printed on both sides using UV light that has been passed through the transparent areas of the photo tool.
These areas are cured by the UV light. When developed in an aqueous solution, they remain on the metal, thereby protecting these areas from being affected by the chemicals in the subsequent etching step. The unexposed areas (opaque on the photo tool) are washed away, thereby exposing the underlying metal.
The chemicals, or etchants, are then sprayed on both sides of the imaged metal blank through a conveyorized spray-etching machine. The chemical oxidation process dissolves the unprotected areas. After the desired part is formed, the cured photoresist that remains must be removed in a stripping solution. The parts are then inspected and, according to customer requirements, sent to secondary operations such as forming and plating.
The process lends itself to high productivity by stepping and repeating the image on the master photo tool, which is used to image large metal blanks of up to 21 inches x 24 inches, as previously described. Obviously, the smaller the part, the higher is the piece count on the metal sheet, thus permitting economies of scale. The multiple parts are held on the sheet via small tabs that are subsequently removed. In some cases, a customer may order parts without tabs. These require special handling techniques, which can lead to a trade-off in part cost.
As with any process, there are limitations. Theoretically, any metal thickness can be chemically blanked. However, the process becomes expensive and impractical at a thickness greater than 0.120 inch, because of greater etch time and lower productivity.
Additionally, the process is such that tolerances are generally 10% of the metal thickness. A 10-mil-thick metal blank will generally be limited to 1 mil part tolerance. Also, the limitation on creating a hole or slot is normally one-and-a-half times the metal thickness. A 10-mil-thick metal blank will generally be held to blanking holes and slots no smaller than 15 mils. It becomes more difficult to spray the etchant into lesser dimensions as the depth of etch increases.
Another factor in the PCM process is that which is known as "undercut." The etchant chemical does not discern a difference between the surface of the metal and the sidewalls of the part, which is forming during the blanking operation. As we etch down, we also etch laterally. This is called "undercut" and is similar to shrinking. It is compensated for when preparing the photo tool. The undercut is defined by a term called "etch factor," which is the depth of etch divided by the lateral etch. The lateral etch is measured by the change in the part line width from the original artwork.
An etch factor of one to two is considered within the process tolerance. Also, due to the lateral etching, the smallest dimension of metal that can be processed (called a "land") is usually one-and-a-half times the thickness of the metal.
Practically all metals can be etched. Stainless steels, iron and copper alloys, aluminum and its alloys, and nickel and its alloys are etched in a reagent called ferric chloride, an almost universal etchant. More exotic alloys based on titanium, and some high heat-resistant and chemical-resistant alloys use somewhat exotic etchants based on hydrofluoric acid. Glass and ceramics also use hydrofluoric acid-based etchants.
Job Shop Specialization
Approximately 40 PCM job shops in the United States are members of the Photochemical Machining Institute. These shops, like those that perform stamping, EDM, and laser cutting, serve many industries. Most PCM job shops overlap to some degree regarding the type of products and industries served. There is, however, a great deal of product and technological specialization from shop to shop.
For example, some job shops may specialize in surgical or automotive parts. Others may concentrate in electronic components, flexible circuitry, and instrumentation, whereas others may specialize in technologically innovative areas such as the etching of exotic alloys and TV shadow masks. Some may specialize in nameplates or decorative products. The PCM industry is varied and all-inclusive and, in many cases, is a niche market. Photochemical machining cuts across all industrial areas that make use of blanked metal components.
Today, there are many more requirements in setting up a PCM job shop than for most of the competitive metal blanking industries. Permit and environmental considerations can be onerous. Waste, a byproduct of the process, must be properly handled and legally disposed. Rinse water must be treated to conform to all local POTW and EPA regulations. The permitting process is involved and quite complex in the United States.
In spite of the specific problems related to the PCM industry, the process offers manufacturers a wonderful method of obtaining short and long runs at minimal tooling cost while being very competitive with other metal blanking processes.
For more information from the Photo Chemical Machining Institute, visit www.pcmi.org
ChemArt, a photochemical-machining job shop, specializes in the manufacture of precision etched parts for electronic and industrial applications that demand exacting tolerances. The ISO 9001-certified firm provides photochemical machining services to the aerospace, automotive, computer, electronics, defense, and medical industries. Primary services include design and engineering assistance, development of precision artwork, and the use of state-of-the-art etching equipment to comply with rigid engineering specifications. The company also offers secondary operations such as forming, heat treating, passivating, and plating.
Dick Beaupre is the founder and CEO of ChemArt Company, a PCM job shop located in Lincoln, Rhode Island. He is a former president of the Photo Chemical Machining Institute and has served on its board for 20 years.
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