Photo chemical machining

Photochemical machining (PCM) is a method of fabricating metal components using reactive etchants to corrosively oxidize selected areas of metal. Also known as photo etching or chemical etching, this process emerged in the 1960s as an offshoot of the printed circuit board industry. Photo etching can produce highly complex parts with very fine detail accurately and economically.

This process can offer economical alternatives to stamping, CNC punching, laser or water-jet cutting or wire EDM for thin gauge precision parts. Tooling is inexpensive and quickly produced.

Photo etching can be used with most metal alloys from .0005” to about .080” thick.

The Process

The process starts with a CAD file showing the “plan” view of the part. The image of the part is duplicated as many times as possible to fit a sheet of metal. The file of multiple images is printed onto optically clear and dimensionally stable Mylar, usually to a laser photoplotter or sometimes to an imagesetter or inkjet printer. The “phototool” consists of two sheets of this film showing negative images of the parts (meaning that the area that will become the parts is clear and all of the areas to be etched are black). The two sheets are optically and mechanically registered to form the top and bottom halves of the tool.

The metal sheets are cut to size, cleaned and then laminated on both sides with a UV-sensitive photoresist. The coated metal is placed between the two sheets of the phototool and a vacuum is drawn to ensure intimate contact between the phototool and the metal plate. The plate is then exposed in UV light that allows the areas of resist that are in the clear sections of the film to be hardened. After exposure, the plate is “developed,” washing away the unexposed resist and leaving the areas to be etched unprotected. The etching line is a multi-chambered machine that has driven-wheel conveyors to move the plates and arrays of spray nozzles above and below the plates. The etchant is typically an aqueous solution of acid, frequently ferric chloride that is heated and directed under pressure to both sides of the plate.

The etchant reacts with the unprotected metal essentially corroding it away fairly quickly. After neutralizing and rinsing, the remaining resist is removed and the sheet of parts is cleaned and dried.

Applications for Photo Etching

Thin gauge (under .050”) parts in a broad range of alloys are candidates for photo etching.

Industrial applications include fine screens and meshes, apertures and masks, battery grids, fuel cell components, sensors, springs, pressure membranes, heat sinks and thermal transfer devices, flexible heating elements, RF and microwave circuits and components, semiconductor leadframes, motor and transformer laminations, metal gaskets and seals, shields and retainers, electrical contacts, encoders and light choppers, EMI/RFI shields, and countless more.

Photo etching is used to produce parts as simple as washers and shims to the most intricate and detailed designs for model building, ornamentation and jewelry.

Economics of Photo Etching

Phototooling is quick and inexpensive to produce. Most phototools costs less than $350 and can be produced in two days or less. Unlike “hard” tools such as stamping and punching dies, phototools are exposed only to light and therefore do not suffer wear.

In photo etching the unit of labor is the sheet. Therefore, it is most economical to plan the largest sheet size possible consistent with the size and dimensional tolerances of the part. The more parts per sheet the lower the unit labor cost per part.

Material thickness affects costs as a function of the length of time to etch through. Most alloys etch at rates between .0005” and .001” of depth per minute per side.

In general, in common alloys of steel, copper or aluminum in thicknesses up to about .020”, part costs will approximate $0.15-0.20 per square inch. As the geometry of the part becomes more complex, photochemical machining gains greater economic advantage over sequential processes such as CNC punching, laser or water-jet cutting, and electrical discharge machining.

Due to the cost of hard tooling for stamping and fine blanking, significant volume is required to justify the expense. Some parts, such as semiconductor leadframes, are so complex and fragile that despite volumes in the millions of pieces can only be produced by photo etching.

Other Fabrication Processes

The spectrum of sheet metal fabricating techniques includes stamping, fine blanking (coining), CNC punching, laser cutting, waterjet cutting, electrodischarge machining (EDM), electrochemical machining, photochemical machining, and electroforming.

Stamping and fine blanking require hard tools which can cost many thousands of dollars and may require weeks or months to produce. These methods are best suited to parts required in high volumes over extended periods of time.

CNC punching, essentially computer guided die stamping, is a relatively fast and flexible process that is well suited to “sheet metal” applications for larger parts often used as covers or housings for equipment. As a “sequential” process –meaning that each cutting action affects a very localized area, the more complex the part, the more time required to produce.

Laser and waterjet cutting have emerged in the last 25 years as alternatives to stamping and punching. The primary advantage is that no hard tooling is required and parts can be programmed directly from CAD files. Laser can be very fast and is capable of producing complex shapes in a variety of materials. By its nature, laser cutting produces significant levels of heat that can damage parts. Waterjet cutting uses abrasive, water-borne slurry under very high pressure to literally blast away material. It is well suited to cutting thick materials. These are also “sequential” processes, and the more complex the part, the more time is required.

Electrodischarge machining is another sequential process that can produce complex shapes in electrically conductive metals. The process is capable of intricate detail but is relatively slow.

Electroforming is an accretive process that deposits molecules –often of nickel or gold in solution onto an electrically charged mandrel or form. This process is capable of extremely precise detail but is relatively slow.

Resources for Photo Etching

Professor David M. Allen, PhD [Cranfield University] [http://www.cranfield.ac.uk/sas/index.jsp?id=redirect] Cranfield, England

White Paper by Dr. David Allen: Photochemical Machining: From Manufacturing’s Best Kept Secret to a $6 Billion Rapid Manufacturing Process [http://conardcorp.com/pdf/Photochemical%20machining%20CIRP%202004%20with%20columns%20v.23.pdf]

The Photochemical Machining Institute [http://www.pcmi.org]

Design Guide for Photo Etching [http://conardcorp.com/pdf/DesignGuide.pdf]