Thin, Intricate Parts Develop from Photochemical Machining
When product manufacturers need thin flat parts made, they usually think of stamping or blanking. But those methods aren't the only ways to make parts from thin metals and other materials. Another process provides advantages not readily available from stamping and blanking--photochemical machining. When outsourcing the manufacture of thin metal parts, photochemical machining is a process worth examining.
Photochemical machining is sometimes confused with chemical milling. The two processes are quite similar because they both use chemicals to remove metal. And some of the same steps are required in both processes.
As the name implies, chemical milling removes material by chemical action rather than by conventional mechanical operations. The process is usually used on three-dimensional parts originally formed by another process, such as forging or casting. It is especially useful if the parts are irregularly shaped. The process can improve surface finish as well as eliminate cracks and other surface defects.
The amount of material removed by chemical milling depends on the amount of time the metal is immersed in the chemical solution. Uncontrolled, the chemical would eat away at the part's entire surface. But it is controlled. As with photochemical machining, areas not to be milled on the part are masked from the action of the chemical milling solution.
Whereas chemical milling alters the shape of formed parts, photochemical machining creates new parts from thin materials, rather than simply smoothing or altering parts formed by other methods. The process is sometimes called chemical blanking because, using chemical action, it cuts parts from sheet materials like mechanical blanking does.
The process requires a chemical-resistant image of the part to be placed on a sheet of material--usually metal. The material is then exposed to a chemical, referred to as an etchant, that dissolves all the material that is not integral to the part. Most such parts are similar to thin-gage stampings, but are usually flat with complex design features.
Sometimes photochemical machining is used to surface etch components with lettering or graphics. Rather than etching completely through the unprotected surface, the etchant works its way to only a certain depth in the material. As in chemical milling, the depth of etch is controlled by the length of time a component is immersed in an etchant, as well as the type of etchant.
Neither photochemical machining nor chemical milling should be confused with photoforming, which is the process of electroplating metals over a mandrel. The photoformed component is removed from the mandrel after the proper thickness of material has been deposited.
Broad Material Range
A broad range of metals and other materials can be photochemically machined. The complete list of suitable metals and alloys is lengthy, but all the common metals including aluminum, copper, zinc, steel, lead, and nickel are photochemically machined. Many exotic metals such as titanium, molybdenum, and zirconium can also be used with the process. Non-metallic materials include glass, ceramics, and some plastics. Because there are so many possibilities, it is best to discuss your need for a thin part with a photochemical machining job shop.
One common attribute necessary for these materials to be photochemically machined is that they must be thin, with a thickness between 0.0005" and 0.0900". Material thickness is usually related to the tolerance needed on the finished part.
Obviously, the materials must also be flat, at least for processing. Parts photochemically machined while flat can later be bent to shape, and assembled to other components. In fact, sometimes photochemical machining is used to score parts along bending lines to ease the forming process.
Very high tempered or brittle materials are excellent candidates for photochemical machining because traditional mechanical action can cause breakage or stress-concentration points. Chemical blanking also works well on springy materials, which are difficult to punch.
Products made by photochemical machining are found in the electronic, automotive, aerospace, telecommunication, computer, medical, and other industries. Typical components include filters and screens, gaskets, lead frames, contacts, connectors, probes, and flat springs.
Photochemical Machining Benefits
When looking at certain parts that have been made by photochemical machining, it is obvious they would be very difficult to produce any other way, either because they are too intricate, too thin, or both. But that's not the case with all photochemically machined parts. Some components could be produced with other processes. The reason they are not is because there are some key benefits derived from photochemical machining.
One benefit is its relatively low cost per unit, especially at low production volumes. Though photochemical machining adds costs for cleaners, photo-resists, and etchants in the per piece cost, its tooling is very inexpensive when compared to the punches and dies for stamping or blanking.
This cost advantage increases as part design complexity increases. That's because increasing design complexity has a minimal effect on photochemical machining costs.
But the cost of punches and dies for stamping increases significantly with part complexity. Of course, as production quantities rise, the cost of hard tooling in stamping or blanking is amortized over more units, making those processes more cost competitive at high volume.
Another advantage of photochemical machining is the quick turnaround time compared to other processes. The main reason is the speed with which photo-tools are made. Lead times are often just days, rather than the much longer times required by processes that require hard tooling. This is especially important in product development, where prove-out of prototypes has a significant influence on time to market.
It is much easier to tweak a part's design with photochemical machining. Changes to tooling are simple, fast, and inexpensive. This gives designers a chance to try out more prototypes. And it simplifies correcting deficiencies found in the design.
Some design changes may not even require modifications to the tooling. Simple alterations to the photochemical machining process, such as time of etch or type of etchant, can change features such as hole sizes and depth of etch.
The process offers other advantages in prototyping. For speed and cost considerations, prototypes are often made using a process different than will be used during production. With photochemical machining, final parts are produced in the same manner as the prototypes. There are no variations from prototype to production that could result in characteristics in the final components that were not present during prototyping.
Also, photochemical machining does not affect the properties of the material being used. It will not change the hardness, grain structure, or ductility of metals, and it is burr free. On the other hand, stamping-type process can impart stresses in the components. And laser cutting can create heat-affected zones.
The process can also help simplify the part design process. There are often instances where a designer will need a large part with a repeated pattern. Because tooling is made by the photographic process, patterns can be reproduced over and over without repetitive redrawing to fill blanking size.
Engineers considering the use of photochemical machining should consult with a photochemical machining job shop on design issues to ensure that components are designed to take advantage of the processes many advantages. Some of the best photochemical machining job shops in the United States are listed at the end of this paper.
Another good source of information about photochemical machining is the Photo Chemical Machining Institute. This is an international organization, headquartered in San Clemente, California, consisting of manufacturers of photochemically machined parts, users of such parts, and suppliers to the industry. It coordinates the efforts of member companies to develop solutions to mutual problems. An excellent source of information on photochemical machining, such as design considerations, the organization can be reached at (714) 493-5702.
Machining with Chemicals
One of the most interesting aspects of photochemical machining is its use of a coating to resist the chemical machining process. This coating is referred to as a maskant--usually a photo-resist maskant for reasons that will be explained.
But applying this coating is the second step in the process. The first step is to clean the material to be machined to ensure it is free of any surface contaminants. This permits the photo-resist to uniformly coat and adhere to the material. A variety of cleaning methods are used including degreasing, pumice scrubbing, electro-cleaning, or chemical cleaning.
Though there are variations, photo-resist maskants are the most common because their precision is the greatest. That's why alternatives, such as cut and peel maskants, are more likely to be used in chemical milling rather than photochemical machining. As the name implies, cut-and-peels are removed from areas to be etched by cutting and peeling away areas that are to be left unprotected from the etchant. Cut-and-peel maskants are often used for chemical milling aircraft, missile, and structural parts.
Photo-resist maskants are applied in extremely thin coatings to enable the fine detail that is usually required of parts made by photochemical machining. The coated material is then exposed to intense light.
However, the light is only permitted to reach the photo-resist in the areas to be protected from the etchant. Light is blocked from specific areas by a photo-tool, a masking device produced photographically on film. The photo-tool contains the detailed image of the part, or parts, to be machined.
Typically, areas to be etched are opaque on the photo-tool, and areas to be protected from etching are transparent on the photo-tool. Opaqued areas block light exposure and remain unprotected by the photo-resist. They will be "machined" away by the etchant.
The photo-resist exposed to the light through the transparent areas becomes hardened against the etchant after developing--photo-hardened.
Photo-tools are made by photographing the part image on photographic film. It is low cost and has an excellent ability to reproduce fine detail. Other media, such as chrome on glass, are used in some applications, but not as frequently.
Most often, photo-tools are used in precisely registered pairs--one on top, one on the bottom, with the material to be processed sandwiched in between. This permits the material to be etched from both sides, minimizing undercutting of the photo-resist and producing straighter sidewalls.
After the photo-tools are in place, the assembly is exposed to intense light for up to 30 seconds, depending on light intensity and the type of resist used. This exposure prepares the photo-resist to be hardened with a developing solution, in the areas to be protected from the etchant.
The exposed image is developed by immersion or spraying. Each photo-resist has its own developing solution, such as water, alkaline solution, hydrocarbons, or solvents. The exposed material is then washed to remove the unexposed photo-resist on the areas to be chemically etched.
Some photo-resists are baked after developing to remove residual solvents or further harden the resist, both of which improve the chemical resistance of the protected image.
The prepared material is now ready for etching, or the dissolving away of the material not protected by the photo-resist. Various etchants are available for different materials. The right choice depends on such issues as cost, quality, depth of etch, and speed of material removal. Photochemical machining job shops have the expertise to select the proper etchant for each application.
After etchants are applied by spraying or immersion, the parts are rinsed and dried. Protective-resist is removed from machined parts with chemicals, or with mechanical techniques along with chemicals.
If you think you have an application that can benefit from photochemical machining, you can locate suppliers by clicking on the link below.
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