There are several standard options available for cosmetic finishes on modified enclosures, all applicable to custom and production enclosures too. For any contract finishing option the price and quality will vary so shop around, get references and be sure to see samples of their work first.
Anti-corrosion coatings: These are strongly recommended for any steel enclosure and also for any die-cast or aluminum enclosure not being otherwise coated. The options for anti-corrosion coatings have become limited by the near-universal adoption of the ROHS (Restriction Of Hazardous Substances) directive which bans or strictly controls the use of many toxic materials. Cadmium, lead, mercury and hexavalent chromium are among the materials that are no longer acceptable. The most common acceptable anti-corrosion coating for steel (and aluminum if a conductive coating is required) is zinc chromate, which contains the acceptable trivalent chromium. Zinc chromate is most commonly applied as either clear or yellow chromate, where the yellow gives more corrosion resistance but the clear has a better cosmetic appearance if paint isn’t applied over it. Neither chromate finish will hide any cosmetic flaws.
Anodizing: Aluminum enclosures can be anodized, a process with a range of physical properties and many colors available. Anodizing is non-conductive, which can be an issue if grounding and/or an EMI shielding contact is required for the enclosure. Anodizing has a couple of significant limits – it will not hide any cosmetic flaws and no ferrous inserts can be present during the process. Ferrous inserts can be installed afterwards but this may be an appearance problem unless they’re covered by a label or something else.
Plating: There’s a wide range of plating options, with the most common for metal enclosures (aside from the trivalent chromates) probably being bright nickel and black oxide. Some useful information about the current plating options can be found here.
Paint: If you have access to a professional grade spray gun and booth, you may be all set. If not, there are still a lot of paint finishing options. Hardware store grade spray paint will yield less than professional results, just to get that out of the way. There are contract finishing houses that can paint a single piece, small run or large production run, whatever you need.
Paint can be divided into wet and dry types. The wet type is either solvent or water-borne and is the more traditional type of paint, generally applied by spraying. Filling surface defects and masking selected areas is straightforward but does add cost, and curing is accomplished either at ambient temperatures or with a fairly low temperature bake. (Note that this bake temperature is too high for most plastic enclosures.) Dry paint is applied as a powder held onto the part by an electrostatic charge and then baked at a fairly high temperature. The range of colors, color effects and surface textures available with this powder-coating method is impressive, but surface defect filling is more difficult due to the required baking temperature and masking is more difficult too. Talk to your vendor about what they can do. Also, some enclosures (often plastic) will have integral environmental seals, which must be masked for any painting process. Threaded areas, inserts and grounding contacts must also be masked, as well as any precision surfaces whose dimensions will be altered by the paint thickness. Some plastics can be successfully powder-coated with some of the lower temperature curing powder coatings, but few off-the-shelf plastic enclosures are suitable for this.
If contract paint finishing houses are beyond your budget, you can try making a deal with a local auto body place to do the spray finishing for you. In that case you may need to work on your own with any special masking materials such as thread plugs and caps, but the tradeoffs could be worthwhile.
This concludes the article about working with off-the-shelf enclosures for electronic products. A new topic will start next week.
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