You’ve designed a new electronic product. You’ve completed the functional design, and everything works as intended. Now it’s time to design the enclosure. But this brings up a couple of fundamental questions. How will you manufacture it? And what materials will you use?
Let’s consider the materials first. The most basic choice is probably between metal and plastic.
Metal is stronger than most plastics, but some engineering plastics are comparable to aluminum in strength. Because a metal enclosure is electrically conductive, it helps to block radio frequency and electromagnetic interference (both incoming and outgoing). If the enclosure is electrically grounded, it can provide shock protection. While it is possible to add a conductive surface coating to a plastic enclosure, it adds cost and an additional step in manufacturing.
The main metal choices are aluminum, carbon steel, galvanized steel, and stainless steel. Fabricating a metal enclosure can be done in several ways. Probably the most obvious is from sheet metal; other fabrication methods include CNC machining, die-casting, and 3D printing.
Plastic is inexpensive and lighter than metal. Since it is nonconductive, an energized wire touching the inside will not make the enclosure electrically live and a shock hazard, or cause a ground fault and trip a circuit protection device.
Plastic is generally unaffected by water, but exposure to ultraviolet light (i.e., sunlight) will degrade some plastics — particularly polypropylene and low-density polyethylene (LDPE). This can be ameliorated by adding UV stabilizers to the plastic or painting the finished enclosure.
Fabricating an enclosure from plastic can be done with injection molding (the most inexpensive choice for large-volume production), CNC machining, or 3D printing.
Sheet metal is robust, scalable, and durable, and sheet metal parts can be made directly from CAD models. Sheet metal generally has a significantly lower material cost than CNC machining, if only because it does not involve removing most of the starting material. It involves multiple steps — cutting, bending, fastening, and sometimes welding or riveting. While initial setup costs can be high, the price per piece drops quickly as the volume increases.
CNC machining makes it possible to build complex structures that might take considerable tooling and multiple steps if made from sheet metal and can make parts to great precision. “The tolerancing between sheet metal and CNC is very different,” says James Hayes, applications engineer at Protolabs. “You get better tolerancing, and you get exactly what you want in regards to that.” In addition, he points out, since a CNC part is machined from a single piece of metal, it has no open seams.
Since CNC machining is generally more expensive than sheet metal, it is used only where really needed. For most applications, sheet metal is more appropriate.
When it comes to metals, aluminum (especially 2007, 2011, and 6020) machines easily, and medium-carbon steels also machine well. Both high-carbon and low-carbon steel can present problems, while stainless tends to be difficult, although AISI grades 303 and 416 are the easiest.
Many plastics are suitable for CNC machining, although it is best to check, as some are not: thermoplastics can be difficult to machine because they tend to melt and stick to the tool. However, some high-temperature thermoplastics, such as PEEK and PEI, can be machined and produce parts that can withstand extreme temperatures. Composites can also be difficult because (depending on materials involved), they can cause excessive tool wear.
Additive manufacturing (popularly called 3D printing) can be used for both metals and plastics. When used with metal, it can replace multi-part assemblies that require additional steps, like brazing and welding, with a single component. It can also eliminate extra material from well-designed parts and makes it possible to develop geometries — for example, with complex internal channels — that are not possible with other methods.
Available 3D printing technologies include DMLS (Direct Metal Laser Sintering), SLS (Selective Laser Sintering), MJF (Multi Jet Fusion), Stereolithography, Carbon DLS, and Polyjet. There are other methods, but they will not be discussed here.
Direct Metal Laser Sintering builds fully functional metal prototypes and production parts in seven days or less. For enclosures, it can be used with stainless steel and aluminum; for other applications, it can be used with copper, titanium, cobalt chrome, and Inconel. It can build lightweight parts because of its ability to produce incredibly complex geometries such as mesh-like structures, honeycombs, or hollowed-out features. Protolabs has recently developed the ability to use DMLS with copper.
Selective laser sintering (SLS) produces accurate prototypes and functional production parts in as fast as one day. Multiple nylon-based materials are available, which create highly durable final parts.
Multi Jet Fusion (MJF) uses an inkjet array to selectively apply fusing and detailing agents across a bed of nylon powder, which are then fused by heating elements into a solid layer. After each layer, powder is distributed on top of the bed, and the process repeats until the part is complete.
Stereolithography (SLA) is used to create concept models, cosmetic prototypes, and complex parts with intricate geometries in as fast as one day. A wide selection of plastics, extremely high feature resolutions, and quality surface finishes are possible with SLA.
Carbon DLS (digital light synthesis) creates functional, end-use parts with mechanically isotropic properties and smooth surface finishes. It can be used with both rigid and flexible polyurethane materials for high impact-resistance components.
PolyJet builds multi-material prototypes with flexible features and complex parts with intricate geometries in as fast as one day. A range of hardnesses (durometers) is available, which work well for components with elastomeric features like gaskets, seals, and housings.
With so many choices, we strongly advise consulting with your chosen prototyping/production house at the earliest possible date. They have expertise that will help you select the best and least expensive material and production method. For more detailed information on materials, you can consult Protolabs’ Material Comparison Guide.