The Landscape of Electronic Housings: Potting, Injection Molding, and Macromelt
Housings are the unsung heroes of consumer electronics. The electronics might pass all tests, and the embedded software might be bug free, but to put your product in the wild it will need a sturdy mechanical housing to keep everything together.
Like any other facet of product development, housings are surprisingly complex beasts. There are several different ways you can go about developing a secure home for your circuits, bits, and bytes. Which you employ will depend on your anticipated manufacturing volume, cost constraints, aesthetic needs, time constraints, and more. And though the following list isn’t exhaustive, some of the most useful technologies for developing mechanical housings are potting, injection molding (clamshell and insert), and Macromelt.
Potting is one of the simplest ways to make your electronics robust enough for the world outside of the lab. You simply place your electronics in a small box, and then fill that box with a sturdy gel (such as silicone or epoxy). This makes your electronics more resistant to shock (think drops and bumps), and also keeps moisture at bay.
The simplicity of potting has advantages and disadvantages. On one hand, potting provides good durability with a low up-front (tooling) cost. On the other hand, the assembly cost and per-part cost are relatively high. This makes potting suitable mostly for low-volume production runs (such as quantities less than 100). An additional drawback of potting is that, once your electronics are potted, they are completely inaccessible. As such, it is nearly impossible to debug or rework any malfunctioning units.
One of the most popular technologies for manufacturing electronic housings is injection molding. In this process, highly pressurized, melted plastic is rapidly forced (shot) into a metal mold (the tool) in the shape of the desired housing. When the plastic cools and hardens, out comes the container. Often, two of these molds are designed such that the resulting plastic pieces snap together, enclosing electronics within. This is known as the clamshell design.
In addition to snapping together, injection molding can also allow you to ultrasonically weld the pieces together, or to assemble with screws. The versatility of this technology makes it possible to create housings with complex geometries. As such, injection molding is the predominant technique for creating housings driven by industrial design (like most modern consumer electronics).
A variant of standard injection molding is insert injection molding. In this process, electronics are inserted into the tool before the plastics are shot in. As such, the plastic cools around the electronics, creating a slick, integrated look.
Injection molding properly is challenging. The case must be designed carefully in CAD and optimized for the injection molding process. Tooling costs (creating the metal molds) can be very high. However, the per-part cost for injection molded components is very low. As such, injection molding is a popular choice for high-volume runs of cosmetic housings.
Macromelt is a housing development process combines elements of potting and insert injection molding. Like insert injection molding, a melted plastic is forced into a mold around electronic components. However, the plastics are at a much lower pressure, and the mold is generally much simpler in design. Macromelt is often used to reinforce connectors and wiring, and for enclosing simpler products (like USB sticks).
Macromelt is also middle-of-the-road in pricing and features. Tooling cost is more expensive than potting, but cheaper than injection molding. The per-part cost is more than injection-molding, but less than that of potting. Because of this, Macromelt can be a good choice for middle-volume production runs (such as between 100 and 10,000 units).
Which technology you utilize will depend heavily on how many units you are manufacturing, how robust your mechanical design is, how sturdy the case needs to be, how much you can afford to spend, and several other factors. We put together a handy chart to help you navigate the possibilities.