Plastic v. aluminum v. plastic-aluminum construction in radios

My current job is designing electronics for use in mines. While searching for inspiration, I started looking at rugged radios, like those from Motorola, Kenwood and others. What surprised me was their hybrid design, an aluminum frame with a plastic shell. I was wondering if someone with more experience could tell me why?

I think I understand the plastic v. aluminum choice. Plastic is highly impact resistant, but flexible. Aluminum is very rigid, but weaker on impact. I would imagine that the hybrid design is partly to ensure a tight waterproof seal and give the product higher rigidity, while the plastic dampens impacts. Any insight?

Below is a picture from the FCC filing for a Motorola design. On the left is the radio. On the right is the electronics and aluminum frame that are inside.

Ruggedized products work by isolating the internal functions from an external shell, often coupling the parts through some flexible brackets, studs, membranes. The external plastic parts let the industrial designers do whatever they want, and the internal aluminum frame supports the electronics. Often the internal frames can be magnesium which (I think) has better impact-guarding properties and can be cast in thinner sections.
Minimal implemented a ‘ruggedized’ aesthetic and assembly very successfully in their LunaTik line of iPhone cases. In this example, consider the outer shell as the ‘cosmetic’ part, and the center webbed-springy-thing as the shock attenuation (same as the couplers or the cast metal body). MINIMAL Inc. Design | Engineering | Brand | Innovation Agency — Taktik

hope this helps.

Ripped apart and designed a few of these while I was at Moto. That internal frame is magnesium, not aluminum.

A few reasons for this design:

Drop properties - Magnesium has great structural properties but god awful finish properties. So the magnesium chassis acts as the structural member liked you mentioned and keeps the PCB from flexing. PCB flex is OK when you have really light drop requirements, but in cases of those big knobs and other components it will cause the solder joint to fail very quickly. For a lot of the mobile computers we designed we used a combination of those floating chassis with internal shock dampers as well so the plastic acts as the external shock absorber, then transfers that load to the internal shock mount which minimizes what hits the actual chassis and components.

Sealing properties - If you need to create flexible enough housing to seal the product against water, then it’s easier to compress a plastic housing around a rubber seal than it is to do it with a metal housing.

RF - For radios it mattered less (giant external antenna) but for the mobile computing products which had cellular/wifi/BT antennas the less metal you could incorporate the better. We did a number of products with insert molded metal housings which acted as a ground plane that the antennas reflected away from.

A few examples of different approaches we tried over the years for ruggedization:

This product had a combination of internal shock mounts, external overmold which acted as both an IP67 seal and shock damper. Looked really good and won a lot of awards, but by the time you make the overmold hard enough durometer to be durable, it’s actually not a very goo shock damper. We had to add additional plastic hit points at the top and bottom very late in the development process to address drop failures for 6’ to concrete. This is what you still sign today when the Fedex guy shows up (~10 years later)

This device used external overmold on a CNC machined aluminum frame. The overmold here wasn’t tied to the seal, and was mechanically interlocked with the frame so it could be a slightly softer durometer, but processing and quality control for this was a nightmare and the engineers probably still curse us.

Happy to geek out over other rugged construction methods.

Thanks Mike for the 200% improvement on my answer.

Thanks Mike! The ruggedization was what interested me. I’ve done a lot of plastics and castings, but never used on the same product.

Any references or other details I should be looking at?

If you have any specific Q’s you can let me know, I don’t know of much external references, most of this was just stuff collected over the years on different products. We really liked insert molding things…

I thought all designers loved insert molding…

Thanks for the offer. If I think of something else, I’ll be in touch.