Going from plastic to aluminum housing

Alright, so I have the Cerevellum Hindsight 35 now into production. The head-unit is injection-molded plastic. It’s a consistent 1.8mm thick. I’d like to make the head-unit out of aluminum. How thin can I make the aluminum? Does anyone have connections to a Chinese supplier that can form these? Even for mass-production, would it be CNC’d or stamped with secondary processes?

Also, the buttons are currently overmolded onto the head-unit body. Can you overmold aluminum?

The thickness is really going to depend on the process.

CNC’ing, while glamorous is still very expensive, especially in low quantities. You’ll save the cost of tooling, but your piece price is probably going to be at least 10X the price of plastic, and you need to take into account the design to “optimize” it for CNCing.

Casting might be a decent alternative, or casting + CNCing the final features.

I don’t see that part working out well as a stamping, since I’m imagining you have lots of internal features. Stamping would be great if you just wanted an external shell to protect over the plastic.

Overmolding on metal really doesn’t work with rubber. You can overmold hard plastic, but the rubber doesn’t have the adhesive strength to want to bond to metal. So you’d end up having to have an aluminum part with hard plastic overmold, and then a second shot of rubber. You’d be better off molding a discrete rubber keypad and inserting it from behind with some adhesive to hold it in place.

Is there a particular reason you want to move to metal?

They are not in China, but you should give Dielectrics a call.

With automation, I’m not so sure you’ll get a better price out of China. Also, Dielectrics has no problem working with product development. They will do things in onesies and twosies before doing several thousands. They can also advise whether machining or stamping would be ideal.

As for overmolding a TPE, if you have holes through the aluminum as secure points, any limited adhesion the material may have can be overcome.

I’ve done this with a rubber cover and a plexiglass window with holes arrayed around the perimeter to mechanically secure it and worked really well.

Instead of aluminum, I would investigate zinc-aluminum and magnesium alloys. You can feasibly get a .02" wall thickness with the right combination of material and design. I’m pretty sure AZ91D (die cast magnesium alloy) would fit your task.

Cell phone frames would be a good reference for die casting.

Can you expand on your reason for the switch to aluminum?

I’ve worked for a non-ferrous foundry for 3 years and I think your best bet would depend on your quantities, but die casting is usually the best as far as surface finish. High pressure die casting would be ideal, but maybe investment casting?

We do low pressure and gravity die casting and with both of those processes you really want at least 2mm wall thickness. Also unless you do investment or high pressure you have porosity to deal with (trapped air and gas) and if you want to machine… you might find there are sub surface holes which are usually exactly where you want to put a boss or thread.

Don’t cast a shape (blank, billet) with the plan to machine off 1mm to get your shape. It will always screw you up. If you’re machining, you want billet that you know 100% is structurally sound.

Azrehan

Does your foundry use graphite permanent molds (GPM) for zinc alloys? I’m investigating that for a small run part I’m designing at the moment. While I am keeping a uniform thickness to around 2mm because of lack of mold pressure, I’ve been reading that this process is better suited to reduce porosity than other gravity fed processes. In particular, I’d hope to use ZA-12. I’m currently waiting on feedback from potential vendors, but it would be nice to hear you comments or suggestions.

We don’t use graphite moulds. Just cast iron, shell and sand moulds for alloys. We use Beryllium copper for the brass low pressure moulds but that stuff is about $4000-$8000 per mould depending on size.

Apparently ZA-12 is similar to DA-12 which is one of the zinc alloys we use. It can be cast fairly thin but depends on the geometry and then gates and risers and venting in the tool.

For really thin parts you would really have to go high pressure die casting which is fairly expensive. A gravity tool is a lot cheaper but the surface finish isn’t as good.