As of 40 years ago, the last time I looked at it in detail, the largest extrusion press in the world was in Arkansas and could accept a die with a hole about 13 inches in diameter. It could make any part you could fit in that circle (even if you had to roll it up: big flat extrusions can be extruded as a partial circle or a spiral and unrolled while they are still soft). That's no problem for you.
The minimum order for that press was about 5000 lbs, which is basically one stroke. There are smaller presses that can accept smaller minimum orders, but you have to make it worthwhile to change the die and insert the ingot.
Most any extrusion house will be glad to provide a design guide that includes suggestions to minimize complexity and cost, and also includes achievable tolerances.
In general, a closed hollow extrusion will cost you a lot more, for the extrusion and for the die, than something that can generally be described as an angle. Fear not; you can design an angle that's self-conjugate, so you can take two pieces of the same length and slide them together to form a closed perimeter, and use the end plate retaining screws (self- threaded into C shaped features) to lock them together. You will still need end plates to make a closed box; they can be stamped sheet metal or molded plastic.
As-extruded shapes are too soft to handle much without damage, so the extruder will recommend a standard temper.
Do not forget to include the cost of cutting the extrusions to a precise length, and especially the cost of deburring the cut surfaces. Some houses will do this for you, but you still get to pay for it.
Don't forget to make room to store a couple thousand pounds of (20 feet long, probably) extrusions from the mill, and protect them from bird poop and roof leaks until you use them up.
... By now you are having second thoughts, right? Extrusions can pay for themselves if they eliminate parts like spacers and standoffs for circuit boards (but you have to design the board to fit into the extrusion and keep the components away from the slot features in the exrusions), and especially if you can use them for more than one end product, so before rejecting them, do take a look at your overall product roadmap and do preliminary designs for the next couple versions or derivative products, and include features for those future products if you can reasonably predict what they will be like.
You may not need fins for cooling, but you might consider adding stubby fins or linear bosses to the exterior surface, so you don't have to worry about tiny scratches being obvious, and so you have trim/finish options like color anodizing the entire part and then sanding or milling off part of the fin/boss surface to add bright highlights or a durable logo.
Mike Halloran
Pembroke Pines, FL, USA
