Man I can't get over that new Mac Pro. And while we all know the "thermal core" part is made from extruded aluminum, how is the exterior (which is also aluminum) made? It's obviously not extruded, as it's got that inward-curving lip up top, and it wouldn't make sense from an efficiency standpoint to CNC-mill the entire thing out of a billet; there would be too much waste. Any guesses?
According to Don Lehman, it's made using the production method known as impact extrusion. Conceptually, the process is more similar to blowmolding than proper extrusion, except a metal punch takes the place of compressed air, and the material used is metal rather than plastic. Here's a quick look at impact extrusion as performed by Illinois-based Best Metal Extrusions, on a product significantly smaller than the Mac Pro. (Looks like a cigar holder, but your guess is as good as mine.)
Here's something a little closer in size to the Mac Pro:Okay, so what's going on here? As you can tell by seeing the guy in the first video inserting the slug with his bare hands, it goes into the die cold. A punch shaped like the negative space of the final object slams downward and, with brute force, forces the material to fill the space of the die. This is why I say it's conceptually similar to blowmolding, and in fact it's how SIGG makes their fee-yancy aluminum bottles.
Why use impact extrusion versus regular extrusion? Because you can get a closed surface (like the bottom of the SIGG bottle) integrated with the sides; with regular extrusions you'd just get a tube. In the case of the Mac Pro, the additional surface is the lipped top, and we assume it goes through a CNC mill afterwards for the top cutout, the purty bevel, the connector cutouts and the nice finish.
Rendering or no, that bevel looks pretty hot
In the first few minutes of this next video, you can see a horizontally-oriented press using impact extrusion to whip out batteries. (The vid is four-minutes-plus, but you needn't watch more than the first few seconds to get the idea.)
The thing I love about seeing impact extrusion in action is how fast the material kinda "squirts" up out of the die. Check out this vid of shiny aluminum cans being made by the same company that does the batteries, Beijing Holland Trading Co.:
In any case, back to the Mac Pro—this thing looks suh-weet in video: