Not everyone had one of those super neato Play-Doh spaghetti maker things, but I’m guessing you all know what I’m taking about. You put the Play-Doh in the hopper and press the handle down and the Doh gets pushed through the little form screen thing, and gets turned into spaghetti. Why the heck would I be talking about such things? Because that process is called “extruding” and it’s sort of alarmingly similar to how alloy rims start life. Alloy rims start life as a stock of (cue Dr. Evil voice) molten hot magma (equivalent to the Play-Doh), and a die (equivalent to the little form screen thing) and an extruder, which is the whole hopper-pusher deal.
The (cue Dr. Evil voice) molten hot magma is composed of a bunch of elements that are mixed together to create the alloy. Alloy is actually more of a word for “mixture of metals” than it is for “a type of aluminum,” although aluminum has certainly become implied when we talk about it in reference to bike stuff. Steel can be alloyed. It just means mixed. Anyhow, the mix determines the properties of the final product. On different aluminum parts, you will often see a number like “7075” or such. These identifiers represent both ingredients and manufacturing processes used to create the aluminum alloy that went into the part. Some are stiffer and stronger but more brittle, some are softer but more resilient, etc. Different rim makers use different alloys to make their rims based on what characteristics they are looking for. FSW rims are based on an alloy called niobium, which is of course the best one to use. In seriousness, it is toward the stiffer/harder end of the scale but still does maintain some resilience. Resilience is good both for ride quality and for preventing catastrophic cracking. Too much resilience and you’ve got a noodle on your hands. It’s a balancing act.
The die creates the section shape of the rim. In the Play-Doh realm, you can make spaghetti that’s got a regular round section, or star section, or flat like linguini, or a bunch of other shapes. Same with rims – the die determines what the section of the rim is going to look like, what the wall thicknesses are going to be, etc. Two different alloy types made in the same die will perform differently, as will one alloy type made in different dies. But once you’ve decided what the section shape is of your rim, you push it through that die and then you come out on the other side with a long, straight piece of something that might look a lot like a rim if it were bent into a circle, instead of being straight and flat.
Now here's a funny sidebar to this. You slam enough molten hot magma through that die and eventually it's going to get a little bit worn down. This, and pretty much only this, is the reason why alloy rim weights vary. Since the dies are relatively expensive to make, rim makers try to get as much life out of them as possible. When they are brand new, they make light rims - the ones that get weighed for the specs page. When they are old, they build heavy rims. More material equals more weight. My first real contact with extrusion was in alloy masts for sailboats. A long story that I am going to shorten the heck out of, but when alloy masts come from only one source and people are trying to go to the Olympics using those masts, you pay VERY close attention to these things. And when your job is to QC those masts, and the company you are buyin gthem from could either pay attention to the millions of feet they are running for some lawn furniture company that really doesn't give a rat's ass about anything other than "round and won't break when heavy-set Aunt Sylvia joins us for July 4th" or pay attention to your order that's 1/1000th that size with 1000x the tolerance demands, you spend a lot of time being frustrated. Extrusion is HEAVY INDUSTRY and there really aren't that many companies that make alloy rims. It's like bikes - 10 or 100 brands for every actual manufacturer.
Guess what the next step is? If you guessed “circle making,” you win. The rim stock is formed into a circle of the appropriate diameter (for road bikes this size is called “700C,” and I have no concept of what either the 700 or the C stand for), and the ends are joined together by either welding or plugging, or plugging and pinning. There are lots of ways to skin that cat, each has its plusses and minuses. Ours are plugged and pinned.
One funny note here is that the lack of truly deep section alloy rims on the market has as much to do with wrinkles as it does with weight. Several rim makers tried to do really deep alloy rims, and what they found was that when the straight stock was bent into a circle, the inner part of the rims got wrinkly as the material got “squished” to the smaller inside part of the circle. People didn’t prefer to buy wrinkly rims, as you might imagine.
After that, there are some finishing details. Brake tracks get machined (the little ribs you see on most brake tracks), which makes the brake track smoother for better braking and also helps dissipate heat by increasing the surface area of the brake track. Rims get either painted or anodized to make them pretty and give the material some corrosion resistance. Holes get drilled. Eyelets, if used, are installed. Etcetera.
Please note that these “How It’s Made” deals are meant not to be a technical guide into setting up your own rim or spoke or whatever factory, but are more like edutainment so you can become a little better informed about where some of your bike parts come from. I do take artistic license, and it’s often way better to give you a clear “90%” picture that you might (ANON CX aside) read and understand and enjoy, versus giving you a 100% picture which will cause your eyes to glaze, eventually putting you to sleep and causing you much embarrassment when your boss comes by to see you asleep on your keyboard with a cycling website open on your screen. No one wants that.