I needed to do a relatively large aluminium casting (about ½kg or so, which is a large casting by my standards!) so I knocked up this little electric crucible heater to do the job. A while ago, I had read on the web about people using spiral electric cooker elements as crucible heaters, and I had bought a 7" 2kW element for £5 or so to experiment with, but never quite got round to it. I finally decided to give it a go. I'll admit I wasn't expecting much from the element, but it surpassed all expectations. Read on...
The crucible is one half of an empty MIG welding gas cylinder. The cylinder was chopped in two and the halfs left in a wood stove for an hour or so to burn the paint crud off them. One half became the crucible, the other ended up being used for a small autoclave. Very useful things! I attached two lifting hooks the top and a pouring hook to the bottom. It would actually be a lot better to use a stainless-steel crucible, since oxide does flake off the steel every time it's heated.
The element proved extremely difficult to re-wind. I tried leaving it on, red-hot, for a while to see if it softened, but it didn't really. It took a good hour of hammering, vises, and mole grips to get it into a cylindrical shape which fitted the crucible nicely. Some element manufacturers actually offer a custom service, so it might be worth trying that, or at least getting a straight element "blank" which should be a lot easier to wind.
For insulation, I used a couple of aereated cement building blocks and hollowed out some holes and channels in them for the elements. It turns out that they are very heat-resistant; when the inside was red-hot, the outside was barely warm to the touch. They do have a tendency to crack up through repeated use, but with careful use they're fine.
So, how did it perform? Pretty impressively. From cold, it can melt a full load of aluminium (about 1kg) in under half an hour! The element obivously runs far hotter than it's normally intended to, but it doesn't seem to mind. Besides, at only £5 or so each, it's almost a disposable item. Here's a couple of photos of some castings I did - they're a horn for an ultrasonic transducer (about 10cm long) and its mounting bracket. There are little bubbles in the casting, but it isn't bad considering I didn't degas the melt, and the miscellaneous aluminium I chucked in the crucible. The horn (the cylindrical thing) was sand-cast, while the bracket was lost-foam cast in dry play sand using a building foam master.
These elements are obivously a great means of getting 700-800°C crucible temperatures quickly and cheaply, and I'll certainly be making more use of them.
17/11/11: The heating element finally burned out, so I converted it into a propane furnace. See here.