Instructables
These techniques will work for pretty much any pourable mold-making material (although with urethanes you will need mold release), I just happen to be using high-temp silicone because it's what I have, and I have it because I can cast pewter (and other low temp alloys) with it. You would also be able to use this method to make waxes which you then cast using investment and more conventional metals (gold, silver, etc.).

The pictures of pouring are from when I molded an aluminum flashlight reflector, and the mold pictured in the last step (and this step) is one of my ring molds
 
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Step 1: Materials

You really don't need all that much stuff to make a mold.

- A part to make a mold of
- Mold Max 60 Rubber (available from Smooth-On), or any other RTV (room temperature vulcanizing) rubber
- A mold box (can be any watertight container that fits your part)
- Clay for the sprue

Step 2: Spruing the Part

IMG_6361.JPG
This is one of the most important parts of the process, because if you get it wrong you’ll either have to spend an inordinate amount of time sanding and grinding miniscule details, or maybe you won’t even get a good casting at all. A mold that yields incomplete castings can be somewhat remedied by cutting air vents, but at the beginning just try to place your sprue in a logical spot (e.g. if you’re casting a clown, don’t put a sprue on its face. Unless you’re deathly afraid of clowns, in which case go right ahead).

In my experience a good configuration is to put a large mass at what will be the bottom of the finished mold. This provides a large reservoir that “sucks in” the molten metal, increasing the chances of getting a complete casting even if you have thin sections in your piece.
To gg1220 ,
I don't know if it's just me being stupid but could you explain how you got the ring shape in the mold, from the pictures and how you explained it, i cant quite figure it out and was wondering if you wouldn't mind helping me out. Again i don't know if it's just me or not.
Thanks Daniel
I'm with Daniel. After viewing this instructable, I am still not clear about how to make a mold for a signet ring.
gg1220 (author)  David41510 months ago
Hopefully this image helps. The ring is mounted on a sprue which is then attached to the bottom of a small sealed container. Here I'm using a 3 oz. cup because it's pretty much the perfect size.
Mix and pour the rubber so it covers the entirety of the ring plus about half an inch. After the rubber cures, peel off the paper cup and cut the mold in half as you see in the last step of the Instructable. 
Hope that helps. Let me know if you need more information!
Ring to be molded w. annotations.jpg
gg1220 (author)  dannywilde941 year ago
Hey Daniel, sorry for the confusion. I had already made the ring mold prior to this Instructable and unfortunately didn't take pictures, so I showed how to make a mold for the reflector just to show the general process.
For the ring in particular, I molded it upside down from what you see in the picture. I attached the top of the sprue to the base of the ring, and the bottom of the sprue to the cup, and then poured the silicone. After cutting the mold open and casting with it a couple times, it was clear that I needed air vents, so I just cut those channels with an X-ACTO knife.
Hope that helps!
grunt16122 years ago
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