Step 5: Resin casting
First lightly dust the inside of the mold with baby powder (talcum powder) and blow off any excess. This will reduce surface tension in the mold and you'll get a cleaner casting. No mold release is required with silicone molds.
Now put some flat sheets on the sides of the mold and put a rubber band around the mold to help hold the mold halves together. It doesn't have to be super tight- you don't want to deform the mold.
Next mix your resin according to the manufacturer's instructions. I use Alumilite regular urethane casting resin. This stuff sets up fast- you have a pouring time of about 90 seconds and after 5 minutes it's completely cured.
Pour the resin slowly into the larger hole- the resin will flow down into the bottom of the mold and then rise up from the bottom, forcing air out of the mold through the vent hole. As you pour give the mold a couple of firm taps down on a table to help release any trapped air. The resin will soon begin to cure and after a few minutes you can remove your ring from the mold. Wait until the resin is fully cured before cutting the sprues off so you don't deform the ring. After the sprues are cut off you can lightly sand the resin casting and paint the ring. Make sure to check that the ring fits first! If it's a bit too tight you can ream out the inside of the ring with a Dremel tool.
Painting is covered later but it is also possible to mix opaque green dye in with the resin during casting to eliminate the need for painting, which makes for a less pretty but far more durable finish.
After casting my resin ring I decided to cast a transparent ring using a clear casting resin with some transparent green dye mixed in. The casting procedure is exactly the same but the clear resin I used takes 24 hours to cure. I've used both clear polyester casting resin as well as two part epoxy casting resin and both have worked well. Clear urethane resin is also available but it can be more difficult to work with as some types of clear urethane require a special type of silicone be used for the mold and/or vacuuming the resin to reduce air bubbles.
All resins are slightly different so make sure to follow the manufacturer's directions reagarding the brand of resin you use. The ring I made used less than 1/4 ounce of resin in each casting so you can make a lot of rings from a small supply of resin.
More info about resins/where to buy it:
For opaque resin castings I use the Alumilite Regular urethane casting resin- it makes great castings that are really durable. The 28oz. kit will make more rings than you can shake a stick at. Alumilite also makes a green dye in one ounce bottles so you can color your resin and avoid the hassle of painting.
For clear casting resin there are three options; epoxy resin, polyester resin and urethane resin.
A company called Cast'n Craft makes both epoxy resin and polyester resin in small kits as well as transparent green dye. Cast'n Craft is sold at places like Michael's and Hobby Lobby. Epoxy resin is a little trickier to mix than polyester resin but it doesn't smell as bad- it does take a lot longer to cure than polyester resin. Alumilite makes a clear urethane resin but they recommend vacuuming to reduce bubbles during casting- urethane resin would ultimately produce the best quality casting but I have yet to try it without vacuuming so I can't really recommend at this time unless you have access to a vacuum chamber.