Step 3: Molding
There are many different ways in which the sprue can be attached. It's often a trade off between what will make the finished casting easier to clean and what will make the mold easier to cut. There is also the issue of which method uses the largest mold as RTV silicone isn't cheap. In the photo below I've shown varius methods of attaching the sprues. To attach the sprue simply heat the end of it and press it onto the wax model.
#1 shows a variation of the way my Hal Jordan ring wax is sprued. It will make the mold much easier to cut open but will require a little bit more clean up work to the finished casting.
#2 shows the sprue going directly to the top of the same ring wax. The mold will be very easy to cut open but the finished casting will require a bit of cutting/shaping to the pattern in the top of the finished cast ring. The mold will be slightly larger and will therefore require more silicone.
#3 and #4 show the Kyle Rayner and Alan Scott rings. These ring waxes should be sprued in this method to because the finished casting would otherwise be very difficult to clean up and/or the quality of the finished casting will suffer due to bad fill.
The important thing to remember is that the fill sprue must be attached to whatever will be the lowest point of the wax model in order to help remove air from the mold during casting.
Once the wax model is sprued I make a mold frame for it by bending some thin aluminum into a squared off "U" shape. Then I superglue the wax model to the bottom of the "U"- this prevents the wax model from the likelyhood of it falling over in the mold during the process of pouring the RTV silicone.
Next I cover the sides of the "U" with some aluminum sheet and wrap a couple of rubberbands around the mold frame. At this point it's important to make sure that the sides of the mold frame are completely sealed- if they aren't use a glue gun to seal any gaps.
This is only one way of making a mold frame/box. You can also use plastic sheet or foam core board or even Legos to make a box for your wax model to sit in. All that matters is that it is taller than the wax model and that it is sealed so the silicone doesn't seep out during molding.
Now I mix the silicone. This is usually where things will go wrong for first timers- you need to mix the RTV exactly according to the manufacturer's instructions and make sure that the rubber and catalyst are thoroughly stirred. I've always used Alumilite HS II RTV silicone to mold wax models and it's worked very well for me- it's very flexible, holds up really well and doesn't require the use of a vacuum chamber. Unless you have access to a vacuum chamber make sure you use a silicone that doesn't require you to vacuum it during the molding process.- otherwise you'll have a thousand tiny air bubbles in your mold.
After mixing the silicone, pour it into the mold by holding it so that a thin strip of silicone can pour down into the mold- this wil help to reduce air bubbles in your mold. I always pour slowly into one corner of the mold and then keep slowly pouring until the entire wax model is covered. After pouring let the mold sit for 24 hours (or as long as the brand of silicone that you are using says to wait) before cutting open the mold.
More info about RTV silicone/where to buy it:
I've used Alumilite products for a long time and have always been very happy with the results. You can buy it directly online from them at- http://www.alumilite.com
For RTV silicone the best choices are the Dow Corning HSII and HSIII. The main difference is the HSIII is a little more flexible, which works well for detailed molds with severe undercuts. The HSII is a little more durable/tear resistant and that's what I used. The 1lb. kit is enough to make three ring molds, unless you have bannana hands, in which case it's good for two molds.