I started casting parts with aluminite then we moved into smooth on and DOW corning rubbers and resins. Aluminite is good to get you started. It is also forgiving in that you do NOT need a vacuum chamber to evacuate air. Casting is great if you need more than 4 of a item, remember you need to take into consideration to cost involved in not only the RTV rubber for the molds but the resins you are pouring. The other resins are useful in that you can adjust the cost to use for each mold by using different products. Molds to age and they DO wear out. After you get comfortable with Aluminite then take a look at Smooth On and DOW corning products.
A vacuum chamber is useful in that it will control if not eliminate the air bubbles what are trapped in the resin pour which can create a "foamed" appearance that is difficult to eliminate. Invest in a scale, as the portions of rubber and resins are best mixed by weight, although a couple of dram measuring cups are great to have around. Most of my casing is dammed up using legos to build a box to pour the RTV rubber into. For a two part mold, you will need some chemically inert modeling clay. Embed you master it to the clay half way. pour your RTV and let it set, then flip the RTV over, remove the clay and then paint petroleum jelly over the RTV, then pour RTV over top to make the second part of the mold. The tricky part is learning your to vent your molds so air escapes as the resin is injected of poured into the mold.
