Here is my basic "how-to" guide to making silicone rubber molds as well as plastic resin casting. Many of my projects, over the last 10 years (B9 Robot, Twilight Zone Mystic Seer and Back to the Future Flux Capacitor) have forced me to learn how to make silicone rubber molds and how to cast plastic resin parts. Hopefully you will pick up a few tips here that will help you avoid some of the problems I encountered. These mold making tips can be used by the hobbyist to make chocolate molds, soap molds and candle molds as well. When making food molds be sure to only use food safe silicone materials.
If you're going to get semi serious in molding, you'll want an industrial vacuum pump. I picked one up on eBay for about $100. Commercial industrial pumps are build very rugged. You may want to buy one of the smaller hobby vacuum pumps available, but I went for the commercial one as it should last me longer, and it actually cost less. You want one that can pull 29 "inches of mercury" for a complete vacuum.
First, a few things you'll need. Get yourself a good 400+ piece basic set of LEGOS. Yes, I said LEGOS! If you're lucky you have a son that went through the LEGOS phase. If you're like me, you may still have yours from when you were young. Either way, they are a must if you want to quickly make molds of various sizes. You want a set with just the basic pieces and none of the fancy special pieces.
You'll also need a vacuum chamber. I looked into getting an actual vacuum bell, which is basically a large inverted blown glass bowl. They sell for several hundred dollars, so I passed. I went to my local Wal-mart and picked up a set of three food vacuum containers for about $25. They sell them as an accessory to the home food vacuum packing sets. It comes with the all-important "Universal Accessory Sealer" vacuum cap. This set does not include the vacuum itself, and that is good because the food vacuum pumps don't have enough suction.
Another more expensive option is to purchase an actual industrial Vacuum canister. For about $110 you can get a stainless steel vacuum chamber that will last you many years. I used the food canisters for several years until I noticed small cracks in the walls. I quickly purchased a stainless steel vacuum chamber like the one pictured here.
You may want to pick up one of the "starter" molding rubber kits and plastic resin kits. I have tried kits from both Smooth-On and Por-a-Cast. Smooth-On has what they call "Super Sampler" kits that contain enough rubber for several small molds and dozens of small resin parts.
When you're serious, you order the Silicone rubber by the gallon. I use the Smooth-On, Mold Max series of silicone rubber for most of my projects. I've also had good luck with the Smooth-On OOMOO rubber as well.
One more piece of equipment that will make your job easier is a digital scale. I happened to have a small digital postal scale that works perfect for this. Some of the silicone molding materials require you to mix the two parts by weight (usually a 10:1 ratio) and others by volume, typically 1:1. If you have the ratio off by too much, it may not cure or it may cure too fast.