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 built 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.
Unfortunately, most of Lego sets now contain so many smaller or specialty pieces that you can't get enough of the longer 2x4, 2x8 and 2x10 pieced needed for mold making. Out of necessity, you need to turn to the alternative Legos brick manufacturers. The imitation Legos! These Lego wannabe's save you some money and provided the larger pieces you will need for mold making. So, take a look at the imitation Lego sets on Amazon. While they are not as good, quality wise, as genuine Legos, the 1000 piece Building Block sets includes many more of the larger pieces you will need when making your molds and they are less than half the price.
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 less than $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 $120 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 "Moldmaking & Casting Pourable Starter Kit" which contains enough rubber for several small molds and dozens of small plastic 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 requires 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.
You will need dozens of Popsicle sticks (craft sticks) as you will be mixing and stirring everything. I may use two or three during each step of the process being sure to use a new one when I am transferring the silicone from one container to another. If any unmixed residue left on the stick makes its way into the final mold you will have an uncured sticky spot. I know from experience. Don't be tempted to scrap every last bit off of the sides of the cup when transferring to the larger container for degassing.