Mold Making Part 1 - The Equipment
I've decided to put together a basic "how-to" guide that covers making liquid silicone molds and plastic resin molding and casting. I learned quite a bit over the last 5 years about silicone rubber while building my B9 Robot and more than 35 Twilight Zone Mystic Seers replicas. Hopefully you will pick up a few tips here that will help you avoid some of the pitfalls. These silicone rubber mold making tips can be used by the hobbyist who want to create molds for making chocolate, or to create soap molds and candle molds as well. When making food molds, be sure to only use food safe silicone materials. If you wanted to know what silicone is or how to use it for molding, you have come to the right place. This is the same type of mold making used by all prototype plastic molders as well as short run plastic molders. I've added links to most of the necessary supplies where available at Amazon.
If you're going to get serious in molding, you'll want to purchase a vacuum pump. I got mine on eBay for about $100. It's an industrial pump and it is 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 about the same. You want one that can pull 29 "inches of mercury" for a complete vacuum. There are several on Amazon that should do the trick.
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.
Bonus Fact: The name "Lego" is derived from the Danish phrase "leg godt", which means "play well".
You may want to pick up one of the "starter" molding liquid silicone rubber kits and plastic resin kits. I have tried kits from both Smooth-On and Por-a-Cast. I prefer Smooth-On as they offer so many different products. Smooth-On has what they call a "Mold Making and Casting Pourable Starter Kit" that contain enough rubber for several small molds and dozens of small resin parts.
You will 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 on those. 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, but you may kill two birds by buying a set that includes both the vacuum pump and vacuum chamber. But, I do not know if the food vacuum pumps have enough suction, so keep your receipt.
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.
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.
You will need some Modeling Clay. Make sure you get clay that is Sulfur free. Silicone rubber does not like to be near clay that contains sulfur and it will inhibit the curing process. I get my clay from Michaels Craft stores. Just look on the label for "Sulfur Free" or a statement that it can be used with silicone rubber molds. The Clay will be used to anchor your original and can be used as a sealant around the edge of the Legos to prevent silicone leakage.
To make sure the silicone does not stick to the bottom half of the mold, you need to spray it with a Silicone Mold Release. If you don't, you won't be able to separate the two halves. Silicone mold release is DIFFERENT from universal mold release. Makes sure you get both if you are making a two piece mold.
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.
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 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.