This Instructable should be used in conjunction with other silicone molding instructables.
Let's face it: molding is an expensive hobby. Materials aren't cheap, tools are expensive, and it takes time to make good molds. It's worth it though to see your army of mass produced thing-a-ma-jigs.
But let's say you only need to make one casting. Do you just want to try this hobby?, Do you really want to track down specialty silicone from obscure vendors in your area? Is it even available?
Here's how to make your own molding silicone from what you can find at your local big box store.
You will need the following materials:
100% Silicone caulking
Plastic Cups and Silverware
Mold Release (Also Optional, but highly recommended)
UPDATE: Cornstarch helps molds to try faster.
UPDATE!: Thanks to JDUFFY54, he has provided the following helpful additions to this recipe.
JDUFFY54: I found that using 2 parts cornstarch, 2 parts silicone caulk, and one part mineral spirits works well, more than doubles the ammount of molding stuff you get per caulk tube, and dries compleatley in under an hour. Not to mention, cornstarch is cheaper than caulk, so half the mold is cheaper. I haven't done a lot with it yet, but I have made some test molds which turned out excellent.
Step 1: Gather Your Materials
Let's take a trip to our local box store, shall we? Grab a shopping basket and pick up the following items:
100% Silicone caulking
Caulking is available in all types and brands, but what you need is simply 100% Silicone caulking. Any brand will do, so buy whatever on sale. I prefer the white kind rather than clear, as it's easier to tell when it's thoroughly mixed. This forms the bulk of your mold. Get a caulking gun too, if you don't have one already.
Cost: Usually $5-$6
Liquid Glycerin is available in the pharmacy as a either a skin protectant, or as a laxative. You may even be able to find it in prefilled syringes. Either way, may sure it's liquid glycerin you're getting, as the suppositories are completely useless to you. This is to add moisture to the mold and allow it to dry throughout. Without this, the molding doesn't work.
Any acrylic paint of any color will do. I use hobby paint because that's that I have lying around. You don't need much of it, so buy as little as possible. This is used to give a consistent color during mixing.
Plastic Cups, Silverware, and Straws
I always use disposable plastic cups to mix my silicone. The silicone won't stick to the cup after it's cured, and you can throw it away when you're done. The straws are helpful if you don't have any pipettes or eyedroppers.
Mineral spirits thin out the silicone,, making ti easier to work with. While not strictly necessary, I find that I get slightly better impressions with it, and it seems to cure faster. The downside is that mineral spirits combined with the acetic acid in the silicone cause some pretty noxious fumes. So do this outside.
Mold Release(also optional, but highly recommended)
This item may be harder to find in your area, and is not strictly necessary unless you're making two part molds.. It does make demolding a lot easier though. You can find it at nearly any craft store like Michael's or Hobby Lobby. This isn't used for making the silicone, it's for getting your molds to release once you're done.
Now that we've got our materials, let's head home and get cracking.
Step 2: Measuring
The following ratio is key to successfully molding with silicone caulking
1oz caulk : 3 drops glycerin : 1 drop paint : .5 oz mineral spirits
A standard tube of caulking is 10oz, so you should get that much molding material out of a single tube. The recipe for an entire tube would be:
10oz Silicone, 30 drops glycerin, 10 drops paint, 5 oz mineral spirits.
Measuring without a scale
A scale isn't strictly necessary to pull off this project, but it is helpful.
To measure silicone without a scale, cut the entire tip off your caulk, and prime it to the top. A standard caulking tube has 20cm of caulk in it, so make your lines at 2cm distances. I say centimeters because it's actually something like 8" and who the heck has 1/5 dividers on their ruler?
To measure mineral spirits, use a shot glass. Half a shot glass is 1oz.
Drops can be done with a straw. Dip your straw into the glycerin, then place your finger over the open end. Squeeze out your drops as necessary.
Step 3: Mixing and Setting
NOTE: The vinegar smell here is normal. Silicone caulking is mixed with acetic acid to help it cure. The fumes are harmless, but you will probably mix this outside.
To mix your molding material add the ingredients in the following order:
1. Measure out your Silicone into plastic cup (1oz)
2. Add appropriate amounts of Glycerin (3 drops)
3. Add appropriate amounts of Paint (1 Drop)
4. Add appropriate amounts of Mineral Spirits (.5oz)
5. Mix with plastic silverware until the paint is dispersed through the mixture with no white left. I like to use a knife, some prefer a spoon.
6. Scoop silicone into casting box. I say scoop becausethis material does not pour like molding silicone. If you're trying to make a two part mold, you're going to have to add pressure to fill in the gaps.
7. Cure times depend on heat and moisture, but is generally 2-4 hours.
That's all there is to it! Follow the directions of some of the other casting instructables, and you should have a mold that you can make your own things with.