For high quality, long lasting moulds, most cold casting methods use an RTV rubber of some kind. These are excellent, and I do use them, but they are expensive and pretty slow to cure. Usually 4 to 24 hours. The polyurethane resin favoured by most small parts casters, such as Easy-Flo or one of the other 50% / 50% mixes are easy to use but need to be ordered in and again can be expensive.
I wanted detailed results VERY quickly so I chose to use Alginate as my mould making material instead. The resin is car repair resin, which I can pick up 5 minutes down the road.
Step 1: Materials
A pattern with very little undercut
Car repair resin
Some plastic cups
Stirring sticks (the ones from the coffee shop are best and free)
The problem with Alginate as a casting mould material is that it is not very strong, it cant handle big undercuts and it's considered to be a 'one shot' mould. Dentists use it to cast your teeth. The advantages of alginate are, that it takes very fine details, it very safe to use, and most of all it cures in 90 seconds flat. Also it is MUCH cheaper than RTV
You may have to think out of the box a little when making your patterns. I wanted to cast a set of small tank wheels. I had to fill some of the undercuts with plasticine, then grind them back with a Dremel after casting. I also built the main pattern so that there were no undercuts at all.
Step 2: Setting Up the Fence
The pattern is mounted using a bit of plasticine, with the main 'face' of the pattern uppermost.
Step 3: Pour the Alginate
once it is unifirm, carefully pour the alginate into the bottom of the cup. Ensure that the pattern is completely covered to a depth of not less than 25mm or 1 inch.
Using this method should ensure that the bubbles rise to the surface and don't cock up the pattern mould.
Step 4: Wait 90 Seconds
Turn the cup over and press out the mould. CAREFULLY (I use tweezers) draw the pattern out of the mould. The mould shape should mean that it is self supporting.
Step 5: Pour the Resin
Be sure to mix the resin until it is a uniform colour before pouring. I mix it in a plastic cup using another coffee stick.
Step 6: Demould
The heat of the curing resin tends to 'leach' a little water from the alginate into the mould. This is as bad as getting bubbles in the rubber. Just blow the mould out with a little air (I use an air can for this).
Step 7: Repeat
Step 8: Cleaning Up
The entire process including casting 10 wheels took 2 hours.