I often receive emails from people looking for some odd shaped mold. Most people assume that you have to pay a fortune for a custom vacuum formed plastic chocolate mold. However, there is a much simpiler and cost effective method.
I highly recommend going to the newly designed MakeYourOwnMolds.com website to look at their silicone moldmaking products. MakeYourOwnMolds.com is a part of Culinart. Amongst their product line up is Silicone Plastique, which is a putty food grade silicone; Silicone Spread, which is a thixotropic food grade silicone that is thick and spreadable; and CopyFlex which is a liquid food grade silicone. They also sell supporting supplies such as sealants and release agents. We will just talk about the Silicone Plastique for now since it is the easiest and cleanest to use.
One example of a common inquiry is a letter in a certain font. If you look at the stock plastic chocolate molds for sale, you will see that there aren't many choices. So, I try to tell people to go to a hobby store and find the letter that you want as a model to make a mold from. This can be made out of wood, clay, or even fabric. It just needs to have some degree of thickness to it.
The model must first be prepped before the Silicone Plastique can be used. Usually this will involve adhering the item to a board using kneading clay. Make sure whatever you are using is FDA approved, or at least non-toxic. The item must also be sealed so that the mold making material doesn't penetrate the surface of the model. Depending on the item you may want to also use a release agent so the silicone does not stick to the model. Again, since you will be using this in the preparation of food, you want an FDA compliant material. These products can all be obtained from the folks at Culinart.
Once the model is ready, you will mix the two parts of the putty together to activate the mold making chemicals. You must work fairly quickly before the materials begin to harden. According to a video on their website, you have about 10 minutes to make your mold once you have mixed the two parts together. If your mold is large, you may want to mix the putty in steps so you can take your time. At any rate, now that the putty is mixed you will want to apply it to your model. It is important to lay the silicone on the model in a manner that does not trap air. After you have applied the materials, it will take approximately an hour to cure, or completely harden. You will need to thouroughly wash and dry the mold prior to use.
I highly recommend that you go http://www.makeyourownmolds.com/ and watch the lace mold making video. It is an excellent demonstration with very good tips that you can use when making any mold.