The type of plastic you choose will be based on different factors, such as durability, price, and ease of use. Beside considering the type of plastic, you must determine the thickness that you want. For quality chocolate molds, you will probably want .040". If you want more detail, such as when doing logos or smaller graphics, you will probably want to go with a thinner plastic, such as maybe a .020"; if you are looking for a more rigid mold, try a .060".
Below are some common types of plastic and a discussion of their characteristics:
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PETG - sold under the name VIVAK by Sheffield Plastics, is a clear thermoplastic that is easily formed using a home vacuum cleaner. It can be bought in very thin sheets. It does not need alot of heat to work with and forms well with simple vacuum cleaners or shop vacs. It is widely used by hobby chocolate mold makers because of its cost. It is much less expensive than polycarbonate, and yet is still very durable. It can be purchased in rolls and then cut, or if you want to buy in larger quantities, you can have the plastic sales company cut it for you.
Polycarbonate - sold under the name MAKROLON FD by Sheffield Plastics, is also a clear thermoplastic. It is much more expensive than PETG. FD is the type of Makrolon that complies with FDA requirements for food contact. Polycarbonate is virtually unbreakable. Most professional chocolate molds are made of polycarbonate. It also can be bought in thin sheets. Check with the sales rep or manufacturer to determine if the polycabonate needs to be dried first. This can be done in your home oven. One other disadvantage of using polycarbonate to make hobby chocolate molds is the high forming temperature. Most home or small vacuum forming units don't have the power needed to properly heat polycarbonate.