Casting resins are two-part polymers that you mix as liquids, which harden into whatever shape you choose. You can use casting resins to make jewellery, to preserve insects or flowers and to make models. Liquid polymer resins come in several varieties, and it's important to choose the right one for your project.
Polyester resin is a liquid plastic. When a few drops of another liquid, called a catalyst, are added to the resin, it changes from a liquid to a solid. Polyester resin is highly toxic and gives off dangerous fumes; work spaces need extra ventilation, and users need to wear masks. Polyester resin is inexpensive and is good for large projects, but it's best left to those with the experience and facilities to handle it safely.
Like polyester resin, polyurethane resin is a liquid plastic to which a hardener is added to make it set. Instead of adding a few drops of hardener to a resin, you mix the parts in a 1:1 ratio, which some find easier than mixing polyester resin. Polyurethane resin is much less toxic than polyester; hobbyists can use it safely in a well-ventilated area. It has good clarity and is ideal for use in jewellery. It generates heat as it cures (hardens), which may discolour organic material such as flowers or insect specimens.
Epoxy casting resin comes in two parts that you must mix in the correct ratio; the exact amounts vary among brands. There are many clear epoxy resin products on the market and not all are suited for casting; ensure that you are purchasing a casting resin and not a resin intended for varnishing. Epoxy casting resins vary in quality; some cheaper brands give inferior results, so look for resins advertised as "water clear" and "jewelry grade." Epoxy resins are more costly than polyester resins but are less toxic and easier to work with --- although they are still irritant and must be used in a well-ventilated work space.
It is sometimes desirable for a casting resin to form a domed, cabochon shape as it hardens. This is useful in making three-dimensional stickers, magnets or jewellery, as the domed surface magnifies any objects cast in the resin. Self-doming resins are viscous and have a high surface tension, naturally holding a domed shape like a large water droplet when poured slightly above the level of the mould.