The video below is pretty dry, but it gets the general point across. Skip to about 4:20 to get the the relevant information, though there are some good tips about mold prep beforehand as well.
For my helmets, I use Smooth-On's 300 or 320 urethane casting resins. Start by mixing 2 cups of resin (1 cup of part A mixed with 1 cup of part B) and pour it into the mold cavity, rotating it slowly by hand as the resin cures.
Repeat this process, moving the mold around to cover all areas of the inside of the helmet, until a uniform thickness of 3/16" is achieved. If some spots are a little thicker, that's fine; slushcasting is a tricky process that can take some getting used to. One helmet usually takes about 32oz of resin.
Warning! I did castings in both polyester and urethane resin - the poly is lighter and thinner, but more brittle. Urethane can deform and its heavier, but much more forgiving when subjected to stress. Whichever resin you choose to go with, read the MSDS (material safety data sheet) and make sure to use a resin that can stand at least 140º of heat. Shipping a helmet in the summer to a chrome shop can subject it to 120º+ in a shipping truck, and the chroming process itself can be quite warm as well. Some resins, like Smooth-On's 65D Roto resin, have lower melting temp and can deform very easily.
After the helmet is fully cured, remove it from the mold and trim out the areas for the visor, ears, mouth and nose vents using a dremel tool and hand files. (pics 3 & 4)
- Urethane casting resin (Smooth Cast 300 used in this tutorial)
- Disposable mixing trowels
- Mixing cups
- Detail files
- Dremel tool
- Cutoff wheel for dremel tool
- Sanding drum for dremel tool
- Rubber gloves & smock