If you decide to build your models out of clay, you will need to create plaster molds to use in the vacuum forming process. Clay molds will not hold up to the pressure and heat of vacuum forming. This process involves creating a negative plaster mold of your clay model and then creating a positive mold from the negative mold . We will need to do this twice for each piece of armor since only half of the piece can be created at a time.
Step 1 Prepare your clay model by brushing on a thin layer of releasing agent. Petroleum Jelly works really well as a releasing agent. This is an important step, it prevents the plaster from adhering to the clay and makes seperation of the two parts possible.
Step 2 Divide your mold into two parts by masking off half of it using thin sheets of plastic. This will help keep your plaster on one side of the mold.
Step 3 Mix up a thin layer of plaster and coat the entire side of the clay model. This thin layer will ensure that we get as much detail out of the piece as possible.
Step 4 Once the first layer of plaster dries add a second thicker coat of plaster to add strength to the mold.
Step 5 Once the plaster dries, remove the clay from the negative mold. At this point you should sand the mold to a nice smooth surface. You can also use this opportunity to fill any air bubble holes that appear in the negative mold. I used Bondo to fill any holes and to make minor repairs. Close off the open end of the mold with a piece of wood or cardboard and apply release agent to the entire inside of the mold.
Step 6 Use the same procedure of adding a thin layer of plaster to the inside of the negative mold. Once that is dry, pour in the plaster to fill the mold.
Step 7 Once the plaster is dry, remove the plaster from the negative mold. This is very difficult to do without destroying the negative mold, but if you applied the release agent correctly, the two pieces should not bond together. You should use some bondo to repair any imperfections and sand the mold to a nice smooth surface. At this point you are ready to vacuum form your plastic over the positive mold.