When making props for plays, musicals, movies or any other sort of production that requires sets, materials that are lightweight and easy to work are ideal. Foam-based props are particularly well suited for these types of productions since they can be easily shaped and moved around. Expandable spray foam, for example, can be carved into shapes much like any other foam. Following a few simple steps will allow you to shape and carve spray foam into whatever forms you desire.
Draw the core of the form onto a piece of corrugated cardboard and cut out the design. Remember to make the core slightly smaller than the final product. While the core won't be seen, it will provide the internal support within the foam shape, so you'll want to make it as close to the finished shape as possible.
Shake a can of spray foam vigorously and spray a thick layer of foam over one side of the cardboard core. Keep spraying until the foam is built up and vaguely resembles the desired finished shape. Let the foam rest for about one hour, or until it is dry and hard. You may need more or less drying time, depending on the size of your form.
Flip the hardened form over and spray foam on the bare side of the cardboard like you did with the first side. Let the foam harden until solid. It is better to use more foam during these initial steps. If you initially make the shape larger than you want, you can always carve it down to size later.
Remove large chunks of the foam using a utility knife. Whittle away large pieces of the foam until the rough form of your design emerges. Fine details will be made later. You just need to approximate the look of your design with the knife.
Carve out the fine details of your form using a Japanese hand saw. Work slowly and check your progress frequently to see where more carving is needed. Brush away the foam dust frequently so that you can see clearly where you have carved and what areas still need work.
Fill any cracks or mistakes in the form with more spray foam and allow the filler to harden. Carve the newly added foam until it blends in with the original material.