Do I need a support shell for a brush on mould

In most cases yes. A brush on mould is a thin skin of five to ten millimetres and has no dimensional stability of its own. Without a support shell the mould sags, deforms while being filled and your castings will not be dimensionally accurate. Only on very small and shallow moulds can you leave it out.

A support shell, also called a mother mould, is definitely needed on moulds larger than roughly twenty centimetres, on tub shapes and anything with high side walls, for heavy castings such as concrete, plaster or filled resins, and on moulds that are used frequently or stored.

Make the support shell only after the silicone mould has fully cured. First apply a release agent to the outside of the silicone mould, certainly when working with plaster or epoxy. Then decide on the split. For a closed or deep shape, make the support shell in two or more parts using a dividing wall of clay or sheet material. Create registration keys on the flanges, meaning recesses or bosses, so the parts can only fit back together one way.

For the support shell itself you can choose plaster with hessian, acrylic resin with fibre, epoxy with glass fibre or rigid polyurethane foam. Plaster is cheap and forgiving but heavy and brittle. Epoxy with glass fibre is light and very rigid but more labour intensive.

Store the silicone mould inside its support shell after use. That is the best protection against deformation during storage.

Read more: complete guide to making a brush on silicone mould