Lightweight concrete moulds are used to cast lightweight concrete. They are typically crafted from rubber compounds which are store bought materials designed specifically for the purpose of mould making. This compound hardens and dries around a model (the object that the mould is created from) and is then removed. From this mould many castings can be made. It is an easy way create your own statues, concrete pavers or anything in-between.
Pour mould-making material, used for creating concrete moulds, over your model. Be sure the mould material completely covers the object and forms a skin around it. In this case, you will use a simple object as your model, such as a sink. However, just about any object can be used for this purpose.
Allow the mould-making material to dry completely, until it forms a hard but rubbery shell. Peel the mould off the sink gently, so that it comes off in one piece.
Carve out a blocking shape (a backer for the rubber mould) into blocks of hobby foam. Use a razor to cut a general shape into the foam which matches the contours and dimensions of the model. Use the rubber mould as a guide and insert it into the carved out shape, once the general shape is cut into the foam. Shave off additional parts of the foam until the blocking shape supports your mould.
Insert the rubber mould into the blocking foam. The rubber mould must sit perfectly inside the foam. Press on the edges to ensure each side of the mould is supported by the foam.
It is important to note that you should brush the rubber mould-making compound onto your model using a paint brush. Reinforce the foam with plywood or metal sheeting as necessary.