The plaster moulding process is commonly used for casting prototypes of aluminium components designed for subsequent production by die casting. Plaster moulded prototype castings possess dimensional tolerances and surface quality comparable to die castings, but without the high tooling costs and lead times. Unfortunately, the mechanical properties of plaster castings are significantly inferior to their die cast counterparts. This is due primarily to the low thermal conductivity of plaster which extends casting solidification time significantly compared to die or even sand casting processes. Long solidification times result in large grained castings demonstrating low strength and hardness. This work is an experimental investigation into the effects of various levels of silica sand and carbon fibre additions on the properties of foaming plaster moulds and associated aluminium castings. Data and statistical analyses are presented and conclusions drawn regarding the wet and dry strength of plaster moulds as a function of sand and carbon fibre content. Likewise, conclusions regarding the associated casting characteristics such as surface finish, solidification time, hardness and tensile strength are presented. The effectiveness of silica sand and carbon fibre additions with respect to improving the properties of plaster moulded prototype castings is also discussed.