Abstract: Mandrels for use in the manufacture of nickel shells by vapour deposition of nickel from gaseous nickel carbonyl. The mandrels comprise a body formed of a composite material of a matrix, for example, a ceramic material or an organic polymeric compound, and a filler, for example, metal, metal oxides, carbon in particulate form and having the same coefficient of thermal expansion as nickel. The invention includes processes of manufacturing the shells using the composite mandrel and shells made thereby. The shells have improved non-deformability in operation and are of use as molds in the mass production of plastic articles.