Fabrication and investigation of ceramic shell moulds for investment casting

Shell moulds for investment casting are formed from several layers, differe nt materials and particle sizes. The fabrication of the shell moulds is a c omplex process comprising a number of different steps. To fabricate close-t olerance castings and prevent premature failure of the shell moulds, it is important to obtain exact details on the behaviour of shell moulds under th e influence of temperature. Dilatometer tests were performed on green and f ired shell moulds made of zirconium and molochite layers and bonded with co lloidal silicic acid, as well as on the individual layers forming the mould s, to determine their sintering behaviour and thermal expansion. From the t hermal expansion and firing shrinkage of the individual layers, inferences can be drawn regarding the dimensional stability of the moulds and the buil d-up of stresses between the layers as a function of the firing temperature . To establish whether the microstructure undergoes any changes as a functi on of the firing temperature, photomicrographs and electron micrographs wer e prepared. For better identification of the phases, radiographic examinati ons of the phases at room temperature as Well as at the firing and applicat ion temperatures of the mould were performed.