Setting expansion in gypsum and gypsum bonded investments

The objective of this study is to analyze the setting expansion of gypsum bonded investment. Gypsum bonded investments are utilized, particularly in dentistry and jewelry production, for casting non ferrous alloys with low melting points such as gold. The composition of the investment material consists of 25-27% ?-gypsum as binder, 70-73% quartz and cristobalite as refractory materials and 2-5% of additives. When water is added to the mixture, the calcinated calciumsulfate hemihydrate (CaSO4 .½ H2O), which is used as binder, regains the 1,5 mole of water it had lost and solidifies after having crystallized as calciumsulfate dihydrate (CaSO4.2H2O). Setting expansion occurs as a consequence of the push between the hydrated and growing needle-like crystals of gypsum and the cavities that result from this action.Gypsum bonded investments expand volumetrically as an outcome of the setting expansion that takes place during the set of the plaster and the polymorphic phase transitions of refractory materials such as quartz and cristobalite at high temperatures. However, this expansion does not compensate for the calcination of the plaster and shrinkage of the solidified alloy during cool down.During the researchs, which were carried out for this study, gypsum bonded investment powders were prepared by inserting various predetermined additives. From these mixtures, appropriate investment mould samples were casted. The dimensional changes of the setting plaster specimens were measured at certain intervals. In light of the values found during these experiments, the influence of additives on setting expansion was analyzed. In order to compensate for the shrinkage of the sollidified alloy, the aim was to obtain the highest expansion values by optimizing the amount of additives. The surfaces of samples that had yielded high amounts of expansion during the casting experiments and those of reference samples were observed with Scanning Electronic Microscope and the micromorphologies of investment surfaces were compared.