Gypsum Setting Expansion Compensation — MFDS Part 1 MCQ

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HardDental MaterialsGypsum Setting Expansion CompensationMFDS Part 1

In the lost-wax casting technique, a refractory mould material contains a calcium sulfate hemihydrate binder and a silica refractory. Its setting and thermal expansion contribute to compensation for shrinkage of the wax pattern and casting alloy. Which material is described?

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Correct answer: E — Gypsum-bonded casting investment

A gypsum-bonded casting investment contains a calcium sulfate hemihydrate binder together with a silica refractory. Conversion of the hemihydrate to calcium sulfate dihydrate produces setting expansion, while the refractory phase contributes thermal expansion during burnout. These dimensional changes help compensate for wax-pattern and alloy shrinkage. Phosphate- and silica-bonded investments use different binder systems and are principally selected where greater high-temperature stability is required. Type IV dental stone is also gypsum-based and has controlled setting expansion, but it is used to construct dies and casts; it is not a refractory casting mould. Epoxy resin is likewise a die material rather than an investment.

Reference: Lodovici E, Meira JBC, Rodrigues Filho LE, Ballester RY. Expansion of high flow mixtures of gypsum-bonded investments in contact with absorbent liners. Dental Materials. 2005;21(6):573-579. https://pubmed.ncbi.nlm.nih.gov/15904701/