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Investment Material Expansion — MFDS Part 1 MCQ

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HardDental MaterialsInvestment Material ExpansionMFDS Part 1

A laboratory uses a phosphate-bonded investment for lost-wax casting. Wax-pattern dimensions, burnout, mould filling and alloy temperature are controlled, yet the completed crowns are consistently undersized. Which dimensional behaviour of the investment is required to minimise this error?

Educational content. Not a substitute for clinical judgement or local policy.

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Correct answer: D — Adequate net mould expansion from setting and heating, calibrated against alloy casting contraction

Explanation lettering: C = shown as A · D = shown as C · A = shown as D

A is correct. An alloy contracts during solidification and subsequent cooling, so the investment mould must undergo sufficient net expansion before casting. This compensation reflects the combined effects of setting and thermal expansion rather than either component in isolation. High hot strength may influence casting distortion, but restraining the contracting alloy is not the primary dimensional compensation mechanism. Options C and D consider only one component of investment expansion and disregard the net dimensional change produced by the complete investing and burnout cycle. Option E would compound the alloy contraction and produce an even smaller casting. The required expansion must also be controlled: excessive or anisotropic mould expansion can produce oversized or distorted castings.

Reference: Ferreira AR et al. Evaluation of the thermal shrinkage of titanium and the setting and thermal expansion of phosphate-bonded investments. Journal of Prosthetic Dentistry. 2007;98(1):24-29. https://pubmed.ncbi.nlm.nih.gov/17631171/