Explains the properties a casting investment must have, such as fluidity, fine grain, strength, refractoriness, chemical inertness, porosity, expansion and heat conductivity, plus ease of breaking away after casting. A table sets out the three stages of alloy shrinkage and how each is handled, and a diagram compares investment expansion with alloy shrinkage for noble and base alloys. It closes with how investments expand and how the casting ring is lined or replaced so the expansion is not restricted.
Topic 18 of 52 in the MU Sofia syllabus for Dental Materials (Dental Medicine, Year 1). The free AceMed note covers it in 4 sections, about 239 words (2 min of reading), with 12 flashcards.
In this note
Introduction
Requirements
Why expansion is so important
Not limiting the expansion
Introduction
Casting investment materials, or refractory investments, are auxiliary materials used to make metal and ceramic restorations by the lost wax technique. They form the heat-resistant mold into which molten metal is cast.
Requirements
Fluid when mixed: a low-viscosity suspension that tightly surrounds the finest details of the wax.
Fine-grained: for a smooth casting. Fine investment painted onto the wax improves this further.
Strong: it must not break or crumble under the pressure of the flowing metal.
Refractory: it must not melt or break down at the casting temperature.
Chemically inert: if it reacts with the alloy, deposits form, and grinding them off loses detail.
Porous: gases in the mold must escape through the pores, or the casting will be incomplete.
Expandable: its expansion must make up for the shrinkage of the alloy.
Heat-conductive: so that the mold expands evenly and the casting does not distort.
Easy to break away after cooling.
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