Dental Investments Quiz

Dental Investments

• Dental investments are ceramic materials used to make a mold for casting molten metals or holding metal appliances in correct relation during soldering procedures.

Lost Wax Technique

• The lost wax technique involves making a wax pattern of the object, investing the pattern to make a mold, burning out the wax in a suitable furnace, and then melting and casting the alloy.

Ideal Requirements for an Investment

• Investments must be able to withstand high temperatures.
• Investments should not chemically react with cast metal.
• The investment mold should have sufficient strength at room temperature.
• Investments should have enough expansion to compensate for shrinkage of wax and metal.
• Investments should be porous enough to allow escape of gases during casting the metal.
• Investments should produce a smooth surface and fine details.
• Investments should break away readily after casting.
• Investments should be inexpensive and easily manipulated.

Composition of Investments

• Refractory material (65-75%): usually silicon dioxide (quartz, tridymite, or cristobalite) that withstands high temperatures and produces thermal expansion.
• Binder material (25-35%): a material that sets and binds the particles of refractory material, such as α-calcium sulfate hemihydrate, phosphate, or ethyl silicate.
• Other chemicals: boric acid and sodium chloride (to reduce thermal contraction of the binder on heating), graphite and copper (as reducing agents).

Types of Investments

• Gypsum-bonded investment: suitable for casting gold alloys, used when the casting temperature does not exceed 700-1000°C.
• Phosphate-bonded investment
• Silicate-bonded investment

Reactions of Gypsum-Bonded Investments

• Refractory material does not undergo any chemical reaction.
• The gypsum binder sets in the usual manner: CaSO4.1/2H2O + 11/2H2O → CaSO4.2H2O + heat.

Expansion of Investments

• Expansion is important to compensate for shrinkage of the wax and metal.
• Wax shrinks on cooling.
• Metal shows two types of shrinkage: from melting to solidification temperature and from solidification to room temperature.

Types of Expansion

• Setting expansion (0.5%): results from conversion of hemihydrate to dihydrate.
• Thermal expansion (1.2-1.6%): results from allotropic transformation of the refractory material by heat.
• Hygroscopic expansion (2-3%): occurs by contacting water during setting.

Factors Affecting Expansion

• Factors affecting thermal expansion: temperature, quantity of allotropic silica used, and thicker mixes.
• Factors increasing setting and hygroscopic expansion: finer particle size of powder, increased spatulation time, increased spatulation rate, higher water temperature, higher silica sulfate ratio, and increased water added during setting.
• Factors decreasing setting and hygroscopic expansion: increasing the age of investment, delaying the time of immersion, and increasing W/P ratio.