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Questions and Answers
What is the primary purpose of dental investments?
What is the characteristic of a refractory material in dental investments?
What is an ideal requirement for an investment material?
What is the purpose of the lost wax technique?
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What is a characteristic of an ideal investment material?
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What is the term for the change of α form to β form?
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Which type of expansion occurs by contacting water during setting?
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What is a type of refractory material used in dental investments?
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What is the effect of increasing the age of investment on setting and hygroscopic expansion?
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What is a desirable property of an investment material?
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What is the purpose of using Type I gypsum-bonded investment?
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What is the purpose of investing procedures?
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What is the effect of higher water temperature on setting and hygroscopic expansion?
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What is the effect of thicker mixes on thermal expansion?
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What is the primary function of the binder material in investments?
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What is the purpose of adding boric acid and sodium chloride to investments?
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Which type of investment is suitable for casting gold alloys?
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What happens to gypsum when it is heated above 700-1000°C?
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What is the purpose of expansion in investments?
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What type of shrinkage do metals exhibit from solidification to room temperature?
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What is the result of the conversion of hemihydrate to dihydrate in gypsum-bonded investments?
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What type of transformation occurs in the refractory material during thermal expansion?
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Study Notes
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.
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Description
Test your knowledge of dental investments, including ideal requirements, composition, and reactions of different types of investment materials, as well as investing procedures.
