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Questions and Answers
What is the primary purpose of dental investments?
What is the primary purpose of dental investments?
What is the characteristic of a refractory material in dental investments?
What is the characteristic of a refractory material in dental investments?
What is an ideal requirement for an investment material?
What is an ideal requirement for an investment material?
What is the purpose of the lost wax technique?
What is the purpose of the lost wax technique?
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What is a characteristic of an ideal investment material?
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?
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?
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?
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?
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?
What is a desirable property of an investment material?
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What is the purpose of using Type I gypsum-bonded investment?
What is the purpose of using Type I gypsum-bonded investment?
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What is the purpose of investing procedures?
What is the purpose of investing procedures?
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What is the effect of higher water temperature on setting and hygroscopic expansion?
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?
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?
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?
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?
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?
What happens to gypsum when it is heated above 700-1000°C?
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What is the purpose of expansion in investments?
What is the purpose of expansion in investments?
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What type of shrinkage do metals exhibit from solidification to room temperature?
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?
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?
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.
