Dental Investments Quiz

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Questions and Answers

What is the primary purpose of dental investments?

  • To make a mold for casting molten metals or holding metal appliances (correct)
  • To withstand high temperature
  • To create a wax pattern of an object
  • To produce a smooth surface and fine details

What is the characteristic of a refractory material in dental investments?

  • It is porous enough to allow escape of gases
  • It breaks away readily after casting
  • It withstands high temperature (correct)
  • It is inexpensive

What is an ideal requirement for an investment material?

  • It must be able to withstand high temperature
  • It should not chemically react with cast metal
  • It should have sufficient strength at room temperature
  • All of the above (correct)

What is the purpose of the lost wax technique?

<p>All of the above (D)</p> Signup and view all the answers

What is a characteristic of an ideal investment material?

<p>All of the above (D)</p> Signup and view all the answers

What is the term for the change of α form to β form?

<p>Inversion (A)</p> Signup and view all the answers

Which type of expansion occurs by contacting water during setting?

<p>Hygroscopic expansion (B)</p> Signup and view all the answers

What is a type of refractory material used in dental investments?

<p>All of the above (D)</p> Signup and view all the answers

What is the effect of increasing the age of investment on setting and hygroscopic expansion?

<p>It decreases (D)</p> Signup and view all the answers

What is a desirable property of an investment material?

<p>It should produce a smooth surface and fine details (B)</p> Signup and view all the answers

What is the purpose of using Type I gypsum-bonded investment?

<p>For making inlays and crowns (B)</p> Signup and view all the answers

What is the purpose of investing procedures?

<p>To invest the pattern to make a mold (B)</p> Signup and view all the answers

What is the effect of higher water temperature on setting and hygroscopic expansion?

<p>It increases (A)</p> Signup and view all the answers

What is the effect of thicker mixes on thermal expansion?

<p>It increases (C)</p> Signup and view all the answers

What is the primary function of the binder material in investments?

<p>To set and bind the particles of refractory material (A)</p> Signup and view all the answers

What is the purpose of adding boric acid and sodium chloride to investments?

<p>To reduce thermal contraction of the binder on heating (B)</p> Signup and view all the answers

Which type of investment is suitable for casting gold alloys?

<p>Gypsum-bonded investment (C)</p> Signup and view all the answers

What happens to gypsum when it is heated above 700-1000°C?

<p>It degrades and reacts with SiO2 to produce CaSiO3 and SO3 (D)</p> Signup and view all the answers

What is the purpose of expansion in investments?

<p>To compensate for the shrinkage of the wax and metal (A)</p> Signup and view all the answers

What type of shrinkage do metals exhibit from solidification to room temperature?

<p>Shrinkage from solidification to room temperature (B)</p> Signup and view all the answers

What is the result of the conversion of hemihydrate to dihydrate in gypsum-bonded investments?

<p>Setting expansion (A)</p> Signup and view all the answers

What type of transformation occurs in the refractory material during thermal expansion?

<p>Allotropic transformation (A)</p> Signup and view all the answers

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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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