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Gypsum-Bonded Investment in Dentistry
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Gypsum-Bonded Investment in Dentistry

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Questions and Answers

What is the purpose of leaving air spaces or vents in the investment?

  • To allow for the wax pattern to expand
  • To fix the wax pattern in space
  • To create a channel for the ingress of molten alloy
  • To permit escape of the air from the mold (correct)
  • What is the purpose of spruing the wax pattern?

  • To fix the wax pattern in space so a mold can be made
  • To create a channel for the elimination of wax during burnout (correct)
  • To reduce air bubbles and porosity in the investment
  • To form a channel for the ingress of molten alloy during casting
  • What is the advantage of using a hollow sprue over a solid one?

  • It is more difficult to attach to the wax pattern
  • It is stronger in its attachment to the wax pattern (correct)
  • It is weaker in its attachment to the wax pattern
  • It is easier to remove from the mold
  • What is the purpose of the casting ring?

    <p>To hold the investment in place during setting and restrict mold expansion</p> Signup and view all the answers

    What is the purpose of the vacuum technique in investing?

    <p>To reduce air bubbles and porosity in the investment</p> Signup and view all the answers

    What is the purpose of applying the casting ring over the crucible former?

    <p>To hold the investment in place during setting</p> Signup and view all the answers

    What is the advantage of using a split rubber ring over a metallic ring?

    <p>It is more flexible and can be reused</p> Signup and view all the answers

    What is the first step in the brush technique of investing?

    <p>The accurate water-powder ratio is mixed under vacuum</p> Signup and view all the answers

    What is the purpose of inverting the bowl during the vacuum technique?

    <p>To fill the ring under vacuum and vibration</p> Signup and view all the answers

    Study Notes

    Gypsum-Bonded Investments

    • Used for casting gold alloys with low melting range (700-1000°C)
    • Composition:
      • Crystalline polymorphs of silica (quartz or cristobalite) - 55-75% (refractory material, regulates thermal expansion)
      • Calcium sulfate hemihydrate (plaster or stone) - 25-45% (binder, imparts strength, contributes to mold expansion)
    • Modifiers:
      • Accelerators, retarders, or reducing agents (e.g., powdered charcoal or copper) to reduce oxide formation in metal during casting
    • Dimensional changes:
      • Normal setting expansion (linear expansion during setting in air, caused by crystal growth of gypsum)
      • Hygroscopic expansion (linear expansion when in contact with water during setting, can be achieved through immersion, wet liner, or addition of water)
      • Thermal expansion (rapid expansions between 200-300°C and 500-600°C due to displacive transformation of silica)
    • Other properties:
      • The total expansion of the mold compensates for the shrinkage on cooling of gold alloys
      • The set investment is porous, preventing porosity in castings
      • The strength of the set material withstands the forces of molten alloy
    • Limitations:
      • Above 1200°C, a reaction occurs between calcium sulphate and silica, causing porosity and corrosion in the casting

    Phosphate-Bonded Investments

    • Used for casting alloys with high melting temperature (1200°C), such as base metal alloys for ceramo-metal restorations
    • Composition:
      • Binder: Magnesium-oxide (basic) and ammonium dihydrogen phosphate
      • Refractory: Colloidal silica in water (increases expansion and enhances casting surface smoothness)
    • Modifiers:
      • Carbon as a reducing agent
    • Setting reaction:
      • Magnesium oxide reacts with ammonium dihydrogen phosphate to form magnesium ammonium phosphate
    • Manipulation:
      • Mixed with water, but no metal casting rings are needed; plastic rings are used and removed after setting
    • Properties:
      • Expansion (setting reaction and thermal expansion)
      • Strength increases during heating due to chemical interaction between silica and binder
      • Porosity is present in the set material

    Silicate-Bonded Investments

    • Used for casting high-fusing partial denture alloys
    • Composition:
      • Powder: Refractory particles of silica and magnesium oxide
      • Liquid: Diluted water-soluble ethyl silicate solution and diluted hydrochloric acid
    • Setting reaction:
      • Stage 1: Hydrolysis (ethyl silicate hydrolyzed to silicic acid)
      • Stage 2: Gelation (silicic acid mixed with powder, leading to gel formation)
      • Stage 3: Drying (heating, shrinkage, and loss of alcohol and water)
    • Properties:
      • Dimensional changes (large thermal expansion due to high silica content)
      • Porosity is negligible due to close packing of silica particles
    • Investing techniques:
      • Brush technique (accurate water-powder ratio, brush applied to wax pattern, and casting ring filled under vibration)
      • Vacuum technique (mix spatulated, then vacuum and vibration used to fill the ring)
    • Importance of spruing the wax pattern:
      • Fixes the wax pattern in space
      • Creates a channel for wax elimination during burnout
      • Forms a channel for molten alloy ingress during casting

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    Description

    This quiz covers the uses and composition of gypsum-bonded investment in dentistry, including its refractory properties and thermal expansion regulation. Learn about the role of calcium sulfate hemihydrate and crystalline polymorphs of silica in this important dental material.

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