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Questions and Answers
What is the purpose of leaving air spaces or vents in the investment?
What is the purpose of spruing the wax pattern?
What is the advantage of using a hollow sprue over a solid one?
What is the purpose of the casting ring?
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What is the purpose of the vacuum technique in investing?
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What is the purpose of applying the casting ring over the crucible former?
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What is the advantage of using a split rubber ring over a metallic ring?
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What is the first step in the brush technique of investing?
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What is the purpose of inverting the bowl during the vacuum technique?
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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.