Gypsum-Bonded Investment in Dentistry

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

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.