Investment Materials Flashcards
What is the purpose of investment materials in dentistry?
what does the technique used to make them involve
Used to produce metal/alloy inlays, onlays, crowns, and bridges.
The technique involves casting molten alloy (under pressure, by centrifugal force) into a mold cavity created with an investment material. (i.e the alloy is surrounded by an investment material)
Describe the steps in the lost wax technique for casting dental restorations.
- Create a wax pattern of the restoration.
- Pour investment material around the wax pattern and allow it to set.
- Eliminate the wax (boiling in water or burning in an oven).
- Force molten alloy into the cavity left by the wax through sprues.
- Allow gases to escape; improper venting can cause back pressure and incomplete casting.
What are the main types of investment materials?
- Dental stone or plaster.
- Gypsum-bonded materials.
- Phosphate-bonded materials.
- Silica-bonded materials (used for acrylic dentures, gold casting alloys, base metals, and ceramics).
What are the key requirements for an effective investment material?
- Expand: Compensate for cooling shrinkage of the alloy.
- Porous: Allow gases to escape and prevent back pressure.
- Strong: Withstand handling at room temperature and casting forces at high temperatures.
- Smooth surface: Ensure easy finishing of the cast.
- Chemically stable: Maintain integrity and surface detail.
- Easy to remove: Minimize technician time.
- Inexpensive: Single-use and disposable.
What are the typical volume contractions of alloys during cooling?
gold, ni/cr, co/cr
Gold alloys: 1.4%.
Ni/Cr alloys: 2.0%.
Co/Cr alloys: 2.3%.
What are the components of investment materials?
what is their role
Binder: Gypsum, phosphate, or silica to form a coherent solid mass.
Refractory: Silica (quartz or cristobalite) to withstand high temperatures and contribute to expansion.
Explain thermal expansion in refractory materials.
Silica (quartz and cristobalite) expands linearly with temperature, contributing to the necessary compensation for alloy shrinkage.
What is the composition of gypsum-bonded investments?
powder
Silica (60–65%).
Calcium sulfate hemihydrate (30–35%).
Reducing agents to prevent oxide formation.
Chemicals like boric acid and NaCl to inhibit shrinkage and control setting time.
Describe the dimensional changes in gypsum-bonded investments.
Silica, Gypsum
Silica: Thermal and inversion expansion.
Gypsum: Setting and hygroscopic expansion; contraction above 320°C.
How does hygroscopic expansion occur?
water molecules attracted between crystals by capillary forces, forcing crystals apart
What factors increase hygroscopic expansion in gypsum-bonded materials?
Lower powder-to-water ratio.
Higher silica content.
Elevated water temperature.
Extended immersion time.
What does sodium chloride and boric acid reduce?
gypsum contraction above 320 degrees
What are the properties of gypsum-bonded investments?
Expansion: Total expansion sufficient for gold alloys (1.4%).
Smooth surface: Fine particles provide good detail.
Porosity: Allows trapped gases to escape.
Strength: Adequate with proper manipulation and correct ratios.
Chemical stability: Satisfactory below 1200°C.
Why is heat soaking required in gypsum-bonded investments?
To complete reactions such as:
CaSO₄ + 4C → CaS + 4CO.
Prevents porosity from gas formation during casting.
What are the limitations of gypsum-bonded investments?
Unsuitable for alloys with melting points above 1200°C due to:
Formation of sulphur trioxide, causing porosity and corrosion.