Investment Materials Flashcards

1
Q

What is the purpose of investment materials in dentistry?

what does the technique used to make them involve

A

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)

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2
Q

Describe the steps in the lost wax technique for casting dental restorations.

A
  1. Create a wax pattern of the restoration.
  2. Pour investment material around the wax pattern and allow it to set.
  3. Eliminate the wax (boiling in water or burning in an oven).
  4. Force molten alloy into the cavity left by the wax through sprues.
  5. Allow gases to escape; improper venting can cause back pressure and incomplete casting.
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3
Q

What are the main types of investment materials?

A
  • Dental stone or plaster.
  • Gypsum-bonded materials.
  • Phosphate-bonded materials.
  • Silica-bonded materials (used for acrylic dentures, gold casting alloys, base metals, and ceramics).
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4
Q

What are the key requirements for an effective investment material?

A
  • 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.
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5
Q

What are the typical volume contractions of alloys during cooling?

gold, ni/cr, co/cr

A

Gold alloys: 1.4%.
Ni/Cr alloys: 2.0%.
Co/Cr alloys: 2.3%.

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6
Q

What are the components of investment materials?

what is their role

A

Binder: Gypsum, phosphate, or silica to form a coherent solid mass.
Refractory: Silica (quartz or cristobalite) to withstand high temperatures and contribute to expansion.

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7
Q

Explain thermal expansion in refractory materials.

A

Silica (quartz and cristobalite) expands linearly with temperature, contributing to the necessary compensation for alloy shrinkage.

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8
Q

What is the composition of gypsum-bonded investments?

powder

A

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.

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9
Q

Describe the dimensional changes in gypsum-bonded investments.

Silica, Gypsum

A

Silica: Thermal and inversion expansion.
Gypsum: Setting and hygroscopic expansion; contraction above 320°C.

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10
Q

How does hygroscopic expansion occur?

A

water molecules attracted between crystals by capillary forces, forcing crystals apart

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11
Q

What factors increase hygroscopic expansion in gypsum-bonded materials?

A

Lower powder-to-water ratio.
Higher silica content.
Elevated water temperature.
Extended immersion time.

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12
Q

What does sodium chloride and boric acid reduce?

A

gypsum contraction above 320 degrees

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13
Q

What are the properties of gypsum-bonded investments?

A

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.

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14
Q

Why is heat soaking required in gypsum-bonded investments?

A

To complete reactions such as:

CaSO₄ + 4C → CaS + 4CO.
Prevents porosity from gas formation during casting.

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15
Q

What are the limitations of gypsum-bonded investments?

A

Unsuitable for alloys with melting points above 1200°C due to:

Formation of sulphur trioxide, causing porosity and corrosion.

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16
Q

What are the components of phosphate-bonded investments?

A

Powder: Silica, magnesium oxide, ammonium phosphate.
Liquid: Water or colloidal silica (increases strength and hygroscopic expansion).

17
Q

What does colloidal silica do?

A
  • increases strength
  • gives “hygroscopic” expansion (2%)
18
Q

What is the chemical reaction during the setting of phosphate-bonded investments?

A

Ammonium phosphate reacts with magnesium oxide and water:

(Product: Magnesium ammonium phosphate).

19
Q

What occurs during heating of phosphate-bonded investments?

A

At 330°C: Water and ammonia are liberated.
At higher temperatures: Complex silico-phosphate compounds are formed, increasing strength.

20
Q

What are the advantages of phosphate-bonded investments?

A
  • High green strength: Eliminates the need for metal casting rings.
  • Porous: Allows gases to escape during casting.
  • Chemically stable: Suitable for high temperatures (1000–1100°C).
  • Easy to manipulate: Convenient handling and preparation.
  • Compatible with alloys: Ideal for alloys with higher melting points.
21
Q

What are the limitations of phosphate-bonded investments?

A

More expensive compared to gypsum-bonded investments.
Handling may require precise control of powder-to-liquid ratio

22
Q

What is the process of preparing silica-bonded investments?

A

Stage 1: Stock solution preparation
* Ethyl silicate, dilute hydrochloric acid, and industrial spirit are mixed to form a silica sol:

Stage 2: Gelation
* Silica powder (quartz or cristobalite) is added to the stock solution to form a gel.
* Powder should be as dense as possible with varied particle sizes for strength.

Stage 3: Drying
- Tightly packed silica particles are formed.

23
Q

Why is MgO included in silica-bonded investment powders?

A

Magnesium oxide ensures alkaline conditions during gelation, which promotes better binding and setting of the silica particles.

24
Q

What dimensional changes occur in silica-bonded investments during heating?

A

Early heating stages: Contraction due to water and alcohol loss from the gel.
Later stages: Substantial thermal and inversion expansion due to the high silica content.

25
Q

What are the advantages of silica-bonded investments?

A

Strength: Sufficient to withstand high casting pressures.
Thermal and inversion expansion: Compensates for shrinkage of alloys during cooling.

26
Q

What are the disadvantages of silica-bonded investments?

A

Non-porous: Requires vents for the escape of trapped gases.
Complex manipulation: Requires precise preparation of stock solutions and gelation.

27
Q

Why are silica-bonded investments non-porous, and how is this issue addressed?

A

Silica particles create a tightly packed structure during drying, resulting in non-porosity. This is addressed by creating vents to allow gas escape during casting.

28
Q

What applications are silica-bonded investments particularly suited for?

A

High-temperature alloy casting, especially base metals and ceramics.

Scenarios requiring precise compensation for alloy shrinkage.