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3 DMS > Investment Materials > Flashcards

Investment Materials Flashcards

1
Q

What are investment materials used for?

A

Producing metal/alloy inlays, onlays, crowns and bridges

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2
Q

What are the stages involving investment materials?

A

Wax pattern made
Investment material poured around wax pattern and allowed to set
Wax is then eliminated by boiling or burning
Molten alloy forces into mould cavity vacated by wax via channels prepared in the investment material

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3
Q

What are the types of investment material and when are they used?

A

Dental stone or plaster - acrylic dentures
Gypsum bonded materials - gold casting alloys
Phosphate bonded materials - base metals/cast ceramics
Silica bonded materials - base metal alloys

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4
Q

What are the main requirements of investment materials?

A

Must expand to compensate for cooling shrinkage of alloy
Must be porous to allow escape of trapped gases
Must be strong at room temp and casting temp

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5
Q

What are some other requirements of investment materials?

A

Smooth surface for easy finishing
Chemically stable
Easy removing from cast
Easy handling
Inexpensive as it will be destroyed after the process

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6
Q

What are the typical contractions by volume for different alloys mp to room temp?

A

Gold alloys - 1.4%
Ni/Cr alloys - 2%
Co/Cr alloys - 2.3%

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7
Q

What are the components of investment materials and what do they do?

A

Binder such as gypsum, phosphate or silica to form a solid mass and provide substance
Refractory such as silica to withstand high temp and give expansion

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8
Q

What is the composition of gypsum bonded investment material?

A

Powder mixed with water
Silica - 60-65%
Calcium sulphate hemihydrate - 30-35%
Reducing agent for oxides
Chemicals to inhibit heating shrinkage and control setting time

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9
Q

What is the setting reaction of gypsum bonded investment material?

A

Calcium sulfate hemihydrate and water react to produce calcium sulfate dihydrate

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10
Q

What dimensional changes are seen in gypsum bonded investment material?

A

Silica - thermal and inversion
Gypsum - setting expansion - hygroscopic expansion and contraction above 320ºC

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11
Q

What is hygroscopic expansion?

A

Water molecules attracted between crystals by capillary forces, forcing crystals apart
Can be up to 5x volume setting expansion

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12
Q

What factors increase hygroscopic expansion?

A

Lower powder/water ratio
Increased silica content
Higher water temp
Longer immersion time in water

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13
Q

What causes the contraction of gypsum above 320ºC?

A

Water loss
The presence of sodium chloride and boric acid

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14
Q

What are the properties of gypsum bonded investment material?

A

Expansion - 1.4% by volume
Smooth surface - fine particles leave a smooth surface
Easy manipulation
Porous
Sufficient strength

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15
Q

Why is gypsum bonded investment material heat soaked?

A

To allow reactions to complete and gases to escape

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16
Q

When is gypsum bonded investment material chemically stable and why is this a problem?

A

Stable below 1200ºC
If alloy melts above this temp, a new investment material is needed

17
Q

What is the composition of phosphate bonded investment?

A

Powder:
- silica
- magnesium oxide
- ammonium phosphate
Liquid:
- water or colloidal silica

18
Q

Why is colloidal silica used over water in phosphate bonded investment?

A

It increases strength and gives hygroscopic expansion of 2%

19
Q

What is the setting reaction of phosphate bonded investment?

A

Ammonium phosphate and magnesium oxide react to produce magnesium ammonium phosphate

20
Q

What happens when phosphate bonded investment is heated between 1000 - 1100ºC?

A

At 330ºC water and ammonia are released
At higher temps complex reactions increase strength

21
Q

What are the properties of phosphate bonded investment?

A

Easy to use
High strength
Porous
Chemically stable

22
Q

What are the stages of using silica investment?

A

Prepare stock solution
Add powder
Drying

23
Q

What are the properties of silica>

A

Sufficient strength
Not porous
Complicated manipulation

3 DMS (8 decks)

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