Investment Materials

Question 1

Gypsum-bonded investment is used for..

Answer 1

Alloys that melt <1300 degrees celsius

Question 2

List the 5 main components of gypsum-bonded investment:

Answer 2

1. α-hemihydrate of gypsum
2. Silica
3. Reducing Agents
4. Modifiers**(impt)
   - prevent most of the shrinkage of gypsum (due to water loss) when heated >300degrees
5. Colouring Agents

+ oxalate is added to protect molten metal from SO2

Question 3

What is the purpose of modifiers in gypsum-bonded investment?

Answer 3

To prevent most of the shrinkage of gypsum due to water loss
- regulate setting expansion & time

• damn impt cuz when the metal cools down itll shrink + if u add the shrinkage of investment material, ure crown will never fit

Question 4

Expansion of _______ compensates for shrinkage in _________ in gypsum-bonded investment.

Answer 4

Expansion of silica compensates for shrinkage of gypsum.

Question 5

Inversion of α type silicas to β type silicas come w _______.

Answer 5

Come w expansion

Question 6

Purpose of reducing agents (carbon, powdered carbon) in gypsum-bonded investment.

Answer 6

Provide reducing atmosphere in mould when alloy is used

Question 7

Inversion temperature of α-Quartz to β-Quartz.

Answer 7

575 degrees celsius

Question 8

Inversion temperature of α-Cristobalite to β-Cristobalite.

Answer 8

200-270 degrees celsius

Question 9

What are the 3 types of investment a gypsum-bonded investment undergoes.

Answer 9

1. Normal setting expansion (SE)
2. Hygroscopic setting expansion (HE)
3. Thermal expansion (TE)

Question 10

Name the types of investment materials.

Answer 10

1. Gypsum-bonded investment
2. Phosphate-bonded investment
3. Silica-bonded investment

Question 11

Types of gypsum-bonded investment:

Answer 11

Type I: inlays & crowns, casting shrinkage mainly compensated by Thermal Expansion
Type II: inlays & crowns, casting shrinkage mainly compensated by Hygroscopic Expansion (room temp)
Type III: for construction of partial dentures w gold alloys