Investment materials

Question 1

Why are the materials used to make investment moulds different for dentures and metallic devices?

Answer 1

Molten substance needs to be poured into the mould = needs to be above melting point

Dentures = below 150 = gypsum used

Metallic devices = 800-1500= gypsum breaks down

Question 2

What are refractory materials? Whats the most common in dentistry?

Answer 2

Stable at high temperatures

Silica

Question 3

Silica is found as particles, what is used to bind the particles to be used as mould for metallic devices?

Answer 3

Gypsum or phosphate

Question 4

What needs to be compensated for during casting?

Answer 4

Shrinkage

Question 5

What dimensional changes occur during casting?

Answer 5

Thermal expansion
Setting expansion
Inversion
Hygroscopic expansion - water enters

Question 6

How to compensate for thermal shrinkage?

Answer 6

Mould space bigger at heating temperature

Question 7

How to compensate for inversion?

Answer 7

Controlled by amount of quartz and cristobalite

Question 8

Disadvantages of gypsum bonded investments?

Answer 8

Breaks down at higher temperatures - unsuitable for high melting alloys e.g. base alloys

Question 9

What investment material is used for acrylic curing

Answer 9

Gypsum

Question 10

What investment is used for casting gold alloys?

Answer 10

Gypsum bonded investments

Question 11

What investment is used for casting base alloys?

Answer 11

Phosphate bonded investment