Investment materials

Question 1

How investment materials are used - lost wax technique

Answer 1

• wax pattern made (e.g. crown, bridge, inlay etc)
• investment material poured around wax patten and allowed to set
• wax then eliminated (e.g. in boiling water or by burning in oven)
• molten alloy then faced into mould cavity vacated by wax via channels prepared in investment material

Question 2

investment material types

Answer 2

• 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

Question 3

investment materials - requirements

Answer 3

expands
- compensates for cooling shrinkage of alloy
porous
- allows trapped gases on casting to escape

strong
room temp - ease of handling
casting temp - withstand casting forces

smooth surface
- easy finishing
chemically stable
- porosity, surface detal
easily removable from cast
- saves time
- relatively inexpensive as it is destroyed after use

Question 4

investment material components

Answer 4

binder - gypsum, phosphate, silica
- to form coherent solid mass
refractory - silica
- withstand high temperatures and gives expansion

Question 5

gypsum bonded investment - composition

Answer 5

powder mixed with water
- 60-65% silica
- 30-35% calcium sulphate hemihydrate
- reducing agent for oxides
- chemicals to inhibit heating shrinkage and control setting time

Question 6

gypsum bonded investment dimensional changes

Answer 6

silica
- thermal and inversion
gypsum
- setting expansion

Question 7

gypsum bonded properties

Answer 7

• smooth surface
• easy manipulation
• low expansion 1.4% - sufficient for gold alloys
• porous
• adequate strength

Question 8

phosphate bonded investment composition

Answer 8

powder
- silica
- magnesium oxide
- ammonium phospahte

liquid
- water or colloidal silica

Question 9

colloidal silica solution function

Answer 9

• increases strength
• gives hygroscopic expansion

Question 10

What is hygroscopic expansion?

Answer 10

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

Question 11

Factors increasing hygroscopic expansion

Answer 11

lower powder/water ratio
increased silica content
higher water temperature
longer immersion time

Question 12

silica investment - stages

Answer 12

1 - prepare stock solution
2 - add powder - gelation
3 - drying = tightly packed silica crystals