Investment material

Question 1

What are the clinical uses of investment materials in dentistry (ILO dental uses)

Answer 1

To produce metal/alloy inlays, onlays, crowns and bridges

Question 2

What technique for using investment materials involves….

Answer 2

Casting molten alloy under pressure by centrifugal force

Question 3

What are the stages when using investment materials?

Answer 3

1) Wax pattern of the required prosthesis e.g crown, inlay (ie a positive replica)

2) Investment material poured around wax pattern and allowed to set (mould, a negative replica)

3) The wax removed – by burning or with boiling water.

4) Molten alloy:
- Poured into mould cavity
– via the sprue – hollow tubes that allow the alloy to flow in.

Question 4

What types of investment material are there?

Answer 4

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

Question 5

What are the ideal properties/requirements of an investment material? (ILO physical properties)

Answer 5

1) Expands - compensate for cooling shrinkage of alloy
2) Porous - allow escape of trapped gases
3) Strong
- Room Temp - easy of handling (“green strength)
-Casting temp - withstand casting forces

Further ideal properties/requirements
- Smooth finish- for easy finishing
- Chemically stable - porosity, surface detail
- Easy removal from cast
- Handling - not complicated
- Relatively inexpensive - destroyed after use

Question 6

What are the typical contractions/shrinkage (by volume) for alloy melting point to room temperature of gold alloy, Ni/Cr alloys and Co/Cr alloys

Answer 6

Gold alloys: 1.4%.
NiCr alloys: 2%
CoCr alloys: 2.3%

Question 7

What are the components of investment material?

Answer 7

1) Binder - Gypsum, phosphate, silica-bonded
2) Refectory - Silica (quartz or cristobalite)

Question 8

What is the purpose of in the binder component in investment materials?

Answer 8

To form a coherent solid mass/provides substance

Question 9

What is the purpose of the refractory component in investment materials?

Answer 9

To withstand high temperatures
to give expansion

Question 10

In the refractory component of investment materials, what form does quartz exist in at different temperatures?

Answer 10

Below 573oC - alpha-quartz (squashed crystalline lattice structure)
Above 573oC - beta-quartz (explodes to it’s maximum volume)

This is called inversion expansion

Question 11

What is the composition of Gypsum-bonded Investment? What do some of the element do?

Answer 11

Power (mixed with water)
- Silica (60-65%) - withstand high temperatures (refractory component)
- Calcium Sulphate hemihydrate (30-35%) - produces calcium sulphate di-hydrate when mixed with water
- reducing agent for oxides
-chemicals to inhibit heating shrinkage and control setting time (Boric acid, Nacl)

Question 12

What is the setting reaction for gypsum-bonded investment?

Answer 12

(CaSO4)2.H2O +3H20 (hemihydrate) –> 2CaSO4.2H20 (dehydrate)

Question 13

What dimensional changes does gypsum-bonded investment material undergo

Answer 13

Silica - Thermal and inversion
Gypsum - setting expansion:
1) Hygroscopic expansion
2) Contraction above 320oC

Question 14

In gypsum, what is hygroscopic expansion?

Answer 14

Expansion:
- 5X change in volume of gypsum-bonded investment materials.
- Expansion due to capillary forces pulling water molecules into gaps between crystals (of calcium sulphate hemi-hydrate) – forcing them apart.

Question 15

What factors increase hydroscopic expansion?

Answer 15

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

Question 16

What temperature does gypsum contract?

Answer 16

above 320oC

Question 17

Why does gypsum contract above 320oC?

Answer 17

a) water loss
b) presence of sodium chloride and boric acid

Question 18

What are the properties of Gypsum bonded investment material?

Answer 18

• Expansion:
  
  • expands by 1.4%
  • good enough expansion for gold alloy
• Smooth surface
  
  • fine particles
• manipulation
  
  • easy
  • setting time controlled
• Porous
  
  • good
• Strength
• adequate if correct powder/liquid ratio and correct manipulation

Question 19

At what temperature does gypsum-bonding investment have an unwanted reaction and why?

Answer 19

above 700oC
Due to wax residue or graphite in the Investment Material leading to production of:
carbon dioxide
sulphur dioxide

CaS04 + 4C –> CaS + 4CO
then
3CaS04 + CaS –> 4CaO + 4SO2

Question 20

What technique it used when unwanted reactions occur in gypsum-bonded investment? And what is it’s purpose?

Answer 20

Heat soaking

Important CO and SO2 produced above 700oC can escape. Heat soaking - when the Investment Material is held at a high temperature fore some time allowing the gases to gradually escape

Question 21

At what temperature is a gypsum bonded investment suitable for cast alloy? And why?

Answer 21

Below 1200oC - satisfies requirement
Above 1200oC - problems due to production of sulphur trioxide which will produce voids in the cast alloy and contributes to corrosion

CaS04+SiO2 –> CaSiO3 + SO3

Question 22

Gypsum-bonded investment are limited to alloys which which melting point

Answer 22

Below 1200oc

Question 23

What is the composition of phosphate bonded investment

Answer 23

Powder
- silica
- Magnesium oxide
- ammonium phosphate

Liquid
- Water or colloidal silica

Question 24

What are the properties of colloidal silica?

Answer 24

Increases strength
Gives hygroscopic expansion (2%) - compensated for alloy shrinkage

Question 25

What is the setting reaction for phosphate bonded investment?

Answer 25

NH4.H2Po4 + MaO + 5H20 ->Mg NH4PO4.6H2O Ammonium phosphate + magnesium oxide -> magnesium ammonium phosphate

Question 26

What happens when you heat phosphate bonded invest to around 1000oC?

Answer 26

At 300oC water and ammonia liberated at higher temperature complex reactions with silico-phosphates formed - increased strength

Question 27

What are the properties of phosphate bonded investment?

Answer 27

High strength porous chemically stable high "green" strength" - don't need metal casting ring for support easy to use

Question 28

In investment material, what types of expansion are there? And what is their purpose?

Answer 28

Thermal, Hygroscopic, inversion Counter alloy shrinkage on cooling

Question 29

Why is strength important for investment material?

Answer 29

To withstand pressures during casting

Question 30

Why is stability important for investment material?

Answer 30

Doesn't degrade

Question 31

What dimensional changes occur for silica-based investment materials?

Answer 31

- contraction at early stages of heating - water and alcohol loss from gel - substantial thermal and inversion expansion - lots of silica present

Question 32

What are the properties of silica-based investment materials?

Answer 32

- strength - not porous - needs vents (alloy will end up porous and weak otherwise) -complicated manipulation