Investment material Flashcards
ILO 1.6c Be familiar with the chemical and physical properties as well as the clinical uses of a range of dental materials
What are the clinical uses of investment materials in dentistry (ILO dental uses)
To produce metal/alloy inlays, onlays, crowns and bridges
What technique for using investment materials involves….
Casting molten alloy under pressure by centrifugal force
What are the stages when using investment materials?
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.
What types of investment material are there?
- Dental stone/plaster - acrylic dentures
- Gypsum bonded materials - gold casting alloys
- Phosphate bonded materials - base metals/cast ceramics
- Silica bonded materials - base metal alloys
What are the ideal properties/requirements of an investment material? (ILO physical properties)
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
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
Gold alloys: 1.4%.
NiCr alloys: 2%
CoCr alloys: 2.3%
What are the components of investment material?
1) Binder - Gypsum, phosphate, silica-bonded
2) Refectory - Silica (quartz or cristobalite)
What is the purpose of in the binder component in investment materials?
To form a coherent solid mass/provides substance
What is the purpose of the refractory component in investment materials?
To withstand high temperatures
to give expansion
In the refractory component of investment materials, what form does quartz exist in at different temperatures?
Below 573oC - alpha-quartz (squashed crystalline lattice structure)
Above 573oC - beta-quartz (explodes to it’s maximum volume)
This is called inversion expansion
What is the composition of Gypsum-bonded Investment? What do some of the element do?
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)
What is the setting reaction for gypsum-bonded investment?
(CaSO4)2.H2O +3H20 (hemihydrate) –> 2CaSO4.2H20 (dehydrate)
What dimensional changes does gypsum-bonded investment material undergo
Silica - Thermal and inversion
Gypsum - setting expansion:
1) Hygroscopic expansion
2) Contraction above 320oC
In gypsum, what is hygroscopic expansion?
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.
What factors increase hydroscopic expansion?
Lower powder/water ratio
increased silica content
higher water temperature
longer immersion time in water
What temperature does gypsum contract?
above 320oC
Why does gypsum contract above 320oC?
a) water loss
b) presence of sodium chloride and boric acid
What are the properties of Gypsum bonded investment material?
- 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
At what temperature does gypsum-bonding investment have an unwanted reaction and why?
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
What technique it used when unwanted reactions occur in gypsum-bonded investment? And what is it’s purpose?
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
At what temperature is a gypsum bonded investment suitable for cast alloy? And why?
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
Gypsum-bonded investment are limited to alloys which which melting point
Below 1200oc
What is the composition of phosphate bonded investment
Powder
- silica
- Magnesium oxide
- ammonium phosphate
Liquid
- Water or colloidal silica
What are the properties of colloidal silica?
Increases strength
Gives hygroscopic expansion (2%) - compensated for alloy shrinkage
What is the setting reaction for phosphate bonded investment?
NH4.H2Po4 + MaO + 5H20 ->Mg NH4PO4.6H2O
Ammonium phosphate + magnesium oxide -> magnesium ammonium phosphate
What happens when you heat phosphate bonded invest to around 1000oC?
At 300oC water and ammonia liberated
at higher temperature complex reactions with silico-phosphates formed - increased strength
What are the properties of phosphate bonded investment?
High strength
porous
chemically stable
high “green” strength” - don’t need metal casting ring for support
easy to use
In investment material, what types of expansion are there? And what is their purpose?
Thermal,
Hygroscopic,
inversion
Counter alloy shrinkage on cooling
Why is strength important for investment material?
To withstand pressures during casting
Why is stability important for investment material?
Doesn’t degrade
What dimensional changes occur for silica-based investment materials?
- contraction at early stages of heating - water and alcohol loss from gel
- substantial thermal and inversion expansion - lots of silica present
What are the properties of silica-based investment materials?
- strength
- not porous - needs vents (alloy will end up porous and weak otherwise)
-complicated manipulation
