Investment Materials Flashcards
when are investment materials used in dentistry
to produce metal/alloy inlays, onlays, crowns and bridges
investment technique involves
casting molten alloy (under pressure, by centrifugal force)
requires a mould cavity of the required shape (i.e. the alloy is surrounded by an investment material)
what is needed to pour in the IM
wax patter (e.g. for crown, inlay etc)
what happens to wax pattern after IM poured in and set
wax is eliminated (e.g. boiling in water or burning in over)
what happens after the wax is burnt out
molten alloy forced into the investment material mould cavity
- via channels (SPRUES) prepared in investment material
stages in using investment material
- A wax pattern of the required prosthesis – crown, inlay – is created (ie a positive replica)
- An InvMater is placed around this wax pattern and allowed to set. It forms a mould (a negative replica)
- The wax is then removed – by burning or with boiling water. So now we have a cavity of the required shape surrounded by the InvMater
- Next, the molten alloy is poured into the mould cavity – and this is done via the sprue – hollow tubes that allow the alloy to flow in.
what needs to be applied to investment material when being poured
PRESSURE must be applied to ensure no gaps or voids form within it
- Material has to be strong enough to withstand the forces generated during this process.
Inevitably, as the alloy is cast, GASES will be produced.
- crucial that these gases are allowed to escape – and be captured by the InvMater.
Otherwise the alloy itself will have voids – and so it will be POROUS.
on cooling, the alloy contracts.
- Therefore, it won’t be the same shape as that determined by the mould cavity.
4 investment types
dental stone or plaster
gypsum bonded materials
phosphate bonded materials
silica bonded materials
dental stone of plaster IM used for
acrylic dentures
gypsum bonded materials IM used for
gold casting alloys
phosphate bonded materials IM used for
base materials/cast ceramics
silica bonded materials IM used for
base metal alloys
requirements of IM (8)
expand
porous
strong
smooth surface
chemically stable
easy removal from cast
handling
relatively inexpensive
why do IMs need to expand
compensate for cooling shrinkage of alloy
why do IMs need to be porous
allow escape of trapped gases on casting
- back pressure effect
why do IMs need to be strong
room temp - ease of handling (“Green strength”)
casting temp - withstand casting forces
why do IMs need to have a smooth surface
allow easy finishing
why do IMs need to be chemically stable
porosity, surface detail
why do IMs need to be easily removed from cast
saves technician time
why do IMs need to be relatively inexpensive
destroyed after use
typical contraction of Gold alloys (by volume) from alloy melting pt to room temp
1.4%
typical contraction of Ni/Cr alloys (by volume) from alloy melting pt to room temp
2.0%
typical contraction of Co/Cr alloys (by volume) from alloy melting pt to room temp
2.3 %
popular alloy
2 components of investment materials
binder - gypsum; phosphate silica
refractory - silica
binder role in IM
to form coherent solid mass
gypsum; phosphate silica
refractory role in IM
withstand high temperatures
also gives expansion
silica (quarts or cristobalites)
quartz thermal expansion
QUARTZ, the linear thermal expansion rose gradually until 570oC; then it climbed more rapidly.
Quartz below this temperature exists in a form called alpha-quartz
- squashed crystalline lattice structure
But beyond this temperature, its structure changes – to beta-quartz –explodes to its maximum volume.
gypsum bonded investment material composition
(mixed with water) :
silica (60 - 65 %) – refractory to withstand high temperatures
calcium sulphate hemihydrate (30 - 35%)
reducing agent for oxides
chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)
setting of gypsum bonded investment material
as in gypsum products
(CaSO4)2 .H2O + 3H2O -> 2CaSO4 . 2H2O
hemihydrate -> dihydrate
dimensional change gypsum bonded investment material
Silica
- thermal & inversion
Gypsum
- setting expansion
- hygroscopic expansion
- contraction above 320OC
hygroscopic expansion
mechanism not fully understood
water molecules attracted between crystals by capillary forces, forcing crystals apart
- the expansion is considered to be due to capillary forces pulling water molecules into gaps between crystals (of calcium sulphate hemi-hydrate) – forcing them apart.
can be up to 5 X setting expansion
hygroscopic expansion increased by (4)
lower powder / water ratio
increased silica content
higher water temperature
longer immersion time
gypsum bonded contraction above 320 degrees effects (2)
water loss
significant reduction by sodium chloride and boric acid
5 properties of gypsum bonded investment materials
expansion
manipulation
porous
sterngth
expansion of gypsum bonded investment materials
total expansion sufficient for gold alloys - shrink by 1.4% on cooling
1.4% by volume
smooth surface of gypsum bonded investment materials
fine particles give smooth surface
manipulation of gypsum bonded investment materials
easy
setting time controlled
porousity of gypsum bonded investment materials
good - can uptake gases released on casting alloys
strength of gypsum bonded investment materials
adequate if correct powder/liquid ration and correct manipulation
heat soaking of gypsum bonded investment materials
above 700OC - reaction between CaSO4 and C (wax residue or graphite in investment)
CaSO4 + 4C -> CaS + 4CO
then
3CaSO4 + CaS -> 4CaO + 4SO2
Hence heat soak to allow reactions to complete & gases escape
CaSO4 and Carbon can release carbon monoxide; and the calcium sulphide that’s also produced, may then react with CaSO4 to produce sulphur dioxide gas.
It’s crucial these gases – CO and SO2 – escape.
- HEAT SOAKING to do so
held at a high temperature for some time – and this enables the gases to gradually escape.
reason for heat soaking gypsum bonded IM
CaSO4 and Carbon can release carbon monoxide; and the calcium sulphide that’s also produced, may then react with CaSO4 to produce sulphur dioxide gas.
It’s crucial these gases – CO and SO2 – escape.
- HEAT SOAKING to do so
held at a high temperature for some time – and this enables the gases to gradually escape.
chemical stability of gypsum bonded IM
below 1200C - satisfies requirements (alloy needs less that 1200C to be cast)
above 1200C - problems: CaSO4 + SiO2 -> CaSiO3 + SO3 - sulphur trioxide: porosity in casting contributes to corrosion
hence limited to alloys with melt. pt < 1200C
phosphate bonded IM composition
Powder
- silica
- magnesium oxide
- ammonium phosphate
Liquid
- water or colloidal silica
colloidal silica
can be liquid used for phosphate bonded IM
increases strength
gives “hygroscopic” expansion (2%)
setting for phosphate bonded IM
NH4.H2PO4 + MgO + 5H2O -> Mg NH4PO4 . 6H2O magnesium ammonium phosphate
Ammonium phosphate reacts with magnesium oxide and water to produce Magnesium ammonium phosphate
heating for phosphate bonded IM (1000-1100 degrees)
at 330C - water and ammonia liberated
at higher temperatures complex reactions with silico-phosphates formed
- increased strength
properties of phopshate bonded IM (5)
High “green” strength:
- don’t need metal casting ring for support
Easy to use
High Strength
Porous
Chemically Stable
Also has sufficient strength to be handled in the preparatory stages of the process. and it’s straightforward to use.
silica IM dimensional change
contraction at early stages of heating
- water and alcohol loss from gel
substantial thermal and inversion expansion
- lots of silica present
properties of silica IM
dimensional change
strength - sufficient
not porous - needs vents
complicated manipulation
key fault in silica bonded IM
though sufficiently strong, is complicated to use.
And crucially, it is not a POROUS material
- not able to capture the gases normally released during the casting process.
- means the alloy itself would end up porous – and weak.
To overcome this, special equipment – WITH VENTS – is used with this material.
not used in GDH
investment materials =
binder + refractory
e.g.
gypsum + silica
properties of IM to asses (4 key)
Porosity
- trap gases released during casting
Expansion (thermal, hygroscopic, inversion)
- counter alloy shrinkage on cooling
Strength
- withstand pressures during casting
Stability
- doesn’t degrade
