investment materials Flashcards
investment materials
sed in the production of inlays, onlays, crowns and bridges that are made of
an alloy
A casting process is needed for these - requiring centrifugal forces.
The casting process requires an INVESTMENT MATERIAL of the required shape, to contain the
molten alloy and withstand the high temperatures involved, and ensure the ALLOY’S dimensions are sustained.
We’ll see that the melting point of an alloy is a determining factor as to which INVESTMENTMATERIAL is most suitable.
stages for shaping investment materials
1) A wax pattern of the required prosthesis – crown, inlay – is created (ie a positive replica)
2) An InvMater is placed around this wax pattern and allowed to set. It forms a mould (a negative replica)
3) 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
4) 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.
when alloy is cast using investment materials
PRESSURE must be applied to ensure no gaps or voids form within it. The
InvMater has to be strong enough to withstand the forces generated during this process.
Inevitably, as the alloy is cast, GASES will be produced.
It’s 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.
Another feature of this process is tha, on cooling, the alloy contracts.
Therefore it won’t be the same shape as that determined by the mould cavity.
ideal properties inv mater
Smooth surface: easy finishing
Chemically stable: porosity, surface detail
Easy removal from cast: technician time
Handling: not complicated
Relatively inexpensive: - destroyed
investment material types
dental stone or plaster = acrylic dentures
gypsum bonded materials = gold casting alloys
phosphate bonded materials = base materials / cast ceramics
silica bonded materials = base metal alloys
typical contractions from alloy melting pt to room temperature
Gold alloys: 1.4 %
Ni / Cr alloys: 2.0 %
Co / Cr alloys: 2.3 %
investment materials components
Binder: Gypsum; Phosphate; Silica
to form coherent solid mass
Refractory: Silica (quartz or cristobalite)
withstand high temperatures
also gives expansion
gypsum bonded InvMat
Powder (mixed with water) =
silica (60 - 65 %)
calcium sulphate hemihydrate (30 - 35%)
reducing agent for oxides
chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)
hydroscopic expansion
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.
several factors that affect HYGROSCOPIC expansion:
a lower powder/water ratio
increased silica content
using water at a higher temperature
and immersing the powder in water for longer
above 320ᵒC, GYPSUM will contract because of
a) water loss
b) the presence of sodium chloride and boric acid
properties does gypsum-bonded InvMater possess.
it expands by 1.4% - which is sufficient for casting GOLD alloys (which will shrink by the same amount on cooling).
its fine particles leave a smooth surface – which is good
it is sufficiently POROUS to uptake the gases released when casting the alloys
it has sufficient strength
unwanted reaction at 700C
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.
To ensure this happens the InvMater undergoes what’s called HEAT SOAKING – that is, it’s held at
a high temperature for some time – and this enables the gases to gradually escape.
phosphate bonded inv mater
Its powder consists of
silica
magnesium oxide
ammonium phosphate
The powder is mixed with water OR colloidal silica.
reason for colloidal silica mix
mix with the PHOSPHATE-BONDED InvMater powder is:
to increase its strength
to achieve 2% expansion – this hygroscopic expansion will compensate for the alloy shrinkage when its
cooled to room temperature.
setting reaction for phosphate bonded inv mater
Ammonium phosphate reacts with magnesium oxide and water to produce Magnesium
Heating PHOSPHATE-BONDED InvMater to around 1,000ᵒC or so results in
H2O and ammonia being released at 330ᵒC
And then complex reactions occur with silico phosphates – which generate increased strength.
The chemistry here isn’t important – but the outcome is
