Investment Material Flashcards
what are investment materials used in the production of
inlays, onlays, crowns and bridges that are made of an alloy
a casting method is required for inlays, onlays and bridges - what forces are required for this process
centrifugal forces
what is an investment material
it is the required shape to contain the molten alloy and withstand the high temperatures involved and make sure the alloy’s dimensions are maintained
what is the first stage in investing material
a wax pattern of the required prosthesis (crown/ inlay) is created - positive replica
what is the second stage in investment material
investment material is placed around the wax pattern and allowed to set to form a mould - the negative replica
what is the third stage in investment material
wax is removed - by burning or boiling water so there is a cavity of the required shape around the investment material
what is the fourth stage in investment material
molten alloy is poured into the mould cavity via the sprue
what must be applied when the alloy is cast and why
pressure so that no gaps or void form in it
what will inevitably be produced when the alloy is cast
gases
why is it important what gases are allowed to escape and captured by the investment material
so the alloy will not have voids and be porous
what happens upon cooling of an alloy
it contracts, and will not be the same shape as determined by the mould cavity
what does investment material need to be with regards to porosity and why
needs to be porous and allow the gases released to be captured/ stored within it which maintaining its own shape
why do investment materials need to have a smooth surface
so the alloys surface wont need much finishing work to be done by the technician
what alloy has the highest shrinkage upon setting and which has the lowest
gold has the lowest
cobalt chrome has the highest
what are the two components in an investment material
refractory component and a binder
what is the refractory component in an investment material
a silica - quartz or cristobalite which withstands high temperatures and undergoes expansion
what is the binder in an investment material
it forms a coherent mass to provide substance
what are the three types of binder available
gypsum, phosphate and silica bonded
what occurs to quartz and cristobalite as the temperature is increased
they expand
below which temperature does alpha quartz exist
below 570
beyond which temperature does beta quartz exist
570
what is alpha quartz
squashed crystalline lattice structure
what is a component in gypsum bonded investment material
calcium sulphate hemi-hydrate in powder form
what percentage of silica is in gypsum bonded investment material
60%
gypsum bonded investment materials also have reducing agents, what do these agents do
inhibit heat shrinkage and controls the setting time
what is the setting reaction for gypsum
calcium sulphate hemi-hydrate added to water which produces calcium sulphate di-hydrate
what expansion does gypsum undergo
hygroscopic expansion
what factors affect hygroscopic expansion
lower powder/ water ratio
increased silica content
using water at a higher temperature
immersing the powder in water for longer
why will gypsum contract above 320 degrees
water loss
presence of sodium chloride and boric acid
gypsum bonded investment material expands by 1.4%, what alloy shrinks by the same amount
gold alloys
does gypsum bonded investment material have sufficient strength and is sufficiently porous
yes
what is the unwanted reaction at about 700 degrees in casting gypsum bonded investment material
calcium sulphate and and carbon react to release carbon monoxide and calcium sulphide which can then produce sulphur dioxide
it is crucial that carbon monoxide and sulphur dioxide gases escape, how is this ensured
investment material goes through heat soaking - which means its held at a high temperature for some time to allow gases to escape
from what temperatures should gypsum bonded investment material not be used
over 1200 due to the calcium sulphate reacting with silica which produces sulphur trioxide which makes voids in the cast alloy
what does phosphate bonded investment material powder contain
silica
magnesium oxide
ammonium phosphate
what is phosphate bonded investment material powder mixed with
water or colloidal silica
why is phosphate bonded investment material mixed with colloidal silica
to increase the strength
and to achieve 2% expansion 5
what is the setting reaction for phosphate bonded investment material
ammonium phosphate reacts with magnesium oxide and water to produce magnesium ammonium phosphate
what are the properties of phosphate bonded investment material
high strength
sufficiently porous
chemically stable
what investment material is not a porous material
silica bonded investment material
