Investment Materials Flashcards
what are investment materials used for
surround an alloy when getting moulded
what is the lost wax technique
investment material poured around wax (of crown e.g.,) and set, wax burnt off and molten alloy forced into mould cavity vacated by wax via channels prepared in investment material
what type of investment is used for acrylic dentures
dental stone or plaster
what type of investment is used for gold casting alloys
gypsum bonded materials
what type of investment is used for base metals/cast ceramics
phosphate bonded materials
what type of investment is used for base metal alloys
silica bonded materials
what are the requirements of an investment material
expand
porous to allow escape of gases
strong to withstand casting forces and for ease of handling
smooth surface
chemically stable
easy removal from cast
uncomplicated handling
inexpensive
what are the 2 components of investment material
binder to form coherent solid mass
refractory to withstand high temperatures and give expansion
what types of material are used as a binder
gypsum, phosphate, silica
what material is used as a refractory
silica
what is the composition of a gypsum bonded investment material
powder mixed with water
silica
calcium sulphate hemihydrate
reducing agent
what is the reaction of a gypsum bonded investment material
hemihydrate to dihydrate
what is hygroscopic expansion
water molecules attracted between crystals by capillary forces, forcing crystals apart
what factors increase hygroscopic expansion
lower powder/water ratio
increased silica content
higher water temperature
longer immersion time
when does gypsum contract
above 320 degrees
what are the properties of a gypsum bonded material
expansion
smooth surface
easy manipulation
porous
adequate strength
what does heat soaking allow
reactions to complete and gases to escape
what temperature limits gypsum and why
below 1200 degrees
becomes chemically unstable above this
what is the composition of phosphate bonded investment material
powder and water/colloidal silica
what does colloidal silica solution do
increases strength and gives hygroscopic expansion
what does the setting reaction of phosphate bonded investment material give
magnesium ammonium phosphate
what are the properties of phosphate bonded investment materials
high green strength
easy to use
high strength
porous
chemically stable
what are the stages of silica investment
prepare stock solution
add powder
drying
what are the dimensional changes of silica
contraction at early stages of heating
substantial thermal and inversion expansion
what are the properties of silica material
sufficient strength
not porous
complicated manipulation
