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
