Investment Materials
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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