investment materials Flashcards
what are investment materials used to produce?
metal/alloy inlays, inlays, crowns and bridges
describe the stages of casting an alloy (lost wax technique)
wax pattern made
investment material poured around wax and sets
wax eliminated
molten alloy then forced into the cavity via sprue prepared in investment material
allow to cool to room temperature
name some investment materials
dental stone or plaster
gypsum bonded materials
phosphate bonded materials
silica bonded materials
what is dental stone or plaster used to invest?
acrylic dentures
what is gypsum bonded material used to invest?
gold casting alloys
what is phosphate bonded material used to invest?
base metals/cast ceramics
what is silica bonded material used to invest?
base metal alloys
required properties of an investment material
expand to compensate for shrinkage of alloy
porous to allow escape of gases
strong to withstand casting forces
what are the two components of an investment material
binder e.g. gypsum to form solid mass
refractory e.g. silica to withstand high temp
properties of gypsum bonded investment material
smooth surface
easy to manipulate
porous
strength
why heat soak a gypsum-bonded investment material
to allow reactions to complete and gases to escape
properties of phosphate bonded investment material
high strength
easy to use
porous
required properties of an investment material
porosity
expansion - to counter alloy shrinkage
strength
chemical stability
3 types of expansion
thermal - occurs when investment is heated
hygroscopic - investment material hardens immersed in water
inversion - quartz changes from low alpha to high beta when heated
what effect does hardening investment material immersed in water have on expansion, what is this known as
expansion up to 5x normal
hygroscopic expansion
