Die and Investment Materials Flashcards
Which ones undergo shrinkage and which under expansion elastomeric impression material die stone wax pattern gold alloy casting investment
elastomeric impression material; shrinkage die stone; 0.1% expansion wax pattern; ranging gold alloy; 2% shrinkage casting investment; 2% expansion
What is the formula for calcium sulfate dihydrate
CaSO4 • 2H20
What is the formula for calcium sulfate hemihydrate
CaSO4 • 1/2H20
This calcining procedure has container open to air
plaster (β form)
This calcining procedure has closed autoclave under steam pressure
stone (α form)
This calcining procedure has boiling calcium chloride solution
high-strength stone (α form)
This has the highest porosity and surface area (spongy appearance)
plaster
This has a rectangular shape, lower porosity and surface area than plaster particles
stone
This has the most uniform shape, lowest porosity and surface area
high-strength stone
What is the typical water to powder ratio of plater, stone, and high strength die stone
plater; 0.4-0.5
stone; 0.3-0.4
HSDS; 0.2-0.3*
What three things happen during the setting reaction for gypsum products
exothermic reaction
reverse of calcining process
end product is gypsum
What is the theoretical minimum water/powder ratio for a balanced setting reaction
0.186 (0.19)
Excess water is required for wetting particles and desired viscosity for mix; most for this material and least for this
most for plaster
least for high-strength stone
What are common accelerators and retardants
accelerator = potassium sulfate
retarder = borax
NaCl; accelerator at low conc. retarder at high
Expansion is caused by what
gypsum particles pushing each other apart
This high strength die stone have the lowest setting expansion and contain larger amounts of modifiers to provide this property
type IV high-strength die stones
Type V die stones are both what and what, designed for modern casting alloys higher melting point alloys with greater casting shrinkage than traditional high-gold alloys
high-strength
high-expansion
Brittle materials have a much higher compressive strength than what
tensile srength
In which condition is there much higher strength
dry conditions have higher strength compared to wet condition
Strength is related to what
the starting W/P ratio used for mixing product, which determines amount of porosity in set material
What happens to the strength if there is a high W/P ratio
decreased strength
Loss of excess water beyond that required for balanced setting reaction causes what
porosity in dry material
Porosity acts to what
concentrate stress during mechanical testing, leading to material fracture
These are termed low-heat investments; used with gold-based casting allows for all-metal restorations
gypsum-bonded casting investments
These phosphate-bonded investments are used with alloys for metal ceramic restorations and some base metal casting allows for RPD
Also, silicate-bonded investments are used with other base metal casting allows for RPD which have high melting temps
high-heat casting investment
This provides mold cavity for allow after burnout of wax pattern or plastic pattern; provides compensating expansion for casting shrinkage when alloy cools
casting investments
What are the three means for investment expansion of mold cavity
normal setting expansion; approx. 0.3-0.4%
hygroscopic setting expansion over 2%
thermal expansion; amount depends on burnout temp, 1%
What is the composition of gypsum bonded casting investment powder
binder is calcium sulfate hemihydrate (stone or α form) 30-45%
refractory is quartz and/or cristobalite; 1/2 to 2/3
chemical modifiers 3% or less
Chemical modifiers can do what two things
control setting time
reducing atmosphere when casting alloy
What is the purpose of the binder component of gypsum bonded investment powder
binds components together; provides structural framework at low temps
This component of gypsum-bonded investment powder does not participate in the setting reaction, its most important role is to provide thermal expansion during heating in burnout furnace; proves strength of investment at temps above 100°C
refractory component
Which one has a much higher thermal expansion; cristobalite investment or quartz investment
cristobalite
Pure gypsum undergoes what over burnout temperature range
contraction
The presence of stone powder component in investment reduces expansion by about how much
1/3
True or False Fused silica (glass) has much lower thermal expansion than crystalline forms
True
This causes a large thermal expansion over a narrow temperature rand
changes in crystal structure
Never heat gypsum bonded investments over what temp
700°C
Why would you not want to overheat gypsum bonded investments
breakdown of calcium sulfate causes sulfur incorporation in casting
results in discoloration and embrittlement
Increasing or decreasing the PW ration increases strength an expansion (more powder less water)
increasing
Why does increasing the PW ration, increase strength and expansion
more dense gypsum framework
Increasing the amount of what increases setting expansion and strength; it breaks down he forming gypsum framework and weakens structural framework
spatulation
What are the two components in the two-component system of PBIs (phosphate bonded investments)
powder and aqueous solution of stabilized colloidal silica
Particles of glasses and other metal oxides provide what in PBIs
bulk and help achieve smooth surface finish
In the setting reaction for PBIa, the binder component undergoes an acid-base reaction between what
ammonium phosphate and magnesia
Water produced by the setting reaction of a PBI does what
lowers viscosity of mix as spatulation continues
To produce higher expansion, manufactures use a combination of what
different particle sizes of silica
True or False
either water or special liquid provided by manufacturer can be used
True
What is the “special liquid”
some form of silica sol in water; provides higher setting expansion and hygroscope expansion
What are three advantages of PBIs
high green strength and high fired strength
sufficient setting and thermal expansion
withstand burnout temperatures
What are four disadvantages of PBIs
when temps excess 1375°C; breakdown
high strength can bauds difficulty in removal
use of additional special liquid results in denser and less porous mold
if bulk powder used instead of remeasured packages, reaction with air may reduce setting expansion
