Investment Materials 2 Flashcards
Why is it important that investment material particle size finer than normal stone
Smooth surface, fine margins
What happens if investment material gives off corrosive gases at high temperature
Chemically injure surface of cast. Cast absorb gases and become brittle or darkened in colour
Why does investment need high temperature strength
Withstand impact force of molten metal spinning
Why do you need sufficient rtp strength of investment
So that investment does not crack when removing sprue
When does significant amount of shrinkage occur for casting
From solid state to room temperature
When should maximum thermal expansion of investment be obtained
Not greater than 700ºC
What is a binder
Hold components together to form mass and allow setting of investment
How much binder present in gypsum bonded investment
25-45%
Function of binder in gypsum bonded investment
Provide rigidity
Provide strength
Function of refractory (silica) in investment
Regulates thermal expansion. When heated, change in crystalline form from alpha to beta, decrease in density with resultant increase in volume
Quartz vs cristobalite
Quartz has lower expansion during conversion
What is function of carbon and copper in gypsum bonded investment
Reducing agent. Fewer oxides formed in casting —> better quality, less darkening
Fucntion of boric acid and sodium chloride in gypsum bonded investment
Modifiers. Prevent most of shrinkage of gypsum when heated above 300ºC
What temperature can you heat type.1 gypsum bonded investment to
650-700ºC
Used for alloys with melting temp below 1300ºC
What temperature can you heat type 2 investment too
482ºC
How much thermal expansion is permitted for type 2 gypsum invstment
0.0-0.6% (should be primarily accomplished by hygroscopic expansion)
Type 1 vs type 3 gypsum bonded investment
Type 1 used for casting of inlays and crowns
Type 3 used for denture metal bases with gold alloy
What casting materials can be used for gypsum bonded investment
Gold alloy (relatively low casting temperature of around 900ºC)
Base metal alloys with fusion temperature below 1300ºC
(Base metal alloy <25wt% noble metal)
Why is oxalate added to gypsum bonded investment
During heating, oxalate decomposes to form co2 in the mould. This protects molten metal from so2 from gypsum
What makes up binder in phosphate bonded investment
Materials soluble in water to yield phosphate ion and materials that react with phosphate ion at rtp
What is function of refractory
Regulate thermal expansion
Provide high temperature strength
Ammonium diacid phosphate + mgo2 —> magnesium ammonium phosphate + water. What does magnesium ammonium phosphate (binder) provide to phosphate investment
Green strength ie room temperature strength to investment
What does silicophosphate (achieved when remaining phosphate react with silica) do
Increase strength of investment at high temperature
Why is carbon added to phosphate bonded investment
Produce clean casting and facilitate separation of casting from investment
When is it appropriate to add carbon to phosphate bonded investment
When casting alloy is gold
When should carbon free phosphate bonded investments be used
Base metal and silver palladium alloys where fusion temperature is above 1504ºC because formation of carbide will embrittle the alloy
Why does thermal shrinkage of phosphate investment occur when heated to 200-400ºC
Decomposition of vinder magnesium ammonium phosphate, accompanied by evolution of ammonia
How to prevent thermal shrinkage of phosphate bonded investment when heated to 200-400ºC
Mix with special liquid (silica sol in water)
(~ modifiers like boric acid and sodium chloride in gypsum bonded investment to prevent thermal shrinkage due to loss of water)
What is the effect of mixing phosphate bonded investment with silica sol ie special liquid
Higher setting and thermal expansion than when mixed with water because no thermal shrinkage due to decomposition of vinder magnesium ammonium phosphate (evolution of ammonia)
Able to expand hygroscopically vs when only water is used (negligible hygroscopic expansion)
Increase strength of investment
[SHT]
Why does silica bonded investment have considerable amount of thermal expansion
Both binder and refractory are forms of silica that can invert during heating
What is green shrinkage of silica bonded investment
Drying of gel below 168ºC will result in loss of water and alcohol, causing volumetric contraction and reduced size of mould
IE SHRINKAGE WHEN DRYING
What types of shrinkage must be compensated for for silica bonded investment
Green shrinkage when drying
Shrinkage when gel forms
Casting shrinkage
What happens if you heat silica bonded investment above 700ºC
Polysilicic acid gel change to silica —> resultant shrinkage
How does silica + gypsum cause greater normal setting expansion than gypsum alone (investment)
Silica interferes with intermeshing and interlocking of crystals as they form
Range of expansion for hygroscopic expansion
1.2 to 2.2%
Thermal expansion should be kept to a minimum 0-0.6%
Explain hygroscopic expansion
Immersion water replaces water of hydration. Prevent confinement of growing crystals by surface tension of excess water
How does silica cause hygroscopic expansion to be greater than when only gypsum is used
Silica has diluent effect
What does gypsum (binder) provide
Room temperature strength
How does water powder ratio affect thermal expansion of investment
Higher ratio, less expansion
How does water powder ratio affect investment strength
Higher ratio lower strength
