Investment Materials 2 Flashcards by Kaley Tan

Why is it important that investment material particle size finer than normal stone

Smooth surface, fine margins

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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

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Why does investment need high temperature strength

Withstand impact force of molten metal spinning

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Why do you need sufficient rtp strength of investment

So that investment does not crack when removing sprue

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When does significant amount of shrinkage occur for casting

From solid state to room temperature

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When should maximum thermal expansion of investment be obtained

Not greater than 700ºC

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What is a binder

Hold components together to form mass and allow setting of investment

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How much binder present in gypsum bonded investment

25-45%

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Function of binder in gypsum bonded investment

Provide rigidity

Provide strength

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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

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Quartz vs cristobalite

Quartz has lower expansion during conversion

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What is function of carbon and copper in gypsum bonded investment

Reducing agent. Fewer oxides formed in casting —> better quality, less darkening

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Fucntion of boric acid and sodium chloride in gypsum bonded investment

Modifiers. Prevent most of shrinkage of gypsum when heated above 300ºC

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What temperature can you heat type.1 gypsum bonded investment to

650-700ºC

Used for alloys with melting temp below 1300ºC

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What temperature can you heat type 2 investment too

482ºC

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How much thermal expansion is permitted for type 2 gypsum invstment

0.0-0.6% (should be primarily accomplished by hygroscopic expansion)

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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