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Dental Materials Ca2 > Investment Materials 2 > Flashcards

Investment Materials 2 Flashcards

1
Q

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

A

Smooth surface, fine margins

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2
Q

What happens if investment material gives off corrosive gases at high temperature

A

Chemically injure surface of cast. Cast absorb gases and become brittle or darkened in colour

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3
Q

Why does investment need high temperature strength

A

Withstand impact force of molten metal spinning

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4
Q

Why do you need sufficient rtp strength of investment

A

So that investment does not crack when removing sprue

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5
Q

When does significant amount of shrinkage occur for casting

A

From solid state to room temperature

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6
Q

When should maximum thermal expansion of investment be obtained

A

Not greater than 700ºC

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7
Q

What is a binder

A

Hold components together to form mass and allow setting of investment

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8
Q

How much binder present in gypsum bonded investment

A

25-45%

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9
Q

Function of binder in gypsum bonded investment

A

Provide rigidity

Provide strength

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10
Q

Function of refractory (silica) in investment

A

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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11
Q

Quartz vs cristobalite

A

Quartz has lower expansion during conversion

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12
Q

What is function of carbon and copper in gypsum bonded investment

A

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

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13
Q

Fucntion of boric acid and sodium chloride in gypsum bonded investment

A

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

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14
Q

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

A

650-700ºC

Used for alloys with melting temp below 1300ºC

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15
Q

What temperature can you heat type 2 investment too

A

482ºC

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16
Q

How much thermal expansion is permitted for type 2 gypsum invstment

A

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

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17
Q

Type 1 vs type 3 gypsum bonded investment

A

Type 1 used for casting of inlays and crowns

Type 3 used for denture metal bases with gold alloy

18
Q

What casting materials can be used for gypsum bonded investment

A

Gold alloy (relatively low casting temperature of around 900ºC)

Base metal alloys with fusion temperature below 1300ºC
(Base metal alloy <25wt% noble metal)

19
Q

Why is oxalate added to gypsum bonded investment

A

During heating, oxalate decomposes to form co2 in the mould. This protects molten metal from so2 from gypsum

20
Q

What makes up binder in phosphate bonded investment

A

Materials soluble in water to yield phosphate ion and materials that react with phosphate ion at rtp

21
Q

What is function of refractory

A

Regulate thermal expansion

Provide high temperature strength

22
Q

Ammonium diacid phosphate + mgo2 —> magnesium ammonium phosphate + water. What does magnesium ammonium phosphate (binder) provide to phosphate investment

A

Green strength ie room temperature strength to investment

23
Q

What does silicophosphate (achieved when remaining phosphate react with silica) do

A

Increase strength of investment at high temperature

24
Q

Why is carbon added to phosphate bonded investment

A

Produce clean casting and facilitate separation of casting from investment

25
Q

When is it appropriate to add carbon to phosphate bonded investment

A

When casting alloy is gold

26
Q

When should carbon free phosphate bonded investments be used

A

Base metal and silver palladium alloys where fusion temperature is above 1504ºC because formation of carbide will embrittle the alloy

27
Q

Why does thermal shrinkage of phosphate investment occur when heated to 200-400ºC

A

Decomposition of vinder magnesium ammonium phosphate, accompanied by evolution of ammonia

28
Q

How to prevent thermal shrinkage of phosphate bonded investment when heated to 200-400ºC

A

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)

29
Q

What is the effect of mixing phosphate bonded investment with silica sol ie special liquid

A

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]

30
Q

Why does silica bonded investment have considerable amount of thermal expansion

A

Both binder and refractory are forms of silica that can invert during heating

31
Q

What is green shrinkage of silica bonded investment

A

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

32
Q

What types of shrinkage must be compensated for for silica bonded investment

A

Green shrinkage when drying

Shrinkage when gel forms

Casting shrinkage

33
Q

What happens if you heat silica bonded investment above 700ºC

A

Polysilicic acid gel change to silica —> resultant shrinkage

34
Q

How does silica + gypsum cause greater normal setting expansion than gypsum alone (investment)

A

Silica interferes with intermeshing and interlocking of crystals as they form

35
Q

Range of expansion for hygroscopic expansion

A

1.2 to 2.2%

Thermal expansion should be kept to a minimum 0-0.6%

36
Q

Explain hygroscopic expansion

A

Immersion water replaces water of hydration. Prevent confinement of growing crystals by surface tension of excess water

37
Q

How does silica cause hygroscopic expansion to be greater than when only gypsum is used

A

Silica has diluent effect

38
Q

What does gypsum (binder) provide

A

Room temperature strength

39
Q

How does water powder ratio affect thermal expansion of investment

A

Higher ratio, less expansion

40
Q

How does water powder ratio affect investment strength

A

Higher ratio lower strength

Dental Materials Ca2 (10 decks)

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