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Dental Materials Ca2 > Waxes And Casting > Flashcards

Waxes And Casting Flashcards

1
Q

Requirements for inlay wax

A

On softening, should be able to mould into homogenous mass without formation of lamination or flakes

Low thermal co efficient of expansion

Wax should burn out of mould without leaving residue

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2
Q

What type of wax do you use for direct technique inlay

A

Type 1 wax

Wax colour should contrast with hard and soft tissue

Fully mouldable at temperature above that of mouth, hard at mouth temperature so that can withdraw without distortion

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3
Q

What wax do you use for indirect technique inlay

A

Type 2 wax

Should not chip or flake on carving to a fine edge at room temperature

Low solidification temperature, melt at low temperature

Solidification temperature should not be too low so that will not flow during carving

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4
Q

What is the main component of inlay wax

A

Paraffin wax which has low melting temperature

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5
Q

Properties of wax

A

Low thermal conductivity, take time to heat uniformly and to cool

High clt hence shrinkage errors when direct wax pattern is made, especially cooling under pressure

Elastic modulus ????

Adequate flow at specific temperatures to record detail, low flow at other temps for dimensional stability

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6
Q

Describe the flows of type 1 and type 2 wax

A

Type 1: flow less than 1% at 37ºC

Type 2: flow less than 1% at 30ºC

Both minimal flow at 45ªC

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7
Q

How should you soften wax for direct inlay technique

A

Soften wax using annealer or flame (use hot air above flame)

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8
Q

What are wax sprues used for

A

Venting sprues to allow air to escape during casting, prevent formation of back pressure porosity

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9
Q

What should you fill core of metal sprie with and why

A

Fill with sticky wax to prevent distortion on spruing and to increase surface contact between external surface of sprue and wax pattern to strengthen attachment

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10
Q

What happens if you use a corrosive metal for a sprue

A

Contaminate alloy which will absorb contaminants, casting contaminated

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11
Q

What is the diameter of the sprue

A

1-2.6mm

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12
Q

What is a wax reservoir

A

Small amount of wax attached to sprue 1mm away from pattern , especially for small sprues

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13
Q

What is the function of wax reservoir

A

Prevent localised shrinkage porosity at junction of wax pattern and sprue

Molten metal at the reservoir last to solidify, immediately fill the voids in the mould at legit wax pattern due to shrinkage

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14
Q

What happens if sprue want large

A

Metal cool down quickly for gases from wax in the mould to be eliminated especially dense investments, result in back pressure porosity

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15
Q

What is back pressure porosity

A

Incomplete casting, casting with rounded porosity, full wall of mould space not in contact with metal

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16
Q

Where should be sprue be placed

A

Bulkiest portion of pattern away from margins

Eg marginal ridge for class 2

Cannot place at stamp cusp

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17
Q

Why do you add wax at the point of contact between sprue and wax pattern

A

Flare the area so that metal flows into mould more evenly through wider neck, reduce porosity at point of contact

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18
Q

Why is sprue placed at 45º

A

Minimise turbulence and time because hit both walls at same time

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19
Q

How long should sprue be

A

Not more than 6.5mm away from open end of ring. If investment is too thick, result in back pressure porosity. If investment too thin, molten metal shoot through and crack wall

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20
Q

When should you invest

A

Immediately after removal from mouth or die

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21
Q

Why should you keep repair and carving of inlay wax pattern to a minimum

A

Introduce stresses that will distort

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22
Q

Requirements of investment material

A

Harden in relatively short time

Produce smooth surface and fine details and margins on casting

Must not decompose and give off corrosive gases

Porous so that gas can escape

High temperature strength

Sufficient expansion

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23
Q

What happens if investment gives off corrosive gases

A

Cast absorb gases and become brittle and darkened in colour

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24
Q

How to remove gyosum bonded investment for gold alloy

A

Throw in cold water and investment break down

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25
Q

How to remove phosphate bonded investment

A

Does not break dow, remove using sand blaster

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26
Q

At what temperature should maximal thermal expansion be atained

A

Temperature not greater than 700ºC

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27
Q

When do you get significant amount of metal thermal shrinkage

A

Solid state to room temperature

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28
Q

What is the expansion gypsum bonded investments must reach

A

1.25 +- 0.1% which is the shrinkage of gold alloys

29
Q

What is the binder in gypsum bonded investmeent

A

A-hemihydrate

25-45% of investment material, provides rigidity to investment material

30
Q

What is the refractory in gypsum bonded investment

A

Silica

Regulate thermal expansion, provide high temperature strength

31
Q

How does refractory cause thermal expansion

A

When silica heated, change in crystalline form from alpha to beta. Decrease in density and resultant increase in volume

32
Q

Compare expansion of quartz and cristobalite

A

Quartz has lower expansion during conversion

33
Q

Role of copper and carbon in gypsum bonded investment

A

Reducing atmosphere to reduce amount of oxides formed, improve casting quality and prevent darkening

34
Q

What is the role of modifier in gypsum bonded investment

A

Eg boric acid, sodium chloride

Prevent most of the shrinkage when heated above 300ºC. If no modifier, reduce amount of shrinkage so that less absolute shrinkage

35
Q

3 types of gypsum bonded investment

A

Type 1: casting of inlay/crown, thermal expansion

Type 2: casting of inlay/crown, hygroscopic expansion

Type 3: investment of denture metal bases with gold alloys, thermal expansion

36
Q

Seraphina is an example of

A

Phosphate bonded investment

37
Q

How does silica cause expansion of investment

A

Interfere with intermeshing and interlocking of crystals as they form

38
Q

Range of hygroscopic expansion

A

1.2-2.2%

39
Q

What must the thermal expansion of type 1 gypsum investment be

A

1.0 to 2.0%

40
Q

What must the thermal expansion of type 2 gypsum investment be

A

0.0 to 0.6% because expansion should be mainly hygroscopic

41
Q

What are the two components that form the binder in cera fina

A

Phosphate bonded investment

Ammonium diacid phosphate (soluble in water to form phosphate ion)

MgO2 (react with phosphate ion at room temperature)

42
Q

What provides green strength to phosphate bonded investment

A

Green strength is rtp strength.

Magnesium ammonium phosphate

43
Q

How does phosphate bonded investment get high temperature strength

A

Remaining phosphate react with silica at high temperature to form silico phosphate

44
Q

How does viscosity change as spatulation continues

A

Water is formed, lower viscosity of mix

45
Q

What is carbon added for in phosphate bonded investment

A

Produce clean casting and facilitate separation of casting from investment

46
Q

What type of alloys should use carbon free phosphate bonded investment

A

Base metal and silver palladium alloys (fusion temperature above 1504ºC)

Only use for gold alloys

47
Q

What happens if you use carbon in investment for base metal alloy

A

Embrittle alloy due to formation of carbide

48
Q

Why is there thermal shrinkage between 200-400ºC when phosphate investment mixed with water

A

Decomposition of binder magnesium ammonium phosphate, with evolution of ammonia

49
Q

What happens when you mix phosphate investment with silica sol instead of water

A

Silica sol is special liquid

Higher setting and thermal expansion than when mixed with water

No thermal shrinkage from decomposition of binder

Mixed investment can expand hygroscopically (negligible when only water used)

Increase strength of investment

50
Q

Should you hand invest or vacuum invest phosphate investment

A

Hand invest

51
Q

What is green shrinkage of silica bonded investment

A

Drying of gel below 168ºC, loss of water and alcohol, volumetric contraction leading to reduced size of mould

52
Q

Why does silica bonded investment have significant thermal expansion

A

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

53
Q

What happens when you heat silica investment over 700ºC

A

Polysilicic acid gel change to silica with resultant shrinkage

54
Q

What is the refractory and binder for titanium investment

A

Magnesium oxide as both refractory and binder

Or

Zirconia particles held together by zirconium acetate

55
Q

Why is it hard to get consistent casting for titanium

A

Formation of titanium oxide

56
Q

Water immersion vs control water added technique

A

Soak ring under water and cause maximum expansion vs calculate amount of water for desired expansion

57
Q

How does type of silica affect thermal expansion

A

Cristobalite expands to greater extent than quartz

Need significantly more quartz to counterblaance contraction of gypsum during heating and casting shrinkage (>75%)

58
Q

How does water powder ratio affect thermal expansion

A

Greater water powder ratio, less thermal expansion

59
Q

What do chemical modfiers do

A

Increase expansion without excessive amount of silica by eliminating contraction caused by gypsum below 700ºC

60
Q

What happens when investment cools after first heating

A

Contracts to less than original dimension because of shrinkage of gypsum when first heated

On second heating reach same maximum but internal cracks may develop

61
Q

How does water powder ratio affect strength of investment

A

Greater water powder ratio, lower strength

62
Q

How do chemical modifidiers affect strength

A

Increase room temperature strength because more gypsum binder can be added without marked reduction in thermal expansion

63
Q

How does particle size affect porosity of investment

A

More unfirom particles, greater porosity

64
Q

How do particles affect hygroscopic expansion

A

Finer particles, more hygroscopic expansion because more particles per unit volume

65
Q

How long should you heat investment for

A

60 to 90min

66
Q

What temperature should you heat type 2 investment to

A

Keep thermal expansion to minimum 482ºC

67
Q

What happens if you heat gypsum investment to too high temp

A

Give off sulfur gas which contaminates the gold colour, cannot be removed by polishing and embrittles casting

Casting rough due to disintegration of investment

Early wear of rings and heating element

68
Q

Rate of heating for investments

A

Type 1 slowly heat from rtp to required temp

Type 2 can heat up slowly or place in pre heated oven

69
Q

What happens if heating is too rapid (of investment)

A

Flaking/fracture of investment due to steam pressure

Cracking due to uneven heating esp cristobalite

Alter size of casting esp quartz

Dental Materials Ca2 (10 decks)

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