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BDS3 DMS > Investment Material > Flashcards

Investment Material Flashcards

1
Q

what are investment materials used in the production of

A

inlays, onlays, crowns and bridges that are made of an alloy

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2
Q

a casting method is required for inlays, onlays and bridges - what forces are required for this process

A

centrifugal forces

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3
Q

what is an investment material

A

it is the required shape to contain the molten alloy and withstand the high temperatures involved and make sure the alloy’s dimensions are maintained

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4
Q

what is the first stage in investing material

A

a wax pattern of the required prosthesis (crown/ inlay) is created - positive replica

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5
Q

what is the second stage in investment material

A

investment material is placed around the wax pattern and allowed to set to form a mould - the negative replica

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6
Q

what is the third stage in investment material

A

wax is removed - by burning or boiling water so there is a cavity of the required shape around the investment material

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7
Q

what is the fourth stage in investment material

A

molten alloy is poured into the mould cavity via the sprue

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8
Q

what must be applied when the alloy is cast and why

A

pressure so that no gaps or void form in it

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9
Q

what will inevitably be produced when the alloy is cast

A

gases

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10
Q

why is it important what gases are allowed to escape and captured by the investment material

A

so the alloy will not have voids and be porous

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11
Q

what happens upon cooling of an alloy

A

it contracts, and will not be the same shape as determined by the mould cavity

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12
Q

what does investment material need to be with regards to porosity and why

A

needs to be porous and allow the gases released to be captured/ stored within it which maintaining its own shape

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13
Q

why do investment materials need to have a smooth surface

A

so the alloys surface wont need much finishing work to be done by the technician

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14
Q

what alloy has the highest shrinkage upon setting and which has the lowest

A

gold has the lowest
cobalt chrome has the highest

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15
Q

what are the two components in an investment material

A

refractory component and a binder

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16
Q

what is the refractory component in an investment material

A

a silica - quartz or cristobalite which withstands high temperatures and undergoes expansion

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17
Q

what is the binder in an investment material

A

it forms a coherent mass to provide substance

18
Q

what are the three types of binder available

A

gypsum, phosphate and silica bonded

19
Q

what occurs to quartz and cristobalite as the temperature is increased

A

they expand

20
Q

below which temperature does alpha quartz exist

A

below 570

21
Q

beyond which temperature does beta quartz exist

A

570

22
Q

what is alpha quartz

A

squashed crystalline lattice structure

23
Q

what is a component in gypsum bonded investment material

A

calcium sulphate hemi-hydrate in powder form

24
Q

what percentage of silica is in gypsum bonded investment material

A

60%

25
Q

gypsum bonded investment materials also have reducing agents, what do these agents do

A

inhibit heat shrinkage and controls the setting time

26
Q

what is the setting reaction for gypsum

A

calcium sulphate hemi-hydrate added to water which produces calcium sulphate di-hydrate

27
Q

what expansion does gypsum undergo

A

hygroscopic expansion

28
Q

what factors affect hygroscopic expansion

A

lower powder/ water ratio
increased silica content
using water at a higher temperature
immersing the powder in water for longer

29
Q

why will gypsum contract above 320 degrees

A

water loss
presence of sodium chloride and boric acid

30
Q

gypsum bonded investment material expands by 1.4%, what alloy shrinks by the same amount

A

gold alloys

31
Q

does gypsum bonded investment material have sufficient strength and is sufficiently porous

A

yes

32
Q

what is the unwanted reaction at about 700 degrees in casting gypsum bonded investment material

A

calcium sulphate and and carbon react to release carbon monoxide and calcium sulphide which can then produce sulphur dioxide

33
Q

it is crucial that carbon monoxide and sulphur dioxide gases escape, how is this ensured

A

investment material goes through heat soaking - which means its held at a high temperature for some time to allow gases to escape

34
Q

from what temperatures should gypsum bonded investment material not be used

A

over 1200 due to the calcium sulphate reacting with silica which produces sulphur trioxide which makes voids in the cast alloy

35
Q

what does phosphate bonded investment material powder contain

A

silica
magnesium oxide
ammonium phosphate

36
Q

what is phosphate bonded investment material powder mixed with

A

water or colloidal silica

37
Q

why is phosphate bonded investment material mixed with colloidal silica

A

to increase the strength
and to achieve 2% expansion 5

38
Q

what is the setting reaction for phosphate bonded investment material

A

ammonium phosphate reacts with magnesium oxide and water to produce magnesium ammonium phosphate

39
Q

what are the properties of phosphate bonded investment material

A

high strength
sufficiently porous
chemically stable

40
Q

what investment material is not a porous material

A

silica bonded investment material

BDS3 DMS (6 decks)

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