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bds2 cds dms > gypsum > Flashcards

gypsum Flashcards

1
Q

what is a study model?

A

positive replica of dentition

produced from the impression

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2
Q

what is the purpose of a study model?

A

records position, shape and dimensions of teeth
aids visualisation/assessment of dentition
enables manufacture of dental prostheses
-partial denture framework, crowns

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3
Q

what are the uses of gypsum?

A

cast (plaster/stone)
die (stone/improved stone)
mould material (stone)
investment binder (stone)

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4
Q

describe the manufacture of gypsum

A

calcium sulphate dihydrate heated to become calcium sulphate hemihydrate and release water

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5
Q

what are the 3 types of gypsum?

A

plaster
dental stone
densite (improved stone)

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6
Q

what determines gypsum properties?

A

crystalline structure

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7
Q

describe plaster

A

heated in open vessel

large porous, irregular crystals

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8
Q

describe stone

A

heated in autoclave

non-porous, regular crystals, requires less water

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9
Q

describe densite

A

heated in presence of Ca and Mg chloride

compact smoother particles

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10
Q

how does gypsum become the different types?

A

processed in different ways influences structure

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11
Q

describe the setting reaction (reverse of manufacture)

A

hemihydrate powder + water = gypsum

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12
Q

what is the water to powder ratio for plaster?

A

50-60ml to 100g

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13
Q

what is the water to powder ratio for stone?

A

20-35 ml to 100g

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14
Q

what happens during the setting process?

A

hemihydrate dissolves
dihydrate forms
dihydrate solubility low-supersaturated solution
dihydrate crystals precipitate on impurities as crystals
more hemihydrate dissolved
continues until all hemihydrate dissolved

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15
Q

describe the initial set

A 
dihydrate crystals come together
-grow until contact each other
expansion starts
properties of weak solid will not flow
can be carved
can be carved
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16
Q

describe the final set

A

strong and hard enough to be worked
strength continues to develop
crystals large dif size & shape

17
Q

what happens to water during setting?

A

excess water trapped in the powder mass

18
Q

what happens to water on completion of setting?

A

excess water evaporates

voids produced-porosity

19
Q

describe gypsums reproduction of surface detail

A

inherently porous resulting in relatively rough surface
-28-40um
good enough for most applications

20
Q

describe gypsums strength

A 
compressive strength
-28-38mPa
develops over 24h
-75MPa
stone stronger as requires less water for workable mix
improved stone strongest
21
Q

describe its surface hardness

A

low
-surface easy to abrade
-not ideal
must be careful when handling

22
Q

describe setting time

A

must be convenient for user

user must be aware of factors that affect this and use accordingly

23
Q

what factors decrease setting time?

A

more powder, spatulation, impurities

24
Q

what factors can increase or decrease setting time?

A

temperature
chemicals
-borax, K2SO4

25
Q

what factors increase expansion?

A

more powder, spatulation, impurities

26
Q

what factor decreases expansion?

A

chemicals

27
Q

why does spatulation decrease setting time and increase expansion?

A

breaks down growing crystals
fragments act as nuclei of crystallisation
more growing crystals come in contact sooner

28
Q

why does increasing powder cause faster set and increase expansion?

A

more nuclei of crystallisation per unit volume

crystals come into contact sooner

29
Q

describe expansion on setting

A 
should be small % otherwise problematic
plaster 0.2-0.2
stone 0.08 -0.1
densite 0.05-0.07
allows model to be a little big so crowns etc. not too tight in mouth
30
Q

what happens when temp increased?

A

rate of diffusions of ions increase with temp

solubility of hemihydrate decreased with increased temp

31
Q

why does potassium sulphate decrease setting time?

A

produces syngenite

crystallises rapidly-encourages growth of more crystals

32
Q

why does borax increase setting time?

A

forms calcium borate- deposits dihydrate crystals

33
Q

describe compatibility with impression material

A

dental stone model surface detail depends on type of impression material

  • needs to be chemically compatible
  • must be wet
  • avoid bubble formation
34
Q

describe bittleness

A

flexural strength 15-20MPa

much more brittle than composite resin 150MPa

bds2 cds dms (16 decks)

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