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DMS :( > investment materials > Flashcards

investment materials Flashcards

1
Q

investment types

A

dental stone/plaster
gypsum bonded materials
phosphate bonded materials
silica bonded materials

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2
Q

dental stone/plaster uses

A

acrylic dentures

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3
Q

gypsum bonded materials uses

A

gold casting alloys

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4
Q

phosphate bonded materials uses

A

base metals/cast ceramics

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5
Q

silica bonded materials uses

A

base metal alloys

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6
Q

requirements

A 
expand
porous
strong
smooth surface
chemically stable
easy removal from cast
simple handling
relatively inexpensive
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7
Q

requirements - expand

A

compensate for cooling shrinkage of alloy

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8
Q

requirements - porous

A

allow escape of trapped gases on casting - back pressure effect

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9
Q

requirements - strong

A

room temp - ease of handling “green strength”

casting temp - withstand casting forces

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10
Q

requirements - smooth surface

A

easy finishing

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11
Q

requirements - chemically stable

A

porosity

surface detail

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12
Q

requirements - relatively inexpensive

A

destroyed

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13
Q

lost wax technique

A

sprue and wax pattern
investment material poured around wax pattern and allowed to set (mould)
heat - eliminate wax - expansion
molten alloy
- cast under pressure by centrifugal force (1000degrees +) - investment material strong
forced into mould cavity left by wax
via channels (sprues) prepared in the investment material
trapped gases escape
- if gases can’t escape, back pressure leads to incomplete casting
cooling to room temp - alloy shrinks
devestment

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14
Q

typical contraction (by vol) from melting point to room temp - gold

A

1.4%

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15
Q

typical contraction (by vol) from melting point to room temp - Ni/Cr

A

2%

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16
Q

typical contraction (by vol) from melting point to room temp - Co/Cr

A

2.3%

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17
Q

components of investment materials

A

binder
refractory
modifiers

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18
Q

binder

A

gypsum, phosphate, silica

- to form coherent solid mass

19
Q

refractory

A 
silica (quartz/cristobalite)
withstand high temps
also gives expansion
 - quartz approx 0.9%
 - cristobalite 1.4%
20
Q

inversion expansion of silica

A

a-quartz to B-quartz

21
Q

gypsum-bonded investment chemistry

A 
powder (mixed with water)
silica 60-65%
calcium sulphate hemihydrate 30-35%
reducing agent for oxides
chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)
22
Q

gypsum-bonded investment - setting

A

hemihydrate to dihydrate

23
Q

gypsum-bonded investment - dimensional changes

A 
silica - thermal and inversion
gypsum
 - setting expansion
 - hygroscopic expansion
 - contraction above 320 degrees
24
Q

gypsum-bonded investment - hygroscopic expansion

A

mechanism not fully understood
H2O molecules attracted between crystals by capillary forces, forcing crystals apart
can be up to 5x setting expansion

25
Q

factors increasing hygroscopic expansion

A

lower powder/water ratio
increased silica content
higher water temperature
longer immersion time

26
Q

gypsum - contraction above 320 degrees

A

water loss

significant reduction by NaCl and boric acid

27
Q

gypsum properties - expansion

A

sufficient for gold alloys

1.4% by vol

28
Q

gypsum properties - smooth surface

A

fine particles give good surface

29
Q

gypsum properties - manipulation

A

easy, setting time controlled

30
Q

gypsum properties - porous

A

good

31
Q

gypsum properties - strength

A

adequate if correct powder/liquid ratio and correct manipulation

32
Q

gypsum - heat soaking

A

above 700 degrees - reaction between CaSO4 and C (wax residue or graphite in investment)
allows reactions to complete and gases escape

33
Q

gypsum stability

A

below 1200 degrees good
above - problems
- get sulphur trioxide = porosity in casting and contributes to corrosion
therefore can only use for alloys with mp <1200 degrees

34
Q

phosphate bonded investment composition

A 
powder
 - silica
 - magnesium oxide
 - ammonium phosphate
liquid
 - water or colloidal silica
35
Q

phosphate bonded investment - colloidal silica

A

increases strength

gives hygroscopic expansion (2%)

36
Q

phosphate bonded investment - setting

A

magnesium ammonium phosphate and water formed

37
Q

phosphate bonded investment - heating (1000-1100 degrees)

A

at 330 degrees - water and ammonia liberated
at higher temps complex reactions with silicophosphates formed
- increased strength

38
Q

phosphate bonded investment - properties

A 
can use for gold, NiCr, CoCr
high "green strength"
 - don't need metal casting ring for support
easy to use
high strength
porous
chemically stable
39
Q

silica investment stages

A

prepare stock solution
gelation
drying

40
Q

silica investment stage 1

A 
prepare stock solution
mix
 - ethylsilicate
 - dilute HCl
 - industrial spirit (to mix silicate with water)

form solution of silica acid and ethyl alcohol liberated

41
Q

silica investment stage 2

A

gelation
gel formed on adding powder (quartz or cristobalite)
add as much powder as possible for strength (range of particle sizes)
need alkaline conditions
- MgO in powder

42
Q

silica investment stage 3

A

drying

tightly packed silica particles

43
Q

silica investment dimensional changes

A

contraction at early stages of heating
- water and alcohol loss from gel
substantial thermal and inversion expansion - lots of silica present

44
Q

silica investment properties

A

strength sufficient
NOT porous - need vents
complicated manipulation

DMS :( (18 decks)

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