investment materials

Question 1

investment types

Answer 1

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

Question 2

dental stone/plaster uses

Answer 2

acrylic dentures

Question 3

gypsum bonded materials uses

Answer 3

gold casting alloys

Question 4

phosphate bonded materials uses

Answer 4

base metals/cast ceramics

Question 5

silica bonded materials uses

Answer 5

base metal alloys

Question 6

requirements

Answer 6

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

Question 7

requirements - expand

Answer 7

compensate for cooling shrinkage of alloy

Question 8

requirements - porous

Answer 8

allow escape of trapped gases on casting - back pressure effect

Question 9

requirements - strong

Answer 9

room temp - ease of handling “green strength”

casting temp - withstand casting forces

Question 10

requirements - smooth surface

Answer 10

easy finishing

Question 11

requirements - chemically stable

Answer 11

porosity

surface detail

Question 12

requirements - relatively inexpensive

Answer 12

destroyed

Question 13

lost wax technique

Answer 13

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

Question 14

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

Answer 14

1.4%

Question 15

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

Answer 15

2%

Question 16

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

Answer 16

2.3%

Question 17

components of investment materials

Answer 17

binder
refractory
modifiers

Question 18

binder

Answer 18

gypsum, phosphate, silica

- to form coherent solid mass

Question 19

refractory

Answer 19

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

Question 20

inversion expansion of silica

Answer 20

a-quartz to B-quartz

Question 21

gypsum-bonded investment chemistry

Answer 21

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)

Question 22

gypsum-bonded investment - setting

Answer 22

hemihydrate to dihydrate

Question 23

gypsum-bonded investment - dimensional changes

Answer 23

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

Question 24

gypsum-bonded investment - hygroscopic expansion

Answer 24

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

Question 25

factors increasing hygroscopic expansion

Answer 25

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

Question 26

gypsum - contraction above 320 degrees

Answer 26

water loss

significant reduction by NaCl and boric acid

Question 27

gypsum properties - expansion

Answer 27

sufficient for gold alloys

1.4% by vol

Question 28

gypsum properties - smooth surface

Answer 28

fine particles give good surface

Question 29

gypsum properties - manipulation

Answer 29

easy, setting time controlled

Question 30

gypsum properties - porous

Answer 30

good

Question 31

gypsum properties - strength

Answer 31

adequate if correct powder/liquid ratio and correct manipulation

Question 32

gypsum - heat soaking

Answer 32

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

Question 33

gypsum stability

Answer 33

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

Question 34

phosphate bonded investment composition

Answer 34

powder
 - silica
 - magnesium oxide
 - ammonium phosphate
liquid
 - water or colloidal silica

Question 35

phosphate bonded investment - colloidal silica

Answer 35

increases strength

gives hygroscopic expansion (2%)

Question 36

phosphate bonded investment - setting

Answer 36

magnesium ammonium phosphate and water formed

Question 37

phosphate bonded investment - heating (1000-1100 degrees)

Answer 37

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

Question 38

phosphate bonded investment - properties

Answer 38

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

Question 39

silica investment stages

Answer 39

prepare stock solution
gelation
drying

Question 40

silica investment stage 1

Answer 40

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

form solution of silica acid and ethyl alcohol liberated

Question 41

silica investment stage 2

Answer 41

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

Question 42

silica investment stage 3

Answer 42

drying

tightly packed silica particles

Question 43

silica investment dimensional changes

Answer 43

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

Question 44

silica investment properties

Answer 44

strength sufficient
NOT porous - need vents
complicated manipulation