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BDS3 DMS > Investment Materials > Flashcards

Investment Materials Flashcards

1
Q

when are investment materials used in dentistry

A

to produce metal/alloy inlays, onlays, crowns and bridges

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2
Q

investment technique involves

A

casting molten alloy (under pressure, by centrifugal force)

requires a mould cavity of the required shape (i.e. the alloy is surrounded by an investment material)

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3
Q

what is needed to pour in the IM

A

wax patter (e.g. for crown, inlay etc)

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4
Q

what happens to wax pattern after IM poured in and set

A

wax is eliminated (e.g. boiling in water or burning in over)

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5
Q

what happens after the wax is burnt out

A

molten alloy forced into the investment material mould cavity
- via channels (SPRUES) prepared in investment material

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6
Q

stages in using investment material

A
  1. A wax pattern of the required prosthesis – crown, inlay – is created (ie a positive replica)
  2. An InvMater is placed around this wax pattern and allowed to set. It forms a mould (a negative replica)
  3. The wax is then removed – by burning or with boiling water. So now we have a cavity of the required shape surrounded by the InvMater
  4. Next, the molten alloy is poured into the mould cavity – and this is done via the sprue – hollow tubes that allow the alloy to flow in.
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7
Q

what needs to be applied to investment material when being poured

A

PRESSURE must be applied to ensure no gaps or voids form within it
- Material has to be strong enough to withstand the forces generated during this process.

Inevitably, as the alloy is cast, GASES will be produced.
- crucial that these gases are allowed to escape – and be captured by the InvMater.

Otherwise the alloy itself will have voids – and so it will be POROUS.
on cooling, the alloy contracts.
- Therefore, it won’t be the same shape as that determined by the mould cavity.

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8
Q

4 investment types

A

dental stone or plaster

gypsum bonded materials

phosphate bonded materials

silica bonded materials

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9
Q

dental stone of plaster IM used for

A

acrylic dentures

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10
Q

gypsum bonded materials IM used for

A

gold casting alloys

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11
Q

phosphate bonded materials IM used for

A

base materials/cast ceramics

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12
Q

silica bonded materials IM used for

A

base metal alloys

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13
Q

requirements of IM (8)

A

expand

porous

strong

smooth surface

chemically stable

easy removal from cast

handling

relatively inexpensive

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14
Q

why do IMs need to expand

A

compensate for cooling shrinkage of alloy

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15
Q

why do IMs need to be porous

A

allow escape of trapped gases on casting

- back pressure effect

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16
Q

why do IMs need to be strong

A

room temp - ease of handling (“Green strength”)

casting temp - withstand casting forces

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17
Q

why do IMs need to have a smooth surface

A

allow easy finishing

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18
Q

why do IMs need to be chemically stable

A

porosity, surface detail

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19
Q

why do IMs need to be easily removed from cast

A

saves technician time

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20
Q

why do IMs need to be relatively inexpensive

A

destroyed after use

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21
Q

typical contraction of Gold alloys (by volume) from alloy melting pt to room temp

A

1.4%

22
Q

typical contraction of Ni/Cr alloys (by volume) from alloy melting pt to room temp

A

2.0%

23
Q

typical contraction of Co/Cr alloys (by volume) from alloy melting pt to room temp

A

2.3 %

popular alloy

24
Q

2 components of investment materials

A

binder - gypsum; phosphate silica

refractory - silica

25
Q

binder role in IM

A

to form coherent solid mass

gypsum; phosphate silica

26
Q

refractory role in IM

A

withstand high temperatures

also gives expansion

silica (quarts or cristobalites)

27
Q

quartz thermal expansion

A

QUARTZ, the linear thermal expansion rose gradually until 570oC; then it climbed more rapidly.

Quartz below this temperature exists in a form called alpha-quartz
- squashed crystalline lattice structure
But beyond this temperature, its structure changes – to beta-quartz –explodes to its maximum volume.

28
Q

gypsum bonded investment material composition

A

(mixed with water) :

silica (60 - 65 %) – refractory to withstand high temperatures

calcium sulphate hemihydrate (30 - 35%)

reducing agent for oxides

chemicals to inhibit heating shrinkage and control setting time (boric acid, NaCl)

29
Q

setting of gypsum bonded investment material

A

as in gypsum products
(CaSO4)2 .H2O + 3H2O -> 2CaSO4 . 2H2O

hemihydrate -> dihydrate

30
Q

dimensional change gypsum bonded investment material

A

Silica
- thermal & inversion

Gypsum

  • setting expansion
  • hygroscopic expansion
  • contraction above 320OC
31
Q

hygroscopic expansion

A

mechanism not fully understood

water molecules attracted between crystals by capillary forces, forcing crystals apart
- the expansion is considered to be due to capillary forces pulling water molecules into gaps between crystals (of calcium sulphate hemi-hydrate) – forcing them apart.

can be up to 5 X setting expansion

32
Q

hygroscopic expansion increased by (4)

A

lower powder / water ratio

increased silica content

higher water temperature

longer immersion time

33
Q

gypsum bonded contraction above 320 degrees effects (2)

A

water loss

significant reduction by sodium chloride and boric acid

34
Q

5 properties of gypsum bonded investment materials

A

expansion

manipulation

porous

sterngth

35
Q

expansion of gypsum bonded investment materials

A

total expansion sufficient for gold alloys - shrink by 1.4% on cooling

1.4% by volume

36
Q

smooth surface of gypsum bonded investment materials

A

fine particles give smooth surface

37
Q

manipulation of gypsum bonded investment materials

A

easy

setting time controlled

38
Q

porousity of gypsum bonded investment materials

A

good - can uptake gases released on casting alloys

39
Q

strength of gypsum bonded investment materials

A

adequate if correct powder/liquid ration and correct manipulation

40
Q

heat soaking of gypsum bonded investment materials

A

above 700OC - reaction between CaSO4 and C (wax residue or graphite in investment)
CaSO4 + 4C -> CaS + 4CO
then
3CaSO4 + CaS -> 4CaO + 4SO2
Hence heat soak to allow reactions to complete & gases escape

CaSO4 and Carbon can release carbon monoxide; and the calcium sulphide that’s also produced, may then react with CaSO4 to produce sulphur dioxide gas.
It’s crucial these gases – CO and SO2 – escape.
- HEAT SOAKING to do so
held at a high temperature for some time – and this enables the gases to gradually escape.

41
Q

reason for heat soaking gypsum bonded IM

A

CaSO4 and Carbon can release carbon monoxide; and the calcium sulphide that’s also produced, may then react with CaSO4 to produce sulphur dioxide gas.
It’s crucial these gases – CO and SO2 – escape.
- HEAT SOAKING to do so
held at a high temperature for some time – and this enables the gases to gradually escape.

42
Q

chemical stability of gypsum bonded IM

A

below 1200C - satisfies requirements (alloy needs less that 1200C to be cast)

above 1200C - problems:
CaSO4  + SiO2 -> CaSiO3 + SO3
- sulphur trioxide:   
porosity in casting
contributes to corrosion

hence limited to alloys with melt. pt < 1200C

43
Q

phosphate bonded IM composition

A

Powder

  • silica
  • magnesium oxide
  • ammonium phosphate

Liquid
- water or colloidal silica

44
Q

colloidal silica

A

can be liquid used for phosphate bonded IM

increases strength

gives “hygroscopic” expansion (2%)

45
Q

setting for phosphate bonded IM

A

NH4.H2PO4 + MgO + 5H2O -> Mg NH4PO4 . 6H2O magnesium ammonium phosphate

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

46
Q

heating for phosphate bonded IM (1000-1100 degrees)

A

at 330C - water and ammonia liberated

at higher temperatures complex reactions with silico-phosphates formed
- increased strength

47
Q

properties of phopshate bonded IM (5)

A

High “green” strength:
- don’t need metal casting ring for support

Easy to use

High Strength

Porous

Chemically Stable

Also has sufficient strength to be handled in the preparatory stages of the process. and it’s straightforward to use.

48
Q

silica IM dimensional change

A

contraction at early stages of heating
- water and alcohol loss from gel

substantial thermal and inversion expansion
- lots of silica present

49
Q

properties of silica IM

A

dimensional change

strength - sufficient

not porous - needs vents

complicated manipulation

50
Q

key fault in silica bonded IM

A

though sufficiently strong, is complicated to use.
And crucially, it is not a POROUS material
- not able to capture the gases normally released during the casting process.
- means the alloy itself would end up porous – and weak.

To overcome this, special equipment – WITH VENTS – is used with this material.

not used in GDH

51
Q

investment materials =

A

binder + refractory

e.g.
gypsum + silica

52
Q

properties of IM to asses (4 key)

A

Porosity
- trap gases released during casting

Expansion (thermal, hygroscopic, inversion)
- counter alloy shrinkage on cooling

Strength
- withstand pressures during casting

Stability
- doesn’t degrade

BDS3 DMS (9 decks)

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