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BDS3 DMS > Investment Materials > Flashcards

Investment Materials Flashcards

(52 cards)

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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

22
Q

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

23
Q

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

Study These Flashcards
A

2.3 %

popular alloy

24
Q

2 components of investment materials

Study These Flashcards
A

binder - gypsum; phosphate silica

refractory - silica

25
binder role in IM
to form coherent solid mass gypsum; phosphate silica
26
refractory role in IM
withstand high temperatures also gives expansion silica (quarts or cristobalites)
27
quartz thermal expansion
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
gypsum bonded investment material composition
(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
setting of gypsum bonded investment material
as in gypsum products (CaSO4)2 .H2O + 3H2O -> 2CaSO4 . 2H2O hemihydrate -> dihydrate
30
dimensional change gypsum bonded investment material
Silica - thermal & inversion Gypsum - setting expansion - hygroscopic expansion - contraction above 320OC
31
hygroscopic expansion
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
hygroscopic expansion increased by (4)
lower powder / water ratio increased silica content higher water temperature longer immersion time
33
gypsum bonded contraction above 320 degrees effects (2)
water loss significant reduction by sodium chloride and boric acid
34
5 properties of gypsum bonded investment materials
expansion manipulation porous sterngth
35
expansion of gypsum bonded investment materials
total expansion sufficient for gold alloys - shrink by 1.4% on cooling 1.4% by volume
36
smooth surface of gypsum bonded investment materials
fine particles give smooth surface
37
manipulation of gypsum bonded investment materials
easy setting time controlled
38
porousity of gypsum bonded investment materials
good - can uptake gases released on casting alloys
39
strength of gypsum bonded investment materials
adequate if correct powder/liquid ration and correct manipulation
40
heat soaking of gypsum bonded investment materials
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
reason for heat soaking gypsum bonded IM
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
chemical stability of gypsum bonded IM
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
phosphate bonded IM composition
Powder - silica - magnesium oxide - ammonium phosphate Liquid - water or colloidal silica
44
colloidal silica
can be liquid used for phosphate bonded IM increases strength gives “hygroscopic” expansion (2%)
45
setting for phosphate bonded IM
NH4.H2PO4 + MgO + 5H2O -> Mg NH4PO4 . 6H2O magnesium ammonium phosphate Ammonium phosphate reacts with magnesium oxide and water to produce Magnesium ammonium phosphate
46
heating for phosphate bonded IM (1000-1100 degrees)
at 330C - water and ammonia liberated at higher temperatures complex reactions with silico-phosphates formed - increased strength
47
properties of phopshate bonded IM (5)
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
silica IM dimensional change
contraction at early stages of heating - water and alcohol loss from gel substantial thermal and inversion expansion - lots of silica present
49
properties of silica IM
dimensional change strength - sufficient not porous - needs vents complicated manipulation
50
key fault in silica bonded IM
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
investment materials =
binder + refractory e.g. gypsum + silica
52
properties of IM to asses (4 key)
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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