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‫هللا‬
‫ِّب ْاَس ِّم‬
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‫{‪}27‬‬
‫اَسِ ِّي‬
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‫احلل ُ‬
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‫{‪}28‬‬
‫يَ ْفقَ ُهوا قَ ْو ِّل‬
‫صدق ﺍﷲ ﺍﻟﻌﻅﻳﻢ‬
Gold
Crown
Reduced
tooth
Impression
Die
Wax
Pattern
Wax Sprue
Wax Pattern
Wax Sprue
Crucible
Former
Ring
+
Investment material
+
Wax pattern
Investment Materials
Ceramic materials used for
making a mould into which the
metal or alloy is cast.
Ideal Requirements:
1) Should withstand high temperature without
decomposition during casting.
2) Should have sufficient expansion to compensate
for the casting shrinkage.
3) Should have sufficient strength at;
- Room temperature, to withstand manipulation
without fracture.
- High temperature, to withstand the force of molten
alloy entering the mold.
4) Should be porous to allow escape of air or gases
during casting.
5) Should produce a smooth surface and fine details
of the casting.
6) Should be broken away easily after casting.
Composition:
1- Refractory material 65% :
[Silica]
Function:
1) It withstands high temperature.
2) It produces thermal expansion of the
investment which is caused by the
displacive transformation of silica from 
to  form during casting.
Effect of Heat on
Refractory Material
Reconstructive
870C
Β-Quartz
(Hexagonal)
525C
1713C
1470C
Β-Tridymite
(Rhombohedral)
Β-Cristobalite
(Cubic)
Β-Fused quartz
(amorphous)
270C
160C
Displacive
αQuartz
αTridymite
αCristobalite
(Hexagonal)
(Rhombohedral)
(Cubic)
(1.4% expansion
at 600C).
(1% expansion
at 600C).
(1.6% expansion
at 400 C )
transformation
2- Binder 30%:
Gypsum
Gypsum
bonded
investment
Magnesium
ammonium
phosphate
Phosphate
bonded
investment
Polysilisic
acid
Silicate
bonded
investment
Role of Binders:
1- Binds the silica particles.
2- Provides more strength to the
material.
3- In some types of investment, It
provides setting and hygroscopic
expansion, which share in the
compensation of the casting
shrinkage of the metal.
3- Modifiers 5% :
Chemicals added in small amount to
modify various physical properties
Functions:
1- They prevent oxidation of the molten
alloy during casting e.g. graphite.
2- They produce smooth surface mould.
Types of Investment Materials
Gypsum
bonded
investment
Phosphate
bonded
investment
Silicate
bonded
investment
Gypsum Bonded Investment
It is suitable for alloys which melt
below 1000C
→ Gold Alloys
WHY???
Effect of temperature on
Gypsum Bonded investment
** Heating from 200- 400ºC it contracts due to the
dehydration of the binder. This can be eliminated by addition
of chemicals (boric acid).
** From 400- 700ºC, it expands again due to the
transformation of the α form to the  form of silica.
** Above 700ºC, CaSO4 will react with carbon in the
investment leading to evolution of sulfur trioxide gas (SiO3)
→ contamination and porosity of the casting.
** Above 1200°C, during melting of high fusing dental alloys
(base metal) such as nickel chromium and cobalt chromium
alloys, this will lead to decomposition of the binder
(gypsum). leading to evolution of sulfur trioxide gas
(SiO3) causing porosity of the casting.
Phosphate and Silicate
Bonded Investment
It is suitable for alloys which melt
Above 1000C
→
Base metals
Titanium
Castable Ceramics
Comparison Between Three Types of Investments
Point of
comparison
Gypsum
Bonded
Investment
Phosphate
Bonded
Investment
1) Form of
supply
Powder / water
Powder / water
&
Powder /
Special liquid
(Silica sol)
Silicate
Bonded
Investment
Powder / 2
liquids
On mixing
powder +
water;
2) Setting
Reactions
CaSO4.1/2H2O
+ H2O
CaSO4.2H2O
-Acts as a
binder
On mixing powder
+ water;
Ammonium
diphosphate +
MgO + H2O
Magnesium
Ammonium
Phosphate
-Acts as a binder
-Gives sufficient
strength at room
temperature.
(Green strength)
On mixing the 2
liquids:
a) Hydrolysis
Ethyl silicate + HCl
Silicic acid sol +
alcohol
-Acts as a binder
b) Gelation
Powder (silica) +
Silicic acid sol
Polysilicic acid gel
c) Dryness
On heating at 170°C
there is loss of
alcohol & water
silica particles tightly
packed together.
Contraction.
Setting
expansion
due to crystal
growth
3)Properties
a)
Dimensional
Hygroscopic
changes
expansion
during
when
setting
allowed to
set under
water, giving
additional
room for
crystal
growth.
Setting
expansion due to
crystal growth,
which is higher
when mixed with
special liquid.
Setting shrinkage
during gelation due
to formation of
coherent Polysilicic
acid gel (change
from sol to gel)
Green shrinkage
during dryness at
170°C.
Hygroscopic
expansion is
negligible when
mixed with water,
- High heat
but high when mixed shrinkage at 700°C
with special liquid
due to transformation
of Polysilicic acid gel
into Cristobalite.
From 200400°C
Shrinkage due
to dehydration
of the binder.
b)
This can be
Dimensional eliminated by
changes
addition of
during
chemicals
heating
(boric acid).
From 400700°C
Thermal
expansion of
silica due to
displacive
transformation
from  to .
Thermal
expansion of silica
due to displacive
transformation
from  to .
Thermal
expansion of high
amount of silica at
higher temperature,
due to displacive
transformation from
 to.
- Strong
c) Strength
- Sufficient
strength at
room & high
temperatures
to withstand
the driving
force of the
gold alloys
during
casting.
- Stronger
- Magnesium
ammonium
phosphate (binder)
gives sufficient
strength at room
temperature.
(Green strength).
-At high
temperature,
Ammonium diacid
phosphate + silica
silico- phosphate
complex the
strength.
- Strongest
- As silica particles
are tightly packed
together.
Porous
Porous
So venting is
required to allow
escape of air from
mold. In order to
prevent
backpressure
porosity &
incomplete casting.
d) Porosity
4) Casting
ring
Non Porous
- Metallic ring
lined with wet
asbestos paper
- Rubber
ring which is
removed after
setting of the
investment
- Rubber ring
which is
removed after
setting of the
investment
5) Uses
- Casting of
gold alloys
As the total
amount of its
expansion is
equal to the
solidification
shrinkage of
gold alloys
(1.6 %).
Casting of;
- Base metal
alloys (when
mixed with special
liquid). - Or gold
alloys (when
mixed with water).
- Casting of Base
metal alloys
As the total amount
of its expansion is
equal to the
solidification
shrinkage of base
metal alloys
(2.3 %).