• The dihydrate form of calcium sulfate, called
• gypsum, usually appears white to milky
yellowish
• and is found in a compact mass in nature.
DR .R.koohkan
2
• As the temperature is raised the remaining
water is removed , and products are formed as
indicated .
1100 –1300
[CaSO4]1/2 H2O
CaSO4+2H2O
GYPSUM
1300 –2000
CALCIUM SULFATE
HEMIHYDRATE
Dehydration by heat or
other means
Mineral
gypsum
2000 –10000
CaSO4
HEXAGONAL
ANHYDRATE
CaSO4
ORTHOROMBIC
ANHYDRATE
formulation
Plasters
Model plaster Lab plaster
Hydrocal
Dental stone
Densite DR .R.koohkan
High-strength dental stone
3
IT IS OF THE FOLLOWING TYPES :
Impression plaster
TYPE
Model plaster.
TYPE
Dental stone.
TYPE
Dental Stone , high strength .
TYPE
Dental stone, high strength &
High expansion.
TYPE
DR .R.koohkan
I
II
III
IV
V
4
• Most gypsum products are obtained from natural gypsum rock.
The reaction is exothermic,
CaSO4 · ½ H2O + 1 ½ H2O →CaSO4 · 2H2O + 3900 cal/g mol
Plaster of Paris Water
Gypsum
Three types of base raw materials are derived from partial dehydration of
gypsum rock, depending on the nature of the dehydration process.:
Plasters:
are fluffy, porous, and least dense, whereas
hydrocal variety :
has a higher density and is more crystalline.
Densite :
• is the densest of the raw materials
DR .R.koohkan
5
ISO Classification of dental gypsum
products
 Type 1 : Impression plaster.
 Type 2 : Dental plaster – model.
 Type 3 : Dental stone – die.
 Type 4 : Dental stone – die, high strength, low expansion.
• Type 5 : Dental stone – die, high strength, high expansion.
• Although these types have identical chemical formulas of calcium sulfate
hemihydrate, CaSO4 · ½H2O,they possess different physical properties,
 Type 4 contains extra salts to reduce its setting expansion
DR .R.koohkan
6
• the main difference being the manner of driving off part of the water of the
calcium sulfate dihydrate.
Plasters :
• gypsum mineral is heated in an open kettle at a temperature of about
110° to 120° C
β-calcium sulfate hemihydrate.
•
(irregular shape and porous)
Hydrocal :
• If gypsum is dehydrated under pressure and in the presence of water vapor at
about 125° C
α-calcium sulfate hemihydrate.
more uniform in shape and denser than the particles of plaster
low- to moderate-strength dental stones..
Densite:
• Boiling gypsum rock in a 30% calcium chloride solution, is washed away with
hot water(100° C) is ground fineness.
•
Types 4 and 5 high-strength dental stones are
•
A high-density
DR .R.koohkan
7
•
α – hemimhydrate
β – hemihydrate
1. Type III , IV &V
2. Produced by wet
calcination.
3. Requires less water for
mixing.
4. Better packing ability.
5. Low surface free energy.
6. Crystal habit of hexagonal
calcium sulfate .
7. High apparent density.
1. Type I & II
2. Produced by dry
calcination.
3. Requires more water form
mixing.
4. Less packing ability.
5. High surface free energy.
6. Crystal habit that of
hemihydrate.
7. Low apparent density.
DR .R.koohkan
8
Gypsum
Mixing Water
(mL/100 g of powder)
Required Water
(mL/100 g of powder)
Excess Water
( mL/100 g of powder)
Model plaster
37-50
18.6
18-31
Dental stone
28-32
18.6
9-13
High-strength dental stone
19-24
18.6
0-5
*Water-powder ratio varies with each product.
 When the set material is dried,
porosity in the structure, weakening it.
the excess water evaporates and leaves
 This difference in the physical shape and nature of the crystals makes it possible to
obtain
the same consistency with less excess water with dental stone than with
model plaster.
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9
SETTING REACTION
• PROPOSED THEORIES :
1- Colloidal theory –
hemihydrate + water --> colloidal state through sol -gel
mechanism.
In sol state hemihydrate particles
are converted to dihydrate
and as the measured amount of water is consumed the mass
converts to a solid gel.
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10
2- Hydration theory:
suggests that rehydrated plaster particles join together through
hydrogen bonding to the sulfate groups to form the set
material.
3-Dissolution – precipitation theory:
• (most widely accepted theory)
the setting reaction of water with calcium sulfate hemihydrate to from calcium sulfate dihydrate is
caused by the difference in solubility between these two
components.
based on dissolution of plaster and instant recrystallization
of gypsum, followed by interlocking of the crystals to form
the set solid.
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 Hemihydrate is 4 times more soluble in water than
is the dihydrate near room temp (20°C). Thus the
setting reaction can be understood as follows:-hemihydrate + water
 Suspension is formed that is fluid and workable.
 Hemihydrate dissolves until it forms a saturated
solution.
 This saturated solution supersaturated in dihydrate,
precipitates out dihydrate.
DR .R.koohkan
12
..
 Thus solution is no longer saturated with hemihydrate,
so it continues to dissolve.
 Dissolution of the hemihydrate and precipitation of
dihydrate proceeds as new crystals form or further
growth occurs on the present crystals.
• In practice about 0.2% to 0.4% linear expansion is
•
obtained.
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13
 The ratio of the water to the hemihydrate powder is usually
expressed as the W/P ratio
 W/P ratio
 setting time
.
, strength , setting expansion
 example : if 100g is mixed
with 60 ml of water, the
W/P ration is 0.6.
DR .R.koohkan
14
The fact that the contraction of gypsum is not visible does
not invalidate its existence, and when the volumetric
contraction is measured by a dilatometer, it is determined
to be about 7%.
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15
 The mixing process, called spatulation, has a
definite effect on the setting time and setting
expansion
 spatulation (either speed of spatulation or time or both)
setting time
 Because an increased amount of spatulation
causes more nuclei centers (dihydrate) to be formed,
the conversion of calcium sulfate hemihydrate to
dihydrate is accelerated.
DR .R.koohkan
16
20° C
30° C
The ratio of the solubilities of calcium sulfate dihydrate and
calcium sulfate hemihydrate
4.5
3.4
the mobility of the calcium and sulfate ions
increase
increase
the rate of the reaction
increase
Over 37° C
100° C
1
increase
increase
decreases
no reaction
I.
The first effect of increasing temperature is achange in the
relative solubilities of calcium sulfatehemihydrate and calcium
sulfate dihydrate, which alters the rate of the reaction.
II.
In general, as the temperature increases, the mobility of the
calcium and sulfate ions
the rate of the reaction ,the setting
time

the temperature is raised over37° C, the rate of the reaction
decreases, and the setting time is lengthened.
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17
 The initial result
is acceleration of setting.
Produce dihydrate, providing more nuclei
for crystallization .
 The final result
is further contamination
by moisture can
reduce the amount of
hemihydrate remaining to form gypsum
retardation of setting will occur
setting
time
DR .R.koohkan
18

Colloidal systems (agar , alginate) retard the setting of
gypsum

by being adsorbed on the hemihydrate and dihydrate

nucleation sites retards the setting
it is more effective on dihydrate nucleation
 potassium sulfate are added
to improve the
surface quality of the set CaSO4 · 2H2O against
agar or alginate.
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19
Liquids with low pH, such as saliva, retard the
setting reaction
Liquids with high pH accelerate setting.
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20
The important properties of gypsum products include
 quality
 fluidity at pouring time
 Setting time
 linear setting expansion
 compressive strength
 hardness
 abrasion resistance
 Reproduction of detail
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21
The time required for the reaction to be completed is
called the final setting time.
The initial setting time is defined by:
 The loss of gloss from the surface
 Increase temperature of the mass
 INITIAL GILLMORE TEST FOR INITIAL SET
The final setting time is defined as the time at which
the material can be separated from the impression
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22
• The smaller needle is most frequently used for
cements but it is sometimes used for gypsum
products .
The mixture is spread out, and the needle is
lowered onto the surface. The time at which it
no longer leaves an impression is called the
initial set , noted as “Initial Gillmore”.
It takes place at approx 13 mins.
DR .R.koohkan
23
 The Vicat apparatus:
 is
commonly
used to
measure the initial setting
time of gypsum products. It
consists of a rod weighing
300g with a needle of 1-mm
diameter.
DR .R.koohkan
24
The setting time can control by:




changing the temperature of the mix water a
changing the degree of spatulation.
Water-powder(W/P) ratio
The easiest and most reliable way to change the setting time is to add
different chemicals.
The setting time is usually shortened for power
mixing compared with hand mixing.
DR .R.koohkan
25
ACCELERATORS:
Sodium Sulphate
- 3-4 %
Potassium Sulphate
- 2-3 %
Sodium Chloride
- 2%
Gypsum
- < 20%
Potassium sulfate
“ROCHELLE
potassium sodium tartrate
SALT”
The acceleration caused by an additive depends on
the amount and rate of solubility of the
hemihydrate versus the same effect on the
dihydrate.
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26
RETARDERS:
Act by forming an adsorbed layer on the hemihydrate and on gypsum
crystals to reduce its solubility.(organic materials)
forms a layer of calcium salt that is less soluble than is the sulfate
salt.(salts)
ORGANIC MATERIALS:
Glue, gelatin and some gums.
SALTS:
Borax
1-2%
(sodium tetraborate decahydrate)
sodium chloride
~ 20%
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27
Manipulative Variable
Setting Time
Consistency
Setting
Expansion
Compressive
Strength
Increase water/powder
ratio
Increase
Increase
Decrease
Decrease
Increase rate of spatulation Decrease
Decrease
Increase
No effect
Increase temperature of
mixing water from
23° to 30° C
Decrease
Increase
No effect
Decrease
DR .R.koohkan
28
 Gypsum products may be formulated with chemicals that modify
their handling characteristics and properties.
Setting time Expansion
K2SO4
↓↓↓
Borax
↑↑
↓
K2SO4 + Borax
↓
↓↓↓
Sodium chloride
?
↓↓↓
in small amounts
Terra alba effective accelerators.
(set calcium sulfate dihydrate)
 sodium citrate is a dependable retarder.
 2% aqueous solution of borax
prolong the setting time (a few hours)
DR .R.koohkan
29
• More voids were observed in casts made from the
stones with the higher viscosities.
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30

The compressive strength is inversely related to the W/P ratio of the
mix.

The more water ,

The excess water is uniformly distributed in the mix and contributes
to the volume but not the strength of the material

the most porous


the lower compressive strength.
the weakest stone
Porous
Model plaster>Dental stone > High-strength stone
strength
Model plaster<Dental stone <. High-strength stone
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31
Compressive strength (MPa)
The dry compressive strength is usually about
twice that of the wet strength.
60
50
40
30
Effect of loss of excess water on
Compressive strength of dental stone.
20
0
DR .R.koohkan
2
4
6
8
Weight loss (%)
10
32
 The surface hardness of un modified gypsum materials is related in a general
way to their compressive strength.
 Increased surface hardness does not necessarily mean improved abrasion
resistance because hardness is only one of many factors that can affect wear
resistance
impregnating the set gypsum by
surface
Hardness
epoxy or methylmethacrylate monomer plaster
compressive scratch
strength
resistance
-
-
-
-
epoxy or methylmethacrylate monomer dental stone
colloidal silica (about 30%) dental stone
DR .R.koohkan
33



ANSI/ADA specification No. 25 requires that:
types 1 and 2 reproduce a groove 75 mm in width
types 3, 4, and 5 reproduce a groove 50 mm in width

Gypsum dies do not reproduce surface detail as well as
electroformed or epoxy dies

gypsum does not wet some impression material (silicon)

use of vibration during the pouring of a cast
Use surfactants in silicone impression materials
Rinsing the impression


DR .R.koohkan
34
 Gypsum product shows linear expansion during the setting due to
outward thrust of crystals that is change from hemihydrate to
dihydrate.
 Low – 0.06 %
High – 0.5 %
[CaSO4]₂ •H₂O + 3H₂O
Molecular mass
290.284
54.048
Density(g/cm³)
2.75
0.997
Equivalent volume 105.556
54.211
Total volume
159.767
DR .R.koohkan
2 CaSO4 •2H₂O
344.322
2.32
148.405
148.405
35
SETTING EXPANSION CONTD . .

Net change in volume is :
(148.405 – 159.767) * 100
159.767
= -7.11%
CRYSTALLIZATION MECHANISM:
crystals grow  outward thrust or stress
develops  expansion of the entire mass .
• Practically the product is greater in external
volume but less in crystalline volume.

DR .R.koohkan
36
) Hygroscopic expansion:
In one technique the investment is immersed in water
after setting has begun. A greatly increased setting
expansion occurs. So less thermal expansion is required.
Increased hygroscopic expansion is obtained in the
following cases:
1) When a lower water/powder ratio is used
2) For an investment material of greater silica content.
3) If water of higher temperature is used.
4) For longer immersion in water
DR .R.koohkan
37
DR .R.koohkan
38

If during the setting process, the gypsum materials are immersed
in water, the setting expansion increases.

Mechanical mixing decreases setting expansion.
Properties of a High-Strength
Dental Stone Mixed by Hand
and by a Power-Driven Mixer
with Vacuum
DR .R.koohkan
39
Power-driven mechanical spatulator with a vacuum
attachment. (Courtesy of Whip Mix Corporation,Louisville, KY.)
Flexible rubber mixing bowl and metal spatula with a stiff blade.
(Courtesy of Whip Mix Corporation,Louisville, KY.)
A Vibrator is designed to promote the release of bubbles in the gypsum
mix and to facilitate pouring of the impression. (Courtesy of Whip Mix
DR .R.koohkan
Corporation,Louisville, KY.)
40
Properties Required of an Investment :
1.
2.
3.
4.
5.
6.
7.
8.
Easily manipulated:
Sufficient strength at room temperature:
Stability at higher temperatures:
Sufficient expansion:
Beneficial casting temperatures:
Porosity:
Smooth surface:
Ease of divestment:
9. Inexpensive.
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41
Composition
 Investment is a mixture of three distinct types
of materials:
1. Refractory material
2. Binder material
3. Other chemicals
DR .R.koohkan
42

During the heating, the investment is expected to
expand thermally to compensate partially or totally for
the casting shrinkage of the gold alloy. Such as:

Quartz
Tridymite
Cristobalite
or a mixture of these.



Microstructure of the surface of a set cristobalite
investment. The large, irregular particles are ,silica,
and the rodlike particles are cristobalite (~3000)
Courtesy oR Earnshdw.)
DR .R.koohkan
43
 The common binder used for dental casting:



α-calcium sulfate hemihydrate (gold alloy)
Phosphate, (for high-temperature casting )
ethyl silicate, (for high-temperature casting)
DR .R.koohkan
44
To produce the desirable properties required of an investment.
such as :
 Sodium chloride
 Boric acid
 Potassium sulfate
 Graphite
 Copper powder
 Magnesium oxide

small amounts of chlorides or boric acid enhance the
thermal expansion of investments bonded by calcium
sulfate.
DR .R.koohkan
45

The final product’s properties are influenced by both
the ingredients present in the investment and the
manner in which the mass is manipulated and used
in making the mold.

The investment may contain 25% to 45% of the calcium
sulfate hemihydrate. The remainder consists of silica
allotropes and controlling chemicals.
DR .R.koohkan
46
• Dimensional change of Three
form of gypsum When heated
• The calciumsulfate portion of
the investment decomposes
into
sulfur dioxide
at temperatures over 700° C
sulfur trioxide

embrittle the casting metal.
Temperature ("C)
DR .R.koohkan
(Courtesy of K. Neiman,Whip-Mix (‘corporation, Louisville, I<Y.)
47
Effect of Temperature on Silicon Dioxide Refractories
 Effect of Temperature on Calcium Sulfate Binders

DR .R.koohkan
48

The percentage of expansion varies from one type to another


cristobalite and quartz each exist in two polymorphic forms
β-Form .Stable at a higher temperature
α-Form Stable at a Lower temperature. stable at room

Tridymite has three stable polymorphic forms.

DR .R.koohkan
49
cristobalite,
the expansion is uniform up to
about 200° C.
Expansion increases
0.5% to 1.2%
573°c
C
,
above 250° C it again becomes more
uniform.
Quartz:
At 573° C also shows a break in
the expansion curve, .
Tridymite :
Shows a similar break at a much lower temperature
105° and 160° C
DR .R.koohkan
50
• Displacive transition temperatures.
• A displacive change involves expansion or contraction in the
volume of the mass without breaking any bonds.
( cristobalite 220° C quartz573° C , tridymite105° and 160° C )
• changing α-form to β-form at all three forms of silica
expand.
• The amount of expansion is highest for cristobalite and lowest for
tridymite.
DR .R.koohkan
51
• Reconstructive transition during which bonds are broken and a
new crystal structure is formed.
• The quartz can be converted to cristobalite and tridymite by
being heated through a
B-quartz
870° C
B-tridymite 1475° C B-cristobalite
1700° C
Fused silica
160° C
573° C
Middle tridymite
220° C
105° C
α-quartz
α-tridymite
α-cristobalite
Displacive transition temperatures
Fused silica
DR .R.koohkan
52
Investment
105° C
up to about 200° C
Thermal expansion
then
Remains unchanged
contracts slightly
anhydrous calcium sulfate
between 200° -700° C.
Depending on the silica and composition of the investment
Registers varying degrees of expansion,
 Dehydration of the dihydrate and a phase change of the
calcium sulfate anhydrite cause a contraction.
DR .R.koohkan
53

When the investment is allowed to cool, the refractory and
binder contract according to a thermal contraction curve that
is different from the thermal expansion curve of the
investment
Thermal expansion and contraction curves for calcium
sulfate–bonded investment (thermal expansion type).
Curve 1 is first heating,
curve 2 is cooling
curve 3 is reheating.
DR .R.koohkan
54
) Hygroscopic expansion:
In one technique the investment is immersed in water
after setting has begun. A greatly increased setting
expansion occurs. So less thermal expansion is required.
Increased hygroscopic expansion is obtained in the
following cases:
1) When a lower water/powder ratio is used
2) For an investment material of greater silica content.
3) If water of higher temperature is used.
4) For longer immersion in water
DR .R.koohkan
55
Thermal expansion curves for calcium
sulfate–bonded investments. A, Hygroscopic
type; B,thermal expansion type.
For hygroscopic expansion,:
the additional water provided must be
presented to the investment during
setting.
The additional water be presented before
the observed loss of gloss, which is when
the setting reaction is not complete. This
allows the additional water to join the
remaining mix water and extend the
water surface so that the action of surface
tension is either delayed or inactive.
DR .R.koohkan
56
melting temperatures
Investment
Phosphate- bonded investments
Over 700° C
Silica-bonded investments
under700° C

calcium sulfate–bonded investments
cobaltchromium
alloys
Gold alloys
Base metal alloys are usually cast into molds at 850° to 110° C. To with
stand these high temperatures, the molds require different types of
binders, such as silicate and phosphate compounds.
( less than 20% binder, remainder of investment is quartz or another form of silica.)
DR .R.koohkan
57

Finer silica produces higher
Setting and hygroscopic
expansions.
The particle size
hygroscopic expansion
calcium sulfate hemihydrate
little effect
silica
significant effect
Silica/Binder Ratio:
Investments usually contain
65% to 75% silica,
25% to 35% calcium sulfate hemihydrate,
2% to 3% of some additive chemicals to control the different physical properties
The strength of the investment
decreases.
The hygroscopic expansion of the investment
increases,
If the silica/stone ratio is increased,
DR .R.koohkan
58
Manipulative Variable
Setting Time
Consistency
Setting
Expansion
Compressive
Strength
Increase water/powder
ratio
Increase
Increase
Decrease
Decrease
Increase rate of spatulation Decrease
Decrease
Increase
No effect
Increase temperature of
mixing water from
23° to 30° C
Decrease
Increase
No effect
Decrease
DR .R.koohkan
59

The water bath has a measurable effect on the wax pattern.
At higher water-bath temperatures
1-the wax pattern expands,
requiring less expansion of the investment to compensate for the total
casting shrinkage.
2- soften the wax
provides less resistance to the expansion of the investment
making the setting expansion more effective.
DR .R.koohkan
60
 Casting techniques involving gypsum bonded investments are often
classified as
1- Thermal techniques
the invested ring
after setting
placing
into the burnout oven
(649° C),
2- Hygroscopic techniques.
the invested ring before setting
(482° C).
immersing in a
water bath
after setting
directs
the burnout oven
Although all gypsum-bonded investments exhibit both thermal and
hygroscopic setting expansion,
DR .R.koohkan
61
 Investments used in the thermal technique usually contain
cristobalite
has a high thermal expansion.
Expansion (%)
 Investments used in the hygroscopic technique usually contain
lower thermal expansions
quartz or tridymite
higher hygroscopic setting expansions.
Thermal expansion of mixed hygroscopicthermal gold casting investment.
DR .R.koohkan
Temperature ( C)
62
Expansion (%)
Setting and hygroscopic expansion of
mixed hygroscopic-thermal gold casting
investment.
Temperature ( C
DR .R.koohkan
63
This type of investment consists of three different components.
1. A water-soluble phosphate ion.
2. The second component reacts with
phosphate ions at room temperature.
3. The third component is a refractory,
such as silica.
ANSI/ADA specification No. 126 (ISO 9694) for
dental phosphate-bonded casting investments
Specifies two types of investments for alloys having a solidus temperature
above 1080° C:
Type 1: For inlays, crowns, and other fixed restorations
Type 2: For removable dental prostheses
DR .R.koohkan
64
an acid-base reaction between acid
monoammonium phosphate+ Mgo
forming a binding medium with filler
particles embedded in the matrix.
Spatulation continues
The water produced by this reaction
at room temperature
Phases formed at high temperatures
DR .R.koohkan
65
• The special liquid is a form of
silica sol in water
• phosphate-bonded investments
possess higher setting expansion
when they are mixed with the
silica sol than when mixed with
water. (increases its strength)
Thermal expansion type
Effect of silica sol concentration on
thermal expansion (solid lines) at 800° C
and setting expansion (dotted lines) of two
phosphate-bonded investments.
A, Thermal expansion type; B, hygroscopic
expansion type.
DR .R.koohkan
66
• This type of investment may derive its silica
bond from ethyl silicate, an aqueous dispersion
of colloidal silica, or from sodium silicate.
DR .R.koohkan
67
Setting reaction:
a) Stage 1 : hydrolysis. Ethyl silicate can be
hydrolysed to silica acid, with liberation of
ethyl alcohol:
Si(OC2H5)4 + 4H2O
Si(OH)4 + 4C2H5OH
ethyl silicate
a colloidal solution of silicic acid and
ethyl alcohol,
In practical, a polymerised form of ethyl silicate
is used, yielding a sol of polysilicate acid.
DR .R.koohkan
68
b) Stage 2: gelation. The sol is mixes with cristobalite or
quartz, then gel formation is made to occur under
alkaline conditions by adding magnesium oxide.
There is a slight shrinkage at this stage.
c) Stage 3: drying. on heating, considerable shrinkage
occurs and there is a loss of alcohol and water,
leaving a mould made of silica particles tightly
packed together.
As alternative of the above, simultaneous hydrolysis
and gel formation can occur, when an amine such as
piperidine is incorporated.
DR .R.koohkan
69
• ANSI/ADA specification No. 126 (ISO 11246)
• The setting time must not differ by more than
30% from the time stated by the manufacturer.
• The compressive strength at room temperature
shall not be less than 1.5 MPa.
• The linear thermal expansion must not differ by
more than 15% from the time stated by the
manufacturer.
• Brazing Investment
• When brazing (soldering) the parts of a
DR .R.koohkan
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ANSI/ADA specification No. 126 (ISO 11244)
 for dental brazing investments defines two types
of investment:




Type 1: Gypsum-bonded dental brazing investments
Type 2: Phosphate-bonded dental brazing investments
Soldering investments are designed to have lower setting
and thermal expansions than casting investments , a
feature that is desirable so the assembled
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INTRODUCTION
Casting is the process by which a wax pattern
of a restoration is converted to a replicate in
dental alloy.
It is used to make dental restorations such as
inlays, onlays, crowns, bridges, and removable
partial dentures
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1 . TOOTH PREPARATION .
2 . IMPRESSION .
3 . DIE PREPARATION .
4 .WAX PATTERN FABRICATION .
- There are 4 methods for making wax patterns
for a cast restoration .
5. SPRUING .
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
Gingival retraction

Retraction cord
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1 .Create a wax pattern of the missing tooth / rim
2 .Sprue the wax pattern
3 .Invest the wax pattern
4. Eliminate the wax pattern by burning it (inside the furnace or
in hot water). This will create a mould.
5 . Force molten metal into the mould - casting.
6 .Clean the cast.
7 .Remove sprue from the cast
8 . Finish and polish the casting on the die
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The management of these dimensional
changes is complex, but can be summarized
by the equation:
wax shrinkage + metal shrinkage = wax
expansion + setting expansion + hygroscopic
expansion + thermal expansion
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
the wax will shrink significantly because of the high
coefficient of thermal expansion of waxes.

Metal shrinkage occurs when the molten metal
solidifies, but this shrinkage is compensated by
introducing more metal as the casting solidifies

Cooling shrinkage may reach 2.5% for an alloy that
cools from a high solidus temperature (1300" to
1400' C), depending on the coefficient of thermal
expansion of the alloy.
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FUNCTIONS OF SPRUE
1 . Forms a mount for the wax pattern .
2 . Creates a channel for elimination of wax .
3 .Forms a channel for entry of molten metal
4 . Provides a reservoir of molten metal to
compensate for the alloy shrinkage
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• 1 . DIAMETER :
It should be approximately the same size of the thickest
portion of the wax pattern .Too small sprue diameter suck
back porosity results .
• 2 . SPRUE FORMER ATTACHMENT :
Sprue should be attached to the thickest portion of the wax
pattern .It should be Flared for high density alloys &
Restricted for low density alloys .
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3 . SPRUE FORMER POSITION
Shape & form of the wax pattern .
Patterns may be sprued directly or indirectly ..
Indirect method is commonly used
4.Reservoir prevents localised shrinkage porosity
. Reservoir And Its Location
Reservoir portion of a Spruing system is a round ball or a bar
located 1mm away from the wax pattern. .
Round ball reservoir & a bar reservoir also called connector
Significance of Reservoirs:
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Reservoir should be positioned in the heat centre of the ring .
This permits the reservoir to remain molten for longer and
enables it to furnish alloy to the pattern until they complete
solidification process .
5. SPRUE FORMER DIRECTION
Ideal angulation is 45 degrees
6 . SPRUE FORMER LENGTH
Depends on the length of casting ring
7.TYPES OF SPRUES
- Wax . II . Solid
- Plastic . Hollow
- Metal .
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VENTING
 Small auxilliary sprues or vents
improve casting of thin patterns
Acts as a HEAT SINK .
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To minimise the irregularities on the investment
& the casting a wetting agent can be used .
FUNCTIONS OF A WETTING AGENT
1 . Reduce contact angle between liquid & wax
surface .
2 .Remove any oily film left on wax pattern
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It serves as a base for the casting ring during
investing .Usually convex in shape.May be metal
, plastic or rubber
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 Most common way to provide investment expansion is
by using a liner in the casting ring .Traditionally
asbestose was used .
Non asbestose ring liner used are :
 1) Aluminosilicate ceramic liner .
2) Cellulose paper liner .
 a resilient liner is to
allow different types of investment expansion
facilitate venting during casting procedure.
facilitate the removal of the investment block after
casting .
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
RINGLESS INVESTMENT TECHNIQUE

Useful for high melting alloys
used for phosphate bonded investments
This method uses paper or plastic casting ring
Solid rings do not permit the investment to expand
laterally during the setting and hygroscopic
expansions of the mold



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




the correct water powder ratio of the investment
mix,
A required number of spatulation turns,
A proper investing technique are essential to obtain
acceptable casting results.
hand investing and vacuum investing.
requires 45 to60 minutes
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Two type of torch tips:
• Multi-orifice
is widely used for metal ceramic alloys. Main advantage is
distribution of heat over wide area for uniform heating of
the alloy.
• Single-orifice tip
concentrate more heat in one area.
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Three fuel sources are used for Casting Torch;
1-Acetylene
2-Natural Gas
3-Propane
CASTING CRUCIBLES
Four types are available ;
1) Clay .
2) Carbon .
3) Quartz .
4) Zirconia –Alumina .
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It is a device which uses heat source to melt
the alloy casting force .
Heat sources can be :
1) Reducing flame of a torch .
( conventional alloys & metal ceramic alloys )
2) Electricity .(Base metal alloys )
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Advantages of electric heating :
-heating is evenly controlled .
-minimal undesirable changes in the alloy composition
- Appropriate for large labs .
Disdvantages :
Expensive .
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1) Air pressure .
2) Centrifugal force .
3) Evacuation technique
Centrifugal casting machine,
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1) Alloy is melted in a separate crucible by a torch flame & is
cast into the mold by centrifugal force .(centrifugal C M )
2) Alloy is melted electrically by a resistance heating or by
induction furnace & then cast centrifugally by motor or
spring action (spring wound CM electrical resistance )
3)
Alloy is melted by induction heating cast into mold
centrifugally by motor or spring action .(Induction CM )
4)
Alloy is vacuum melted by an argon atmosphere
Torch melting / Centrifugal casting machine
Electrical resistance /Heated casting machine
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Induction melting casting machine,
water cooling induction coil.
Induction melting casting machine,
vertical crucible positional within the
induction coil
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Spring-wound casting
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