investment material

1. GOOD MORNING 1

2. Investment
Materials

3. INTRODUCTION
Casting procedure :- It is a principal laboratory method used
to form metal inlays, onlays, crowns and bridges by casting
molten alloys, under pressure, or under vacuum with the help
of centrifugal force.
It involves:-
(1) a wax pattern of the object to be reproduced
(2) a suitable mold material, known as investment, which is
placed around the pattern and permitted to harden
(3) suitable furnaces for burning out the wax patterns and
heating the investment mold
(4) proper facilities to melt and cast the alloy.
3

4. DEFINITION
An investment can be described as a ceramic material that is
suitable for forming a mold into which a metal or alloy is cast.
The operation of forming the mold is described as investing.
(CRAIG’S)
Dental casting investment: A material consisting primarily of
an allotrope of silica and a bonding agent. The bonding
substance may be gypsum (for use in lower casting
temperatures) or phosphates and silica (for use in higher
casting temperatures).
(GPT 8)
4

5. Metal Casting
• Lost wax technique (Taggart, 1907)
5
A  crucible former
B  sprue
C  wax pattern
D  investment material
E  ring liner
G  thickness of investment at
the top

6. Ideal Properties Required for an Investment
1. Easy to manipulate: it should be easy to mix & manipulate
and to paint the wax pattern. It should also harden within a
relatively short time.
2. Sufficient strength : The investment should provide
enough strength at higher temperatures to withstand the impact
force of the molten metal and the inner surface of the mold
should not break down at a high temperature.
3. Stability at higher temperatures: Investment must not
decompose to give off gases that could damage the surface of
the alloy.
4. Sufficient expansion: It must expand enough to compensate
for shrinkage of the wax pattern and metal during the casting
procedure. 6

7. 5. Beneficial casting temperatures: it should have a good
thermal expansion over a wide range of casting temperatures.
6. Porosity: It should be porous enough to permit the air or
other gases in the mold cavity to escape easily during the
casting procedure.
7. Smooth surface: Fine detail and margins on casting should
be preserved.
8. Ease of divestment: The investment should break away
readily from the surface of the metal and should not react
chemically with it.
9. Inexpensive. 7

8. Composition in general
An investment is a mixture of three distinct types of materials:
Refractory material
silica based materials
 regulates thermal expansion
Binder material –
Gypsum(α-calcium sulfate hemihydrate), Phosphate, Ethyl-Silicate
To hold or bind the refractory material particles
Other chemical / Modifiers
NaCl, boric acid, potassium sulfate, graphite, copper powder,..etc.
 accelerates or retard the setting reaction , increase expansion
8

9. Types of Investment
According to refractory material used :
• Quartz investment
• Cristobalite investment
• Tridymite investment
• Mixture of quartz, cristobalite and tridymite
• Mixture of alumina, magnesia, zirconia.
9

10. According to the binder- materials used :-
•Calcium sulfate-bonded or Gypsum-bonded – used for
casting of gold alloy inlays, onlays, crowns and fixed
partial dentures(FPDs).
•Phosphate-bonded – for some base metal alloys and
framework of metal - ceramic prosthesis.
•Ethyl- silicate bonded – for removable partial dentures
with base metal alloy (Co- Ni based alloy)
10

11. Gypsum bonded
investment
 Employed for casting of gold alloys.
 Because of their tendency to decompose at
high temperatures, these investments are not
suitable for casting high-melting alloys.

12. ADA Specification No. 2
Types of gypsum-bonded casting investments (based on
whether the appliance is fixed or removable & on the method
of expansion.
Type I: For casting inlays and crowns.
Thermal expansion
Type II: For casting inlays, onlays and crowns
Hygroscopic expansion
Type III: For casting partial dentures with gold alloys.
12
PHILLIP’S

13. • Binder: α- calcium sulfate hemihydrate (25%-35%)
• Refractory: Silica (SiO2) Quartz, Cristobalite (65%-75%)
• Modifiers: coloring agent, reduction agents e.g. carbon,
Cu (2%-3%)
13
Composition of gypsum bonded
investment

14. 14
• Gypsum bonded investment (GBI) shows shrinkage when
heated above 700o
C and is most likely due to
decomposition and release of sulfur gases.
• This decomposition not only causes shrinkage but also
contaminates the castings with the sulfides of the non-
noble alloying elements.
• Hence gypsum investments should not be heated above
700o
C.
CaSO4  SO2 + SO3 (above 700o
C )

15. •Colouring matter
•Reducing agents - carbon and powdered copper.
USE - To provide a non-oxidizing atmosphere in the mold
when the gold alloy is cast.
•Boric acid and NaCl - regulate setting expansion, setting time
and prevents shrinkage of gypsum.
15
Modifiers

16. Refractory material
Silica based materials which exist in following forms:-
•Silicon dioxide (SiO2)
• Quartz
• Tridymite
• Cristobalite
• Mixture of these
16

17. REFRACTORY MATERIAL
FUNCTION
During the heating, the investment is
expected to expand thermally to
compensate partially or totally for the
casting shrinkage of the gold alloy.
Gypsum shrinks considerably when it is
heated, whether it is set plaster or stone.
If the proper forms of silica ,is employed
in the investment, this contraction during
heating can he eliminated and changed to
an expansion. 17
DIMENSIONAL CHANGE OF
GYPSUM WHEN HEATED

18. When quartz, tridymite, or cristobalite
is heated, a change in crystalline form
occurs at a particular transition
temperature.
QUARTZ
α β
low form High form
CRISTOBALITE
α β
As α form changes to β form when
heated DENSITY , resulting in
VOLUME leading to increase in
the linear expansion. 18
SiO2
α ↔ β
α ↔ β
1.6%
1.4%
MECHANISM OF EXPANSION
573 C∘
200- 270 C∘

19. Effect of Temperature on Silicon Dioxide Refractories
19
Each of the polymorphic forms of silica—quartz, tridymite,
and cristobalite—expands when heated, but the percentage of
expansion varies from one type to another.
Pure Cristobalite expands to 1.6% at 200°C- 270° C
Quartz expands about 1.4% at 573°C
Tridymite expands to less than 1% at 600°C.
The amount of expansion is highest for cristobalite and
lowest for tridymite.

20. • The quartz form of silica is found abundantly in nature, and
it can be converted to cristobalite and tridymite on heating
through which bonds are broken and a new crystal structure
is formed.
• These transitions are shown in the following equation.
20

21. Effect of Temperature on Calcium Sulfate
Binders
During the investing process, some of the water mixed with the
investment reacts with the hemihydrate and is converted to
calcium sulfate dihydrate, whereas the remainder of the water is
uniformly distributed in the mix. as excess water. During the
early stage of heating, the excess water evaporates.
On further heating investment expands initially when it is first
heated from room temperature to about 105° C, then contracts
slightly or remains unchanged up to about 200° C, and registers
varying degrees of expansion, between 200° and 700° C.
21

22. It is explained as :-
Up to about 105° C, ordinary thermal expansion occurs.
Above 105° C, the calcium sulfate dihydrate is converted to
anhydrous calcium sulfate by loosing water . Dehydration of
the dihydrate and a phase change of the calcium sulfate
anhydrite cause a contraction.
However at elevated temperatures, α-forms of silica present
in the investment are converted to the β-forms, which causes
some expansion and compensates for the contraction of
calcium sulfate.
22

23. Setting time
According to ANSI/ADA Specification No. 2-
For dental inlay casting investment, the setting time
should be 5 min-25 min.
Usually, the modern inlay investments sets in 9 to 18 min.
Sufficient time should be allowed for mixing and
investing the pattern before the investment sets.
23

24. Advantages:
• Adequate strength
• Adequate porosity
• Controlled large setting and thermal expansion
• Simple methods of manipulation and casting procedure
• Not very expensive.
24

25. Disadvantages:
• Sulphur dioxide gas produced at the walls of the mold by
decomposition of gypsum during heating causes
tarnishing or discoloration of gold alloys. Disintegration
causes large contraction.
• Hence this is not suitable for casting of high fusing Noble
and High Noble alloys. Thus phosphate-bonded
investment material is preferred.
25

26. • Too high casting forces and careless wax burnout
methods fractures and produces cracks in the investment
(result: fins)
• The GBI is hygroscopic. Hence large quantity should not
be purchased and it should be stored carefully in air tight
containers.
• Cannot be used for titanium alloys.
26

27. Expansion
All the calcium sulfate–bonded investments have both setting
and thermal expansion.
•Setting Expansion
 Normal setting expansion
 Hygroscopic setting expansion
•Thermal Expansion
27

28. Normal setting expansion- If the investment is setting
surrounded by air, the expansion is referred to as normal
setting expansion.
Hygroscopic setting expansion- If the mixed investment is
setting in contact with water, the expansion is substantially
greater and is called hygroscopic setting expansion.
28

29. Setting Expansion
(normal vs. hygroscopic)
• Stage I: initial mix represented by three
round particles of hemihydrate surrounded
by water
Stage II: crystals of dihydrate are formed
• Lt: water around the particles is reduced
and the particles are drawn more closely
together by the surface tension action of
the water.
• Rt: hydrated water is replaced
Stage III: crystals grow
• Lt: water is decreased, particles are
drawn together
• Rt: water is replenished from outside,
crystals grow freely
Stage IV and V:
Lt: intermeshed and entangled crystals
Rt: crystals grow much freely 29

30. Normal Setting Expansion (0.4-0.6%)
• A mixture of silica and calcinated gypsum (CaSO4
hemihydrate) results in setting expansion greater than that of
the gypsum product used alone.
• Because silica particles probably interfere with the
intermeshing and interlocking of the crystals as they form.
Thus the thrust of the crystals is outward during growth, and
they increase expansion.
• It can be regulated by accelerators and retarders .
30

31. Variables other than the exothermic heat of reaction
which can influence the effective setting expansion are:-
• After setting and gaining sufficient strength investment
produces dimensional change in the wax pattern and mold
cavity.
• And if the pattern has a thin wall, the effective setting
expansion is somewhat greater than for a pattern with
thicker walls, because the investment can move the thinner
wall more readily.
• Also, the softer the wax, the greater the effective setting
expansion, because the softer wax is more readily moved
by the expanding investment. 32

32. Hygroscopic Setting Expansion (1.2- 2.2%)
• Occurs when the gypsum product is
allowed to set under or in contact with
water.
• Greater in magnitude than the normal
setting expansion (> 6 times).
• Can be obtained by
 Immersing the invested ring in
water bath.
Adding water before the loss of gloss
or completion of the setting reaction.
Placing wet ring liner in the casting
ring. 33
Normal setting
expansion
Hygroscopic setting
expansion

33. 34
TRADITIONALLY
Asbestos ring liners were
used.
Disadvantage
• High carcinogenic potential
• Asbestos fiber bundles were
found to cause lung
diseases.
NEW ALTERNATIVE RING
LINERS
- Aluminosilicate ceramic
liner
- cellulose liner
VARIOUS TYPES OF RING LINERSVARIOUS TYPES OF RING LINERS

34. Factors to Control Normal and Hygroscopic
Setting Expansion
Composition
• Proportional to the SiO2content
• The finer the particle size of the SiO2, the greater the hygroscopic
expansion.
•Silica/Binder Ratio
silica/CaSO4 ratio is increased, the hygroscopic expansion increases,
but strength decreases.
•Spatulation
Mixing time reduced  decrease expansion
•Shelf life of the investment
Older investment  lower expansion 35

35. Time of Immersion
• Delayed immersion (after initial set)  decrease expansion
Confinement
•Both the N & HSE are confined by opposing forces, such as the
walls of the container in which the investment is poured or the walls
of a wax pattern.
•However, the softened wax provides less resistance to the
expansion of the investment, thus making the setting expansion
more effective.
Water bath temperature
•Softens and expands wax pattern, requiring less expansion of the
investment to compensate for the total casting shrinkage.
36

36. Water : Powder Ratio
• Higher W:P (thinner mix) less normal and hygroscopic
expansion
Amount of Added Water
• The magnitude of the expansion is in directly proportional
to the amount of water added during the initial setting
period.
37

37. Linear hygroscopic setting expansion vs. Amount of water
added
38

38. Thermal Expansion (1.0-1.6%)
• Directly related to the amount
of SiO2 present and to the type
of SiO2 employed (quartz vs.
crystobalite).
• Cristobalite shows more
thermal expansion as compared
to quartz.
• The effect of SiO2 will balance
the contraction of the gypsum
during heating.
39
SiO2

39. • The maximum thermal expansion for inlay investment
must be attained at a temperature not higher than
700o
C. As noted earlier gold alloys can become
contaminated at a mold temperature higher than this.
• Type I investments, which rely principally on thermal
expansion for compensation, the thermal expansion must
be not less than 1% nor greater than 1.6%
40

40. Factors to Control Thermal Expansion
Effect of the Water/Powder
Ratio
Thermal expansion is directly
proportional to the amount of solid
(silica) present. Hence more water
used in mixing  less thermal
expansion.
41

41. • Effect of Chemical Modifiers
Silicas do not prevent gypsum shrinkage but counterbalance it,
whereas chlorides actually reduce gypsum shrinkage below
temperatures of approximately 700∘C. A large amount of
silica can weaken the investment also.
Hence chemical modifiers, e.g., sodium, potassium and lithium
chlorides, are added to eliminates the contraction and
increases the expansion without the presence of an excessive
amount of SiO2.
42

42. Cooling of the Investment
On cooling to room temperature,
the investment contracts to less
than its original dimension.
On reheating, it does not expand
thermally to the previous level.
Moreover, the second time
heating of investment causes
internal cracks that can affect the
quality of the casting.
43
Curve 1 is first heating, curve 2 is
cooling, Curve 3 is reheating.
Thermal expansion and contraction
curves for CaSO4 bonded investment

43. STRENGTH
The strength of the investment must be adequate to
prevent fracture or chipping of the mold during heating
and casting of the gold alloy.
Compressive strength : 2-4Mpa
44

44. Factors affecting strength
• The strength of the investment is affected by the W/P
ratio in the same manner as any other gypsum products;
the more water that is employed in mixing the lower the
compressive strength.
• Heating the investment to 700∘Cmay increase or
decrease strength depending on composition.
• After investment has cooled to room temp. its strength
decreases---------fine cracks
45

45. Other gypsum investment considerations:
Fineness: a fine particle size is preferable to a coarse one,
because the finer the investment, the smaller the surface
irregularities on the casting.
Porosity: The more gypsum crystals that are present in the
set investment, the less its porosity.
• It follows that the lower the hemihydrate content and the
greater the amount of water used to mix the investment,
the more porous it becomes.
• The particle size of the investment is also a factor. The
more uniform the particle size, the greater its porosity. 46

46. Storage
• The investment must be stored in airtight and moisture-
proof containers.
• During use, the containers should be opened for as short
time as possible.
47

47. Phosphate-bonded
investment

48. • These are high temperature investments.
• The main use of high-temperature investments is for
casting dental alloys which usually need to be heated to
casting temperatures of about 1200°C to 1300°C.
• Alloys that fit this description include many high-gold
and palladium-based alloys used for fabrication of
porcelain-veneered FPD and those dental alloys based on
Ni, Co-Cr, or Ti.
49

49. 50
• Casting of alloys
• Soldering and porcelain veneering.
Applications:

50. 51
• PBI have been used to make molds into which high-
melting temperature dental alloys are cast.
According to ADA Specification No.126 phosphate
bonded investment are 2 types :
• Type I: employed for casting of inlays, crowns and other
restorations
For alloys: gold, palladium, platinum, Co-Cr, to which
porcelain is fused in the construction of esthetic fixed
restorations.
• Type II: is used for casting of removable partial
dentures.
Casting of alloy

51. 52
• Another traditional use of
PBI is to make “soldering”
fixtures that hold prosthetic
components in alignment
while they are being joined
with solders, brazing
alloys, or welding alloys.
Soldering and porcelain veneering

52. 53
1 Compressive strength
• Type 1- 2.5 Mpa
• Type 2- 3 Mpa
2 Thermal expansion
• 0.8% when 50:50 mixture of liquid and water
Properties

53. Composition
54
Refractory materials – 80% Silica
(quartz , cristobalite or a mixture of two)
Purpose
• To provide high temperature thermal shock resistance
• To provide high thermal expansion.
Binder (<20%)
• Magnesium oxide, phosphoric acid (liquid) Or
monoammonium phosphate (can be incorporated into
powder).

54. 55
Modifiers :
• Carbon is often added - to produce clean casting and
facilitate divesting.
• When silver palladium or base metal alloys are invested
with the investment containing carbon, it embrittles the
alloys even though the investment is heated to the
temperature that burn out the carbon.
• Palladium reacts with carbon even at temp 1500°C, so
carbon free phosphate bonded investment began to be used
for higher temp.
• These PBI can be used with water or Colloidal silica
solution ( greater expansion) both.

55. 56
It is available as two component systems
1- It is a Powder - Refractory materials
Binders
Modifiers
2- Liquid- Aqueous solution of colloidal silica.
or Water
Colloidal silica suspension facilitate greater expansion of the
investment which can compensate the greater casting shrinkage of
alloys.

56. 57
The chemical reaction for the binder system that causes the
investment to set and harden is generally as follows:
NH 4H 2PO 4 + MgO + 5H 2O NH 4MgPO 4.6H 2O
Magnesium ammonium
phosphate
SETTING REACTION
Monoammonium
phosphate
Magnesium
oxide

57. On heating, the binder of the set investment undergoes thermal
reactions as follow:
MgO + NH 4H 2PO 4 + H 2O
(NH 4MgPO 4.6H 2O )n
MgO
NH 4H 2PO 4
H 2O
(NH 4MgPO 4.6H 2O )n
(NH 4MgPO 4.H 2O )n
(Mg 2P 2O 7)n
Mg 2P 2O 7
Mg (P O )
58
Room temperature
Collloidal type particles
Prolonged setting at 25 0
C
or dehydration at 50 0
C
Dehydrated at 160 0
C
Heated from 350- 650 0
C
Noncrystalline polymeric phase
Heated above 1040 0
C

58. 59
Final products are-
•Crystalline Mg 2P 2O 7 and some excess MgO
•Unchanged quartz, cristobalite or both
•Overheating or when molten metal contacts the mold
cavity surfaces - Mg 3 (P 2O 4)2

59. 60
Setting and Thermal Expansion
When PBI are mixed with water- initial shrinkage is seen
same that of GBI within the temperature range of (200˚ C to
400°C).
The early thermal shrinkage of phosphate investments is
associated with the decomposition of the binder, magnesium
ammonium phosphate, and is accompanied by evolution of
ammonia, which is readily apparent by its odour.

60. • This contraction is practically eliminated when a colloidal
silica solution is used for mixing phosphate bonded
investment instead of water.
61

61. Expansion and strength of these
investment can be modified by:-
• altering the liquid:powder
ratio or
• Increasing the concentration
of the special liquid.
•But if more liquid is used it
caused dense nonporous
investment which can effect
casting.
62
Influence of liquid concentration on
the setting and thermal expansion

62. Working and Setting Time
• Markedly affected by temperature
The warmer the mix, the faster it sets.
• Increased mixing time and mixing efficiency result in a
faster set and a greater rise in temperature.
• In general, the more efficient the mixing, the better the
casting in terms of smoothness and accuracy.
• Mechanical mixing under vacuum is preferred.
63

63. Other Properties
• Detailed reproduction and surface smoothness of
phosphate-bonded investment is inferior than gypsum-
bonded investment.
• Increasing the special liquid:water ratio used for the mix
markedly enhances casting surface smoothness but can
lead to oversized extracoronal castings.
64

64. 65
• High green strength
• High fired strength – less mold cracking and few fins on
casting
• Can withstand temperature up to 1000°C for short period
of time (useful or performing metal-joining operations).
They can also provide setting and thermal expansions
high enough to compensate for the thermal contraction of
cast-metal prostheses or porcelain veneers during
cooling.
Advantages

65. • When used with higher-melting alloys (casting temp.
higher than 1375°C) - results in mold breakdown and
roughen surface of castings.
• Difficult to divest (removal of casting from the investment)
• When higher expansion is required, more of the silica
liquid is used with the result that a more dense and less
porous mold is produced. This can result in incomplete
castings if a release for trapped gases is not provided.
66
Disadvantages

66. 67
Silica bonded investment:
It comprises the second type of investment used in
dentistry.
Used since the early 1930’s in the construction of the
high-fusing base metal partial denture alloys.

67. 68
Refractory material – Silica
Binder –Silica gel or ethyl silicate
Modifier –
Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator
Composition

68. 69
Binder silica gel reverts to silica (cristobalite) on heating.
Methods to obtain silica or silicic acid gel binders:-
1)By adding an acid or an acid salt to sodium silicate.
2)By the adding an accelerator, such as ammonium chloride to
an aqueous suspension of colloidal silica.
3)By hydrolyzing ethyl silicate in the presence of HCl, ethyl
alcohol, and water.
Binder

69. 70
The reaction can be expressed as follows:
Ethyl Silicate Colloidal Sol Of
Polysilicic Acids
When sol is mixed with powder (quartz or cristobalite and
magnesium oxide) a coherent gel of polysilicic acid is
formed.
During drying at temp below 168°C , soft gel loses alcohol
and water to form a concentrated, hard gel which lead to
volumetric contraction (GREEN SHRINKAGE) which
ultimately reduces the size of the mold. This shrinkage
occurs in addition to setting shrinkage.
SOFT GEL HARD GEL

70. 71
It is supplied as a powder and liquid or two liquids
If supplied as a powder and liquid
Powder consists of refractory particles of silicas and
glasses along with magnesium oxide and some other
refractory oxides in minor amounts.
Liquid contains stabilized alcohol solution of silica gel

71. 72
If supplied as 2 liquids
1 bottle contains diluted water-soluble silicate solution.
2 bottle contains a properly diluted solution of hydrochloric
acid.
Before use, mix an equal volume from each bottle and wait
for a prescribed time acc. to manufacturer’s instructions, so
that hydrolysis can take place and freshly prepared silicic
acid forms.

72. 73
The powder is added to hydrolyzed ethyl silicate liquid,
mixed quickly and vibrated into a mold , that has an extra
collar to increase the height.
The mold is placed on a vibrator that has a tamping action
this allows the heavier particles to settle while the excess
liquid and some of the finer particles rise to the top .
In about 30 mins, the accelerator in the powder hardens the
settled part, and the excess at the top is poured off. Thus the
L/P ratio in the settled part is greatly reduced, and the setting
shrinkage is reduced to 0.1 %.
Its low setting expansion minimise distortion.
Manipulation

73. 74
Can be heated to temp ( 1090˚ C-1180˚ C) and hence
compactible with higher fusing alloys.
Care should be taken while handling and burnout as
inflammable alcohol is given off.

74. 75
• It offers the ability to cast high temp Co-Cr and Nickel –Cr
alloys, and attain good surface finishes, low distortion, and
high thermal expansion (good fit).
• High permeability(gel form), yields sharply defined castings
• Low fired strength – easy removal of casting.
Advantages

75. 76
Disadvantages
•Ethyl silicate has the disadvantage of giving off flammable
components during processing hence extra precaution is
needed while using this investment.
•The low strength and high thermal expansion require a more
precise burnout process and firing schedule to avoid cracking
and destruction of a mold.

76. Soldering Investments

77. 78
 When soldering the parts of a restoration, such as clasps on
a RPD, the parts must be surrounded with a suitable
ceramic or investment material before the heating
operation.
 The assembled parts are temporarily held together with
sticky wax until they are surrounded with the appropriate
investment material, after which the wax is softened &
removed.
 The portion to be soldered is left exposed & free from
investment to permit wax removal & effective heating
before it is joined with solder.

78. 79
 2 types of brazing investments are:-
Type 1- Gypsum bonded dental brazing investments
(low-melting point alloys)
Type 2- Phosphate bonded dental brazing investments.
(high-melting-point alloys)
 Soldering investments are designed to have lower setting
and thermal expansions than casting investments, so that the
assembled parts do not shift in position during the setting &
heating of the investment.
 Do not have as fine a particle size as the casting investment
because the smoothness of the mass is less important.

79. 80
It is a combination of Die stone and gypsum bonded investment
material. The powder is mixed with colloidal silica.
•The die is made from this mix and the wax pattern is then
constructed on it. The whole complex is then invested in a
mixture of Divestment & water.
•This combination used to compensate distortion of wax pattern
of long span bridges or RPD frameworks during removal from
die.
•Special GBI or PBI investment materials are used.
Divestment (whip-mix, Louisville, KY)
Phillip’s

80. 81
PROPERTIES
•Setting expansion - 0.9%
•Thermal expansion - 0.6% (at 977°c)
AdvantagesAdvantages
•The wax pattern and die are invested simultaneously with out
removal of pattern. Useful with gold alloys
DIVESTMENT PHOSPHATE(DVP)DIVESTMENT PHOSPHATE(DVP)
•Similar to divestment, but used for casting post and core, crowns
of base metal alloys without any need of removal of wax pattern.

81. 82
PROBLEM CAUSE SOLUTION
Rough surface Breakdown of
investment
Don’t overheat mold or alloy
Weak surface of
investment
Avoid use of high
water/powder ratio of
investment.
Avoid dilution of the
investment material from
application of too much
wetting agent
Fins Cracking on
investment
Avoid too rapid heating
investment.
Casting defects because of investment material:

82. 83
PROBLEM CAUSE METHOD OF
AVOIDING FAULT
Casting too large Excessive mold
expansion
• Use correct
temperature
• Use correct type of
investment material
Casting too small Too little mold
expansion
Heat the mold
sufficiently.

83. 84
PROBLEM CAUSE METHOD OF
AVOIDING FAULT
Irregular voids Inclusion of particles of
investment material
Heat mold upside down so
that particles fall out of the
mold
Rounded margins;
Regular large voids
Back-pressure effect;
air unable to escape
from mould
1.Use sufficient casting
force
2.Use investment of
adequate permeability
3.Avoid presence of
residue of wax in mould
4.Place pattern no more
than 6-8mm away from
end of casting ring.

84. 85
• Newer investments have been aimed at the casting of titanium
and titanium based alloys.
• This is because molten titanium is highly reactive with oxygen
and is capable of reducing some of the oxides commonly found
in investment.
• Titanium can react with residual oxygen, nitrogen and carbon
from investment. These elements harden and embrittles
titanium in the solid state.
Recent advances:

85. 86
As a result, either modifications of existing refractory
formulations and binder or new refractory formulations and
binder system are required.
A variety of investment formulations for the casting of
titanium have been developed over the past several years.
These investments might be classified as PBI,ESBI and
Cemented (magnesia bonded by an aluminous cement,
which contains a mass fraction of 5% zirconia)
In these investments many kinds of refractories such as
silica, alumina, magnesia and zirconia have been used.

86. 87
Titavest, an aluminia-magnesia-system. Mixed with a
special liquid the material becomes a spinel structure.

87. 88
• Craig’s Restorative Dental Materials
13th
edition by Ronald L. Sakaguchi and
John M. Powers.
• Philip’s science of dental materials, 11th
edition by
Kenneth J. Anusavice.
References

88. THANK YOU