3. CONTENTS
Introduction
History
Definition
Properties of an ideal investment material
Composition
Gypsum bonded investments
Phosphate bonded investments
Ethyl silicate bonded investments
Die stone investment combination.
Hygroscopic thermal gold casting investments.
Soldering investments.
Investments for all- ceramic restorations.
Investment of titanium & titanium based alloys.
Summary & Conclusion.
References.
4. INTRODUCTION
An investment is a refractory material that is used to form a
mould around a wax pattern.
Following the production of a wax pattern either by direct or
indirect method; the next stage in many dental procedures
involves the investment of the pattern to form a mould.
A sprue is attached to the pattern and the assembly is located in
a casting ring. Investment material is poured around the wax
pattern while still in a fluid state.
When the investment sets hard, the wax and sprue former are
removed by burning out to leave a mould which can be filled
with an alloy or ceramic using a casting technique.
5. Lost wax casting art was widely known in
ancient times.
The Aztec gold smiths of Pre-Columbian
Mexico used it to create their elaborate
jewellery.
11th century – Theophilus first described
the lost wax technique.
1558 – Benvenuto Cellini, used wax and
clay for preparation of castings.
1884 – Aguilhon de saran – used 24K gold
to form inlays
1897 – Philibrook described a method of
casting metal fillings
1907 – Taggart devised a casting machine
HISTORY
6. 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.
- Craig
--Refractory material used to form a mould casting for
cast metals or hot pressed ceramics.
- k J Anusavice.
7. REQUIREMENTS OF AN INVESTMENT MATERIALS
Easy manipulation
Should provide
smooth surface to
the casting
Must not
decompose at
higher
temperatures
Sufficient
strength
Should break away
readily from metal
surface
Porous to permit
air and other gases
in mould to escape
Sufficient
expansion
Casting
temperatures
should not be
critical
Economical
10. Binder Material
Since the refractory materials alone do not form a coherent
solid mass, some kind of binder is needed.
The common binders used for dental casting gold alloy is α-
CaSO4 hemihydrate, Phosphate, ethyl silicate.
11. Other Chemicals:
These are added in small quantities to modify various
physical properties.
Sodium chloride.
Boric acid.
Potassium sulfate.
Graphite.
Copper powder.
Magnesium Oxide.
ex:
small amounts of chlorides or boric acid enhance the
thermal expansion of investments bonded by
calcium sulfate
12. Classification according to Binders used:
• GYPSUM BONDED
INVESTMENTS
• PHOSPHATE BONDED
INVESTMENTS
• ETHYL- SILICATE BONDED
INVESTMENTS
13. GYPSUM BONDED
INVESTMENTS:
American Dental AssociationAmerican National Institute For
Standards specification no: 2 for casting investments used for
gold alloys.
Type 1: For casting inlays and crowns.
Type 2: For casting complete and partial
removable denture bases.
14. Composition
Binder:
25% to 45% - -calcium sulfate hemihydrate.
Refractory material:
65% to 75% - quartz or cristobalite.
MODIFIERS : - 2% to 3%
- chemical modifiers.
- Reducing agents
15. GYPSUM:-
- 25% to 45% of -hemihydrate is present.
- -hemihydrate acts as a binder to hold the ingredients
together & to provide rigidity.
- Used in casting gold alloys with melting ranges below 1000ºC.
Above 1000ºC greater shrinkage & frequent fractures takes
place.
16. All forms of gypsum shrink after dehydration between 200ºC
to 400ºC .
A slight expansion takes place between 400oC and 700oC And
a large contraction then occurs.
The shrinkage on heating is due to the dehydration of the set
gypsum in two stages.
2CaSO4 · 2H2O (CaSO4)2 H2O + 3H2O
(CaSO4)2 · H2O 2CaSO4 + H2O
17. Shrinkage is due to the transformation of calcium
sulphate from the hexagonal to the orthorhombic
configuration.
18. SILICA:
55% to 75%.
It acts as a refractory material during the heating of the
investment & regulates the thermal expansion.
If the proper form of silica is used in the investment, the
contraction of gypsum during heating can be eliminated &
changed to an expansion.
20. When quartz, cristobalite or tridymite is heated, a change in
crystalline form occurs at a transition temp. characteristic of
the particular form of silica.
Ex: When quartz is heated
5750c
Quartz β-quartz
Cristobalite undergoes an anologous transition between
200ºC & 270ºC from to β form.
2 inversions of Tridymite occurs at 117ºC & 163ºC
respectively.
21. Density decreases as α form changes to β form, with a
resulting increase in volume exhibited by a rapid increase in
linear expansion.
On the basis of the type of silica employed, dental
investments are classified as:-
Quartz investment.
Cristobalite investment.
22. EFFECT OF VARYING COMPOSITION
Increasing the proportion of silica in the investment powder
Increases :
manipulation time,
initial setting time,
setting expansion both in air & water
thermal expansion
Decreases:
compressive strength.
- The rate of setting reaction is unchanged.
23. The increase in manipulation & setting time occurs because
the particles of the refractory filler, interfere with the
interlocking of the growing gypsum crystals & making this less
effective in developing a solid structure.
The compressive strength of the investment is reduced for
the same reason.
24. 3) MODIFIERS:-
a) Modifying Agents:-
Regulates the setting expansion & setting time.
They prevents most of the shrinkage of gypsum, when it
is heated above 300ºC.
eg. NaCl , Boric acid.
25. b) Reducing agents:- They are used in some
investments to provide a non- oxidizing atmosphere
in the mold when the gold alloy is cast.
eg. Carbon, powdered graphite or powdered copper.
26. SETTING TIME
It can be measured by,
1. Loss of Gloss test for initial set.
2. Initial Gilmore test for initial set.
3. Vicat test for setting time.
4. Gilmore Test For Final Setting Time.
27. According to ADA/ANSI specification no. 2 for dental inlay
casting investment, the setting time should not be less than 5
min. or more than 25 min.
Usually, the modern inlay investments set initially in 9 to 18
min.
Sufficient time should be allowed for mixing & investing the
pattern before the investment sets.
28. NORMAL SETTING
EXPANSION
Definition:-
The volumetric or linear increase in physical dimensions of an
investment caused by chemical reactions that occur during
hardening to form a rigid structure.
Regardless of the type of gypsum product used, an expansion
of the mass can be detected during the change from
hemihydrate to dihydrate.
29. A mixture of silica & gypsum hemihydrate results in setting
expansion greater than that of the gypsum product when it is
used alone.
The ADA Specification no. 2 for Type 1 investment permits a
maxi. setting expansion“in air”of only 0.6%. The setting
expansion of modern investments is 0.4%.
The setting expansion of an investment with a comparatively
high gypsum content is more effective in enlarging the mold
than with a low gypsum content.
30. FACTORS AFFECTING NORMAL
SETTING EXPANSION
If the pattern has thin walls, the effective setting expansion is
somewhat greater, than for a pattern with thicker walls.
The softer the wax, the greater is the effective setting
expansion.
If a wax other than the Type 2 inlay wax is used, the setting
expansion may cause a serious distortion of the pattern.
The lower the W:P ratio for the investment, the greater is the
effective setting expansion.
31. HYGROSCOPIC SETTING
EXPANSION
If the setting process is allowed to occur under water(slurry),
the setting expansion may be more than double in magnitude
because of the additional crystal growth permitted.
ADA Specification no. 2 for Type 2 investments requires a
mini. setting expansion in water of 1.2% & maxi. expansion
permitted is 2.2%.
32. PURPOSE:- To expand the casting mold to compensate for the
casting shrinkage of the gold alloy
The HSE differs from the NSE in that, it occurs when gypsum
is allowed to set under or in contact with water & that it is
greater in magnitude than NSE.
33. FACTORS CONTROLLING THE
HYGROSCOPIC EXPANSION:-
Effect of Composition
- Finer silica particle size, greater expansion
- α hemihydrate, in presence of silica,
produces greater expansion when expansion is
unrestricted.
Effect of Water/Powder Ratio
- Higher ratio, less expansion.
Effect of Spatulation
- Reduced mixing time, decreased expansion.
Shelf Life
- Older investment, lower expansion.
34. Effect of Time of Immersion
- More expansion observed if immersion takes
place before the initial set.
Effect of Confinement
- More pronounced
- less relative to the expected expansion than is
normal setting expansion.
35. Effect of Added Water
- Magnitude of the hygroscopic expansion is
in direct proportion to the amount of water
added during the setting period until a
maximum expansion occurs.
The term ‘hygroscopic’ is a misnomer
as although the added water may be
drawn into the setting material by
capillary action, the effect is not
related to hygroscopy
36. THERMAL EXPANSION
DEFINITION:-
It is the increase in dimension of a set investment
due to temp. increase during burnout.
The expansion of a gypsum bonded investment is
directly related to the amount of silica present & to
the type of silica employed.
37. A considerable amount of quartz is necessary to
counterbalance the contraction of the gypsum during
heating. Even when the quartz content of the
investment is increased to 60% with the balance
being hemihydrate binder, the initial contraction of
the gypsum is not eliminated.
The contraction of the gypsum is entirely
balanced when the quartz content of the investment
is increased to 75%.
38. The thermal expansion of quartz investment are
influenced by:-
- The particle size of the quartz.
- Type of gypsum binder
- W:P ratio
Since greater expansion occurs during the inversion
of the crystobalite, the normal contraction of the
gypsum during heating is easily eliminated.
39. According to ADA/ANSI Specification no. 2 for
Type 1 investment which rely on the thermal
expansion for compensation, the thermal expansion
must not be <1% nor >1.6%.
Type 2 investment, which rely on hygroscopic
expansion for compensation of the contraction of the
gold alloy, the thermal expansion be between 0% &
0.6% at 500ºC .
40. Thermal Contraction
When an investment is allowed to
cool from 700°C,its contraction
curve follows the expansion curve.
Due to inversion of the beta
quartz to its stable form at room
temperature , It shrinks to less
than its original dimension
because of the shrinkage of the
gypsum when first heated.
In practice, the investment should
not be heated a second time,
since internal cracks may develop
which affect the quality of the
casting.
CURVE -1 IS FIRST HEATING
CURVE -2 IS COOLING
CURVE -3 IS RE-HEATING
41. FACTORS AFFECTING THERMAL
EXPANSION:-
1) W:P ratio:- The magnitude of thermal expansion is related to
the amount of solids present.
W:P ratio thermal expansion.
42. 2) Effect of Chemical Modifiers –
- A disadvantage of an investment is the weakening
effect of silica.
- The addition of small amount of Na, K or Lithium
chloride to the investment eliminates the contraction
caused by the gypsum & increases the expansion.
- Boric acid hardens the set investment but, it
disintegrates during the heating of the investment &
a roughened surface of the casting may result.
43. 3) Strength
Increases rapidly as the material hardens after
initial setting time. The free water content of the
set product affects its strength.
The strength of an investment must be adequate to
prevent fracture or chipping of the mold during
heating & casting of the gold alloy and is measured in
terms of its compressive strength.
44. The compressive strength is increased according to
1) Type & amount of gypsum binder present
2) Use of chemical modifiers.
According to ADA Specification no. 2, the
compressive strength for the inlay investment should
not be less than 2.4 MPa when tested 2hrs after
setting.
45. Factors affecting strength :
1) W:P ratio:-
W:P ratio Porosity CS .
2) Heating the investment to 700ºC may / the
strength as much as 65%, depending on the
composition.
3) After the investment has cooled to room temp.,
its strength decreases considerably, because of
the fine cracks that form during cooling.
4) The addition of an accelerator or retarder lowers
both the wet & dry strength.
46. OTHER GYPSUM INVESTMENT CONSIDERATIONS
Fineness:- Finer the investment, the smaller are the
surface irregularities on the casting.
Porosity:- More gypsum crystals present in the set
investment, less porosity.
More uniform the particle size, greater is its
porosity.
Storage:- The investment should be stored in
airtight & moisture proof containers.
48. EXPANSION CONTROL
The expansion of the compound can be controlled by
varying the amount of liquid added to compensate
the contraction of the casting alloy.
The less water used, the greater the total expansion of
the investment compound.
Distilled water Setting
expansion
(average)
Thermal
expansion
(average)
Total expansion
32 ml 0.4% 1.2% 1.6%
40 ml 0.1% 1.2% 1.3%
49. MANIPULATION
Mix the investment compound by hand for
approximately 30 seconds with a spatula, allow to
stand under a vacuum for approx. 30 seconds. Then
mix under vacuum for 60 seconds with a mixing and
evacuation unit.
After mixing, place the mixing bowl on a vibrator
(approx. 30 seconds) and allow the investment
compound to flow together. Then let the compound
to flow into the mould, shaking gently and ensuring
that no bubbles form. After the mould has been filled,
switch off the vibrator.
Allow the mould to set for 45-60 minutes, depending
on the size If the mould is preheated too soon the
investment compound can crack.
50. PRE-HEATING
After the recommended drying time the wax can be eliminated
from the mould in accordance with the table below and heated to
the pre-heating temperature appropriate to the alloy being used.
The maximum heat-up rate should not exceed 7°C per minute.
Pre-heating Size of casting flask
1x 3x 6x 9x
setting/drying time 45 min 50 min 55 min 60 min
initial temperature of pre-
heating furnace
200 °C 200 °C 200 °C 200 °C
wax-elimination time (at 290
°c)
30 min. 40 min 50 min 60 min
dwell time at final
temperature
20 min 30min. 45 min 60 min
51. During wax elimination place the mould in the oven in
such a way that the wax can run out of the mould easily,
otherwise wax residue will be absorbed by the
investment compound capillaries and will carbonize
damaging the cast object.
The mould which have dried overnight should be wetted
slightly before casting rings are placed in furnace.
Devesting- Allow the casting ring to dry slowly in the air
at room temperature after casting and remove the
investment compound under running water with a plier.
52. PHOSPHATE BONDED INVESTMENTS
The rapid growth of the use of metal ceramic
restorations, cast removable partial dentures and
higher melting alloys resulted in an increased use of
phosphate bonded investment materials.
1) Type I - For crowns, inlays & other
fixed restorations
2) Type II - For partial denture & other
cast removable restorations
53. COMPOSITION:-
Consists of :-
1) Filler - Silica, in the form of Cristobalite, Quartz,
or a mixture of the two and in a
concentration of 80%.
- Provides high temperature thermal shock
resistance (refractoriness) and a high
thermal expansion.
2) Binder -
- The binder consists of magnesium oxide and
and a phosphate.
54. 3) Colloidal Silica Liquid Suspensions:-
- The colloidal silica suspensions are used with the
phosphate investments in place of water to
compensate for the greater contraction of the high
fusing alloys during solidification , as it increases the
setting expansion of the investment.
- For base metal alloys, a 33% dilution of the colloidal
silica is required.
55. 4) Carbon:-
- Often added to the powder to produce clean
castings to facilitate the divesting of the casting
from the mold.
- Appropriate if the casting alloy is gold.
- Evidence indicates that palladium react with carbon
at temperature above 1504°C. Thus, if the
temperature exceeds this temperature during
casting, a carbon free investment should be used.
56. SETTING REACTION
- Magnesium ammonium phosphate formed is
polymeric. An excess of magnesia is usually
present, and some of it is never fully reacted.
- So, colloidal multimolecular aggregate around
excess MgO and fillers is formed.
- On heating, the binder of the set investment
undergoes thermal reactions.
NH4H2PO4 + MgO + 5H2O NH4MgPO4 6H2O
57. NH4. H2PO4 + mgo + 5H2O Mg.NH4.PO4.6H2O
Mg.NH4. PO4. 6H20
NH4. MGPO4. H2O
Mg2. P2O7 + 2NH3 + 13H20
Mg3(P2O4)2 crystaline form
Dehydration on heating at
temperature 160 c
Heated about 300c-
6500c ammonia is
released
Room temperature
1040c
58. SETTING & THERMAL EXPANSIONS:-
-Instead of shrinkage , the reaction entails slight
expansion which increases by using colloidal silica
solution instead of water.
- 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.
- Some of the shrinkage is masked because of the
expansion of the refractory filler especially in
cristobalite.
59. Phosphate investments when mixed with H2O have shrinkage in
essentially the same temperature range, as gypsum bonded
investments (200-400C (400-750F).
This contraction is practically eliminated when a colloidal silica
solution replaces the water, expansion of a phosphate bonded
investment is decreased by increasing the liquid to powder
ratio.
The early thermal shrinkage in phosphate investments is due to
the decomposition of the binder magnesium ammonium
phosphate and is accompanied by evolution of ammonia’s which is
readily apparent by its odour.
60. WORKING & SETTING TIME:-
- Phosphate investments are affected by temperature
as warmer the mix, faster it sets.
- The setting reaction itself gives off heat, and this
further accelerates the rate of setting.
- Increase in mixing time and mixing efficiency
results in faster set & a greater rise in temperature.
- Increase in w:p ratio, increases the working time
61. Miscellaneous properties
Increasing the special liquid to water ratio used for
the mix markedly enhances casting surfaces
smoothness but can lead to oversized extra coronal
castings.
62. 1. Technical data
Mixing ratio
(powder: liquid)
100g :22 ml-23 ml
Total expansion
(linear)
1.2%-2.4%
Mixingtime
(under vacuum)
60 sec
Compressive strength
(depending on concentration
ofthe mixing liquid)
4MPa-8MPa
64. PREHEATING WITH HOLDING TIMES
•A uniform increase in temperature is important; in the ranges of
the cristobalite and quartz phase shifts (at about 270 °C and 560
°C respectively) the increase in temperature should be carried out
slowly (ideally 7 °C/min).
Heat treatment Size of the casting ring
1x 3x 6x 9x
Setting/drying time 20min 30min 40min 50min
Wax elimination temperature 300 °C**
Duration of wax elimination 30min 40min 50min 60min
Burnout temperature 730 - 850 °C"
Duration of preheating after final temperature
is reached*
20min 30min 45min 60min
65. ADVANTAGES:-
They have both green strength & fired strength
which makes them easy to handle without breaking
before burnout & strong enough afterwards to
withstand the impact & pressure of centrifugally cast
molten alloy.
Provide setting & thermal expansion high enough to
compensate for the thermal contraction of cast
metal prosthesis or porcelain veneers during cooling.
Have the ability to withstand the burnout process
with temperature that reach 900ºC.
66. DISADVANTAGES:-
When used with higher melting alloys i.e. those with
casting temp. ≥ 1375ºC, these investments results in
mold breakdown & rougher surface on castings.
The high strength of these investments can make
removal of the casting from the investment difficult.
67. Devesting :-
After casting let the casting ring cool down to room
temperature and devest. For this purpose the
investment material mold must be soaked for approx.
15 min and carefully split open at several points using
plaster pliers. When this method is employed, the
dental casting can be easily removed from the investing
compound, and formation of dust is prevented. Final
residues of investing compound in the crowns can be
picked out or carefully blasted out with aluminum
oxide, granularity 100 µm to 150 µm.
68. ETHYL SILICATE-BONDED INVESTMENT:-
Losing popularity - complicated and time consuming
procedures involved.
Consists of powder & liquid.
- Powder contains refractory particles of silica &
glass along with calcined MgO & some other oxides in
minor amount.
- 2 Liquids are ethyl silicate & acidified solution of
denatured ethyl alcohol.
- Binder is a silica gel that reverts to silica
( crystobalite) on heating.
69. Several methods may be used to produce silica or silicic acid gel
binder.
1. Na SiO2 pH + acid or acid salt
bonding silicic acid gel forms
Magnesium Oxide is added to increase strength
2. Aqueous solution of colloidal silica + ammonium chloride silica gel
70. 3.By hydrolising ethyl silicate + HCl + ethyl alcohol+water
colloidal sol of polysilicic acid
Cristobalite or quartz +MgO (alkaline)
Added
Coherent gel of polysilicic acid forms accompanied by setting shrinkage (soft gel )
Dried at temperature 168◦ C
Loses alcohol, water hard gel
Volumetric contraction because of drying (green shrinkage)
Total shrinkage = setting shrinkage + green shrinkage
71. A volumetric contraction accompanies the drying
which reduces the size of the mold.
“Green shrinkage” occurs in addition to the setting
shrinkage.
Investments of this type are designed to reduce the
layer of silica gel around the particles and can be
heated between 1090°C to 1180°C.
72. ADVANTAGES:-
The investments have the ability to cast high temp.
cobalt- chromium & nickel- chromium alloys with good
surface finish, low distortion & high thermal
expansion.
They are less dense than Phosphate bonded
investments & thin sections with fine details can be
reproduced.
The low fired strength makes removal of casting
from investment easier than with Phosphate bonded
investments.
73. DISADVANTAGES:-
Added processing attention & extra precaution
needed in handling the low strength fired molds.
The low strength & high thermal expansion requires
a more precise burnout process & firing schedule to
avoid cracking.
74. DIE STONE INVESTMENT COMBINATION:-
In this the die material & the investing medium have
a comparable composition.
A commercial Gypsum Bonded material called
“Divestment” is mixed with colloidal silica liquid.
The setting expansion of the material is 0.9% &
thermal expansion is 0.6%, when heated to 677ºC.
Not recommended for high fusing alloys, as used for
metal ceramic restorations.
75. HYGROSCOPIC THERMAL GOLD CASTING
INVESTMENT:-
Designed for use with either hygroscopic or thermal
type of casting technique.
Thermal expansion of this investment takes place in
the range 482ºC & 649ºC .
This expansion is high enough to use the investment
with the thermal casting technique without water
immersion. But when it is immersed in a water bath,
then the investment expands hygroscopically.
With the hygroscopic technique the investment only
needs to be heated to 482ºC to provide appropriate
expansion.
76. SOLDERING/BRAZING INVESTMENTS:-
When soldering the clasps on a RPD, the parts must
be surrounded with a suitable investment material
before the heating operation.
The assembled parts are temporarily held together
with sticky wax until they are surrounded with the
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.
77. Types of brazing investments are:-
- Type 1 - Gypsum bonded.
- Type 2 - Phosphate bonded.
The investment for soldering is similar to casting
investment.
They are designed to have lower setting & thermal
expansions than casting investments, a feature that
is desirable so that the assembled parts do not shift
in position during the setting & heating of the
investment.
78. INVESTMENTS FOR ALL-CERAMIC
RESTORATIONS:-
Type I – Used for the cast glass technique composed
of phosphate bonded refractories.
Type II – Refractory die type of material, used for all-
ceramic veneers, inlays & crowns.
Refractory dies are made by pouring the investment
into impressions. When investment is set, the die is
removed & is heated to remove gases that may be
detrimental to the ceramic (degassing).
A refractory die spacer may be added to the
surface.
79. Then, the porcelain or other ceramic powders are
added to the die surface & fired.
Materials must accurately reproduce the impression,
remain undamaged during the porcelain firing & have
a thermal expansion compatible with that of ceramic,
otherwise the ceramic could crack during cooling.
These materials are phosphate bonded & they
generally contain fine grained refractory fillers to
allow accurate reproduction of detail.
80. INVESTMENT OF TITANIUM & TITANIUM
BASED ALLOYS:-
Ti is highly reactive with oxygen & is capable of
reducing some of the oxides commonly found in
phosphate and silica bonded investments.
Ti can also dissolve residual oxygen, nitrogen &
carbon from the investment.
These elements can harden & embrittle Ti in the
solid state.
Hence, modification in the existing refractory
formulation & binder are required.
81. COMPOSITION:-
- These investments can be classified as:-
1) Phosphate bonded
2) Ethyl silicate bonded
3) Cemented
REFRACTORIES:-
- Silica(SiO2)
- Alumina(Al2O3)
- MgO
- Zirconia(ZrO2)
82. PHOSPHATE BONDED TITANIUM
INVESTMENT:-
- To achieve expansion without the use of reactive
powders, a PBI that contains both magnesia & alumina
as refractories was developed.
- This investment can achieve large expansion by the
reaction of alumina & magnesia, when it is burned out
at 1150ºC - 1200ºC .
ETHYL- SILICATE BONDED INVESTMENT:-
- Reactions with the liquid Ti has been reported to be
somewhat less than that of Phosphate bonded
investments due to the use of highly refractory
oxides in the powder.
83. CEMENTED TITANIUM INVESTMENT:-
- This investment use magnesia bonded by an
aluminous cement & contains 5% zirconium powder by
weight.
- The aluminous cement serves as binder for the
magnesia refractory & it sets by mixing with water.
- Oxidation of the zirconium powder to zirconia
during the burnout process provides irreversible
expansion to compensate for the shrinkage of the
casting during cooling from the solidification
temperature.
- The zirconia formed is highly stable & it does not
contaminate Ti. Ti castings from this investment have
smooth surface which are free of contamination.
84. Brush technique:
1. pattern is first painted with surface tension reducer; the
surface must be wet completely.
2. Add investment powder to the liquid in the mixing bowl and
quickly incorporate it by hand.
3. Attach the vacuum hose to the bowl, evacuate the bowl, and
mechanically spatulate.
4. If phosphate bonded investments are used, additional
vibration under vacuum helps minimize nodules.
85. 3. Coat the entire pattern with investment, pushing the material
ahead of the brush from a single point. Gently vibrate through
out the application of investment, being especially careful to
coat the internal surface and the margin of the pattern.
4. After the pattern has been completely coated, the ring is
immediately filled by vibrating the remaining investment out
of the bowl.
86. 4. When the investment reaches the level of the pattern, tilt the
ring several times to cover and uncover the pattern, thereby
minimizing the possible entrapment of air.
5.Investing must be performed quickly within the working time
of the investment. If the investment begins to set too soon,
rinse it off quickly with cold water. The wax pattern can then
be replaced on the die, and its margins can be reflowed
again.
6.After the ring is filled to the rim, allow the investment to set.
7.If the hygroscopic technique is used, the ring is placed in a
37C (100F) water bath for 1 hour
87. VACUUM TECHNIQUE
1. First, hand spatulate the mix
2. With the crucible former and pattern in place attach the ring to the mixing
bowl
3. Attach the vacuum hose and mix according to the manufacturer’s
recommendations
4. Invert the bowl and fill the ring under vibration
5. Remove the vacuum hose before shutting of the mixer
6. Remove the filled ring and crucible former from the bowl
7. Immediately clean the bowl and mixing blade under running water.
88. Safety information
Mixing with other liquids or materials during the use must not
be carried out, since during this process health hazards as
well as impairments to the casting quality cannot be excluded.
All dental investment compounds contain proportions of
crystalline quartz and/or cristobalite. These substances can
adversely affect health if they are breathed in. Always wear a
mask.
If contact is made with the skin, immediately wash off with a
large amount of water. If contact is made with the eyes, wash
them out immediately and consult a doctor.
89. REVIEW OF LITERATURE
Thomas E.M (1952) conducted studies on hygroscopic and setting expansion
of investment and find out that a confined compensating expansion of at least
1.5 % is necessary to compensate for the casting shrinkage of the available
inlay gold alloys. Hygroscopic expansion when taking place at 1000º F will
compensate for the casting shrinkage and shape of the wax pattern has no
influence on the amount of expansion required
90. Delgado et al (1953) studied hygroscopic expansion of investment
andStated that:
1. The use of mechanical spatulation or hand spatulation does not
affect the amount of hygroscopic setting expansion, when a
water bath at mouth temperature is used.
2. Mechanical spatulation gives higher expansion values for thick
mixes than hand spatulation when a water bath at room
temperature is used.
91. Hygroscopic expansion obtained when water bath
is at room temperature rather than mouth
temperature.
Addition of borax to the investment mix decreases
the amount of expansion appreciably depending
on the concentration.
Increased mechanical spatulation increases the
amount of hygroscopic Setting expansion.
92. Lacy et al (1985) stated that machine mixing under vaccum is
more effective than hand mixing in reducing the number of
bubbles from investment.
•They also stated that increasing the mixing time had a little
effect on reducing the air bubbles but decrease liquid powder
ratio favors reduction of incidence of air bubbles.
•Debubblizer is effective in reducing air bubble adhering to the
surface
93. SUMMARY & CONCLUSION:-
Of the three main types of casting investment
materials, the phosphate bonded products are most
widely used.
Silica bonded materials are rarely used as they are
less convenient to use than the other products & the
ethanol produced in the liquid can spontaneously
ignite or explode at elevated temperatures.
The investment which is best able to retain its
integrity at the casting temp. & able to provide the
necessary compensation for casting shrinkage is
chosen.
94. REFERENCES
1. Kenneth J. Anusavice. Phillips Science of Dental materials. 11 th Ed. 2003.
2. Craig’s Restorative Dental Materials 13th Edition.
3. William J. O'Brien Dental Materials and Their Selection - 3rd Ed.
4. DELGADO V.P. PYTOM F.A. - The hygroscopic Setting Expansion of dental
casting Investment. J. Prosthet. Dent 1953; 3-423.
5. Stephen F. Rosenstiel. Contemporary fixed prosthodontics III Ed. 1995.
6. LACY. M. A and MORA. A. incidence of bubbles on sample cast in
phosphate bonded investment. J. Prosthet. Dent 1985; 44, 367-369.
5. THOMAS E.M. - Resume of the expansion required to compensation for
casting gold shrinkage. J. Prosthet. Dent 1952; 550-56.
