The Indian Dental Academy is the Leader in continuing dental education , training dentists in all aspects of dentistry and
offering a wide range of dental certified courses in different formats.for more details please visit
INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
Properties of an ideal investment material
Gypsum bonded investments
- Setting time
- Normal setting expansion
- Hygroscopic setting expansion
- Thermal expansion
- Control of expansion
- Thermal contraction
- Other gypsum investment considerations
Phosphate bonded investments
- Setting reaction
- Setting & thermal expansion
- Working & Setting time
- Advantages & disadvantages
Ethyl silicate bonded investments
- Advantages & disadvantages
Lost wax technique
Ringless casting system
Die stone investment combination
Casting ring liner
Hygroscopic thermal gold casting investments
Investments for all- ceramic restorations
Investment of titanium & titanium based alloys
Review of Literature
Summary & Conclusion
The adoption of casting practice in dentistry for
making gold inlays, crowns, bridges & other
restorations represents one of the major advances in
In recent years, base metal alloys have been cast
Crowns, bridges & removable partial denture
All such casting operations involve :-
1) A wax pattern of the object to be reproduced.
2) A suitable mold material known as “investment,”
which is placed around the pattern & permitted to
3) Suitable furnaces for burning out the wax patterns
& heating the investment mold.
EVOLUTIONIt is remarkable that the art of lost wax casting was so widely
known in ancient times. It is not an easy process & calls for
considerable skills in its execution.
The Aztec gold-smiths of pre-Colombian Mexico used lost wax
process to create much of their elaborate jewellery.
In the city of Benin,now a part of Nigeria, the brass smiths
continue to produce lost wax castings using a method passed
down through the ages from one generation to the next.
The brass cutters begin with a core of clay kneaded into a
mass. They shape the clay into the approximate size & shape
of the article to be made.
These cores are then dried in sun for several days.
The brass smith creates a pattern for the casting by covering
one of these cores with beeswax.
After finishing the wax, it is covered in a thick coating of clay.
layer is applied as a very fine slip. Before the pattern is
fully sealed in the coating, a thin roll of wax is added to form a
channel into which the molten metal will be poured.
The thicker layer of clay is added for investing the form
completely, creating a mold.
This tech. was largely ignored by modern industry untill the
dawn of the 20th
century when it was “rediscovered” by the
dental profession for producing crowns & inlays.
The true significance of the use of investment casting in
dentistry was not realised untill the research of Dr. William
Taggart of Chicago was published in 1907.
He described a tech., formulated a wax pattern compound of
excellent properties, developed an investment material & also
invented an air pressure casting machine.www.indiandentalacademy.com
In a solid mold technique, a wax sprue was placed in a steel
casing & surrounded by a setting slurry.
Drawbacks were, extremely long pre-heat, size limitations &
poor dimensional tolerances.
In 1538, molds for large statues were made which, if one
desired to make them of bronze, are first made of wax by the
In “ordinary procedure”, the the original model was created in
wax. The image was then coated with the milky slurry of
plaster, building up successive layers untill a strong shell
enveloped the wax.
Alternatively, the image could be dipped or invested in a
“bucketful” of freshly mixed plaster which set up rock-hard in a
very short time. After melting the wax & casting molten metal
into the void, a perfect duplicate of the original pattern was
INVESTMENT :- It can be defined as a ceramic
material that is suitable for forming a mold into which a
metal or alloy is cast.
The operation of forming a mold is described as
The investment should be capable of reproducing the
shape, size & details recorded in the wax pattern.
The investment mold should be capable of maintaining
its shape & integrity at elevated casting temperatures.
The investment should have a sufficiently high value
of compressive strength at the casting temp. so that it
could withstand the stresses set up when the molten
metal enters the mold.www.indiandentalacademy.com
PROPERTIES REQUIRED FOR AN IDEAL INVESTMENT:
1) Easily Manipulated
2) Sufficient strength at room temperature
3) Stability at higher temperature
4) Sufficient Expansion
5) Beneficial casting temperature
7) Smooth surface
8) Ease of Divestment
In general, an investment is a mixture of 3 distinct
types of materials:-
1) Refractory Material:- This material is usually a
form of silicon dioxide such as:-
- Cristobalite or a mixture of these.
2) 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 & other similar materials
also serve as binders for high- temperature casting
3) Other Chemicals:- Chemicals such as NaCl, boric
acid, potassium sulfate, graphite, copper powder or
MgO are often added in small quantities to modify
various physical properties.
In general, there are 3 types of investments materials
1) GYPSUM BONDED INVESTMENTS :-
For conventional casting of gold alloy inlays,
onlays, crowns & FPD’s.
2) PHOSPHATE BONDED INVESTMENTS:-
For metal ceramic restorations, for pressable
ceramics & for base metal alloys.
3) ETHYL- SILICATE BONDED INVESTMENTS:-
For casting of removable partial dentures with
base metal alloy.
1) GYPSUM BONDED INVESTMENTS
GYPSUM:- is a mineral that is mined in
various parts of the world. Chemically the
gypsum that is produced for dental purposes
is nearly pure calcium sulfate dihydrate.
The gypsum bonded investments are classified by
the ISO(1990) as:-
TYPE 1:- Thermal expansion type.
For casting inlays & crowns.
TYPE 2:- Hygroscopic expansion type.
For casting inlays & crowns.
TYPE 3:- For casting complete & partial denture
bases with gold alloys.
Essential ingredients of the dental inlay investment
employed with the conventional gold casting alloys are
α- hemihydrate, allotropic forms of silica & chemical
1) GYPSUM:- α-hemihydrate acts as a binder to hold
the ingredients together & to provide rigidity.
- Most investments contain α-hemihydrate because
greater strength is obtained which depends on the
amount of binder present.
- 25% to 45% of α-hemihydrate is present.
- Used in casting gold alloys with melting ranges below
1000ºC. Above 1000ºC greater shrinkage & frequent
fractures takes place.www.indiandentalacademy.com
- All forms of gypsum shrink after dehydration between
200ºC to 400ºC .
- Slight expansion takes place between 400ºC & 700ºC.
- A large contraction then occurs
.- Second shrinkage is most likely caused by:-
The release of sulphur gases, such as SiO2.
- This decomposition causes shrinkage & also
contaminates the castings with the sulphides of non-
noble alloying elements, such as silver & copper.
- Therefore, it is imperative that gypsum investments
should not be heated above 700ºC.
- For gypsum products containing carbon, the max. temp.
should be 650ºC to obtain proper fit & uncontaminated
2) SILICA:- Present 55% to 75% & 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.
- Silica exists in 4 allotropic forms:-
- Quartz - Cristobalite
- Tridymite - Fused quartz
- When quartz, cristobalite or tridymite is heated, a
change in crystalline form occurs at a transition temp.
characteristic of the particular form of silica.
- eg. When quartz is heated, it inverts from α-quartz to
β-quartz, at a temp. of 575ºC(1067ºF).
- Cristobalite undergoes an anologous transition
between 200ºC & 270ºC from to β form.
- 2 inversions of Tridymite occurs at 117ºC & 163ºC
- Density decreases as α form changes to β form, with
a resulting increase in volume exhibited by a rapid
increase in linear expansion.
- Fused quartz is an amorphous & glasslike in
character, & it exhibits no inversion at any temp.
below its fusion point. It has low coefficient of thermal
expansion & is of little use in dental investments.
- Quartz, cristobalite or a combination of the 2 forms
may be used in a dental investment.
- On the basis of the type of silica employed,dental
investments are classified as:-
EFFECT OF VARYING
- Increasing the proportion of silica in the investment
powder increases the manipulation time, initial
setting time, setting expansion both in air & water &
thermal expansion & decreases the compressive
- The rate of setting reaction is unchanged.
- 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
The compressive strength of the investment iswww.indiandentalacademy.com
- The setting expansion is increased when interlocking
of the growing gypsum crystals is inhibited by the
refractory particles, because more of the crystal
growth is directed outwards.
- The thermal expansion is increased, since
increasing the proportion of the expanding
component increases the observed expansion.
3) MODIFIERS:- Consists of:-
a) Modifying Agents:- They regulate the setting
expansion & setting time & also prevent most of the
shrinkage of gypsum, when it is heated above
300ºC, eg. Boric acid, NaCl.
b) Coloring agents
c) 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.
EFFECTS OF MODIFYING
In 1982 & 1986 studies were done by Mori T. on the
thermal behaviour of gypsum binder in dental
casting investments & effect of boric acid on the
thermal behaviour of this binder was found & he said
that – All of these act mainly by reducing 2 large
contractions of gypsum binder that occur on heating
to temp. above 300ºC.
- Boric Acid:- When heated above 150ºC, it forms a
viscous liquid. This viscous liquid retards the
evaporation of water i.e. delay α β transformation
of CaSO4. The presence of the viscous liquid phase,
also reduces the high temp. contraction that resultswww.indiandentalacademy.com
- The presence of modifiers added to increase the
thermal expansion also affects the strength changes
of the investment that occur on heating, because of
their effect on the CaSO4 binder.
Ohno et. al. has done a study in 1982 on the effect
of phase transformations of silicas & CaSO4 on the
compressive strength of gypsum bonded
investments at high temp. & they said that:-
1) On heating GBI without these additives, show a
rapid increase in compressive strength of about
100% in the range of 100ºC to 175ºC on drying.
2) Between 175ºC - 225ºC there is decrease in
compressive strength, because of the dehydration
3) Relatively minor strength fluctuations occur during
subsequent heating to higher temp. because of:-
-Further phase changes in the binder.
- α β inversion of the refractory.
- Sintering of the binders.
- These investments are heated to temp. in the range
of 670ºC- 700ºC & show compressive strength
changes ranging from +10% to -40%.
- Investments containing boric acid, when heated to
the same temp. range show increase in
compressive strength ranging from +40% to +50%.
Setting time – measured & controlled in the same
manner as plaster.
According to ADA/ANSI specification no. 2 for dental
inlay casting investment, the setting time should not
be < 5 min. or >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.
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.
PURPOSE:- To aid in enlarging of the mold, to
compensate partially for the casting shrinkage of the
Regardless of the type of gypsum product used, an
expansion of the mass can be detected during the
change from hemihydrate to dihydrate.
A mixture of silica & gypsum hemihydrate results in
setting expansion greater than that of the gypsum
product when it is used alone.www.indiandentalacademy.com
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.
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
If a wax other than the Type 2 inlay wax is used, the
setting expansion may cause a serious distortion of
The lower the W:P ratio for the investment, the
greater is the effective setting expansion.
HYGROSCOPIC SETTING EXPANSION
The hygroscopic setting expansion was 1st
in connection with an investigation of dimensional
changes of a dental investment during setting.
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
ADA Specification no. 2 for Type 2 investments
requires a mini. setting expansion in water of 1.2% &
maxi. expansion permitted is 2.2%.
PURPOSE:- To expand the casting mold to
compensate for the casting shrinkage of the gold
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.
The HSE may be 6 or more times the NSE of a
FACTORS CONTROLLING THE HYGROSCOPIC
1) Effect of Composition:-
- HSE α silica content.
- Finer silica particles HSE.
- α- hemihydrate(in presence of silica) HSE.
2) Effect of W:P ratio:- W:P ratio eα 1/HSE.
3) Effect of Spatulation:- Mixing time α HSE.
4) Shelf life of investment:-
5) Effect of time of Immersion:-
6) Effect of Confinement:-
7) Effect of the amount of added water:-
HSE α amount of water added
8) Water bath Temperature:-
9) Effect of Particle size of Silica:- Particle size affects
the smoothness of the mold cavity surface & the
inherent porosity of the mold.
Finer particles of silica ensures smooth mold surface
& smooth casting.
THERMAL EXPANSIONDEFINITION:- 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.
A considerable amount of quartz is necessary to
counterbalance the contraction of the gypsum during
The thermal expansion of quartz investment are
- The particle size of the quartz.
- Type of gypsum binder
- W:P ratio www.indiandentalacademy.com
Since greater expansion occurs during the inversion
of the crystobalite, the normal contraction of the
gypsum during heating is easily eliminated.
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%.
For 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 .
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.
2) Effect of Chemical Modifiers:-
- A disadvantage of an investment that contains
sufficient silica to prevent any contraction during
heating 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 – similar effect. It hardens set investment,
but, it disintegrates during the heating of the
investment & a roughened surface of the casting may
- Silicas do not prevent gypsum shrinkage but
counterbalance it, whereas chlorides reduce gypsum
shrinkage below temp. of 700ºC.
CONTROL OF EXPANSION
1) Composition:- By the choice of refractory & binder
& by the use of certain additives.
2) W:P ratio:- W:P ratio Setting expansion &
3) Period of exposure to water:- In hygroscopic
expansion techniques, additional control can be
obtained by varying the length of time the setting
investment is exposed to an aqueous environment.
THERMAL CONTRACTION/ COOLING OF THE
When an investment is cooled from 700ºC, its
contraction curve follows the expansion curve during
the inversion of the β-quartz or β-cristobalite to its
stable α form at room temp.
The investment contracts to less than its original
dimension, because of the shrinkage of gypsum when
it is first heated.
If the investment is reheated, it expands thermally to
the same, maxi. reached when it is 1st
In practice, the investment should not be heated a
second time, since internal cracks may develop which
affect the quality of the casting.www.indiandentalacademy.com
STRENGTHThe strength of an investment increases rapidly as the
material hardens after initial setting time. The free
water content of the set product affects its strength.
2 strengths of a gypsum product are:-
1) Wet Strength
2) Dry Strength
The dry strength is 2 or more times the wet strength.
The strength of an investment must be adequate to
prevent fracture or chipping of the mold during heating
& casting of the gold alloy.
The strength of an investment is measured in terms of
its compressive strength which should not be too high.
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 2.4MPa when tested 2hrs after
FACTORS AFFECTING STRENGTH
1) W:P ratio:-
W:P ratio Porosity CS & TS.
2) Heating the investment to 700ºC may / the
strength as much as 65%, depending on the
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.
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.
2) PHOSPHATE BONDED INVESTMENTS
The rapid growth of the use of metal ceramic
restorations & use of cast removable partial dentures
& the increased use of higher melting alloys have
resulted in an increased use of phosphate bonded
Phosphate bonded investments are of 2 types:-
1) Type 1- For crowns, inlays & other fixed restorations.
2) Type 2- For partial denture & other cast removable
Consists of :-
1) Filler:- The filler is silica, in the form of
Cristobalite, Quartz, or a mixture of the two and in a
concentration of approx 80%.
- Purpose:- To provide high temp. thermal shock
resistance & a high thermal expansion.
2) Binder:- It is less than 20%.
-The binder consists of magnesium oxide and a
phosphate that is acid in nature.
- MgO reacts with the filler at room temp.
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.
- It is often added to the powder to produce clean
castings to facilitate the divesting of the casting
from the mold.
- This is appropriate if the casting alloy is gold.
- The latest evidence indicates that palladium does
react with carbon at temperature above 1504°C.
Thus, if the temperature exceeds this temperature
during casting, a carbon free investment should be
- Magnesium ammonium phosphate formed is
polymeric. Although the stoichiometric quantities are
equal molecules of magnesia and monoammonium
phosphate, an excess of magnesia is usually
present, and some of it is never fully reacted.
- On heating, the binder of the set investment
undergoes thermal reactions.
SETTING & THERMAL EXPANSIONS
- Theoretically, the reaction should entail a shrinkage,
but in practice there is a slight expansion by using a
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 the
case of cristobalite.
WORKING & SETTING TIME
- Phosphate investments are affected by temperature.
- 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 temp.
- Increase in w:p ratio, increases the working time
They have both green strength & fired strength.This
makes them easy to handle without breaking before
they are placed in a furnace for wax burnout & strong
enough afterwards to withstand the impact &
pressure of centrifugally cast molten alloy.
They can provide setting & thermal expansion high
enough to compensate for the thermal contraction of
cast metal prosthesis or porcelain veneers during
They have the ability to withstand the burnout
process with temp. that reach 900ºC. They can
withstand temp. upto 1000ºC for short periods.www.indiandentalacademy.com
When used with higher melting alloys i.e. those with
casting temp. greater than 1375ºC, these
investments are at a disadvantage. These temp.
coupled with high mold temp., result in mold
breakdown & rougher surkace on castings.
The high strength of these investments can make
removal of the casting from the investment difficult.
When higher expansion is required, more of the
silica liquid is used with the result that a more dense
& less porous mold is produced. This can result in
incomplete castings if a release of trapped gases is
3) ETHYL SILICATE-BONDED INVESTMENT
Losing popularity - complicated and time consuming
Consists of powder & liquid.
- Powder contains refractory particles of silica & glass
along with calcined MgO & some other oxides in
- 2 Liquids one is ethyl silicate & the other an acidified
solution of denatured ethyl alcohol.
Binder is a silica gel that reverts to silica on heating.
Several methods may be used to produce the silica
or silicic acid gel binders:-
- When the pH of sodium silicate is lowered by the
addition of an acid or an acid salt, a binding silicic
acid gel forms.
- An aqueous suspension of colloidal silica can be
converted to a gel by the addition of an accelerator,
such as ammonium chloride.
- Another system for binder formation is based on
ethyl silicate. A colloidal silicic acid is formed by
hydrolyzing ethyl silicate in the presence of
hydrochloric acid, ethyl alcohol and water.
Si(OC2H5)4 + 4H2O Si(OH)4 + 4C2H5OHwww.indiandentalacademy.com
A polymerised form of ethyl silicate is used, to form
a colloidal sol of polysilicic acid.
The sol is then mixed with the quartz or cristobalite,
to which is added a small amount of finely powdered
MgO to make the mixture alkaline.
A coherent gel of polysilicic acid then forms,
accompanied by a setting shrinkage. The soft gel is
then dried at temp. below 168ºC.
During drying process the gel loses alcohol & water
to form a concentrated, hard gel.
A volumetric contraction accompanies the drying
which reduces the size of the mold.
This contraction is known as “green shrinkage” and it
occurs in addition to the setting shrinkage.
Investments of this type are designed to reduce the
layer of silica gel around the particles.
This type of investment can be heated between
1090°C to 1180°C and is compatible with the higher
The investments have the ability to cast high temp.
cobalt- chromium & nickel- chromium alloys & good
surface finish, low distortion & high thermal
They are less dense than PBI & thin sections with
fine details can be reproduced.
The low fired strength makes removal of casting
from investment easier than with PBI.
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
LOST WAX TECHNIQUE
Introduced by TAGGART in 1907.
Elimination of the wax pattern from the mold of set
investment material is reffered as “Burnout.”
It should not be started until investment is set for
minimum of 1hr. If preheated, causes cracks in
Ideally it should be kept in oven when mold is still little
wet as water trapped prevent absorption of wax &
when water vaporises, it flushes out wax.
If the burnout procedure is delayed for several hrs., it
should be kept in humidor to avoid excessive drying,
as molten wax during burnout may get absorbed by
If the rubber crusible former is used it is removed
To facilitate a rapid & clean burnout, the ring can be
placed on a raised object within the oven, allowing
free release of molten wax & gases.
RINGLESS CASTING SYSTEM
To provide maxi. Expansion of investment, a
ringless system is available commercially.
The system called “powder cast ringless system,”
consists of 3 sizes of rings & formers, preformed
wax sprues & shapes, investment powder, & a
special investment liquid.
The tapered plastic rings allows for the removal of
the investment mold after the material has set.
This system is suited for the castings of alloys that
require greater mold expansion than traditional gold
DIE STONE INVESTMENT COMBINATION
In this the die material & the investing medium have a
A commercial GB 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.
Since divestment is a GB material, it is not
recommended for high fusing alloys, as used for
metal ceramic restorations.
The PBI for divestment is used in the same manner
as the Divestment material & is suitable for use with
high fusing alloys.
CASTING RING LINERSWith the use of solid metal rings or casting flasks,
provision must be made to permit investment
expansion. The mold may actually become smaller
because of the reverse pressure resulting from the
confinement of the setting expansion.
The most common technique to provide investment
expansion is to line the walls of the ring with a ring
liner. Earlier, asbestos was the material of choice,
but it can no longer be used because its
carcinogenic potential makes it a biohazard.
2 types of non-asbestos ring liner materials are:-
- An aluminium silicate ceramic liner
- A cellulose paper liner.www.indiandentalacademy.com
4) HYGROSCOPIC THERMAL GOLD CASTING
It is designed for use with either hygroscopic or
thermal type of casting technique.
Thermal expansion of this investment takes place in
the range bet. 482ºC & 649ºC .
This expansion is high enough to use the investment
with the thermal casting tech. without water
immersion. But when it is immersed in a water bath,
then the investment expands hygroscopically.
With the hygroscopic tech. the investment only needs
to be heated to 482ºC to provide appropriate
5) SOLDERING/BRAZING INVESTMENTS
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
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.
2 types of brazing investments are:-
- Type 1- Gypsum bonded dental brazing investments
- Type 2- Phosphate bonded dental brazing
The investment for soldering is similar to casting
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.
6) INVESTMENT FOR ALL-CERAMIC RESTORATIONS
2 types of investment materials have been developed
recently for producing all- ceramic restorations:-
- Type 1 – Used for the cast glass technique composed
of phosphate bonded refractories.
- Type 2 – Refractory die type of material, used for all-
ceramic veneers, inlays & crowns.
Refractory dies are made by pouring the investment
into impressions. When the 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.www.indiandentalacademy.com
Then, the porcelain or other ceramic powders are
added to the die surface & fired.
These 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
These materials are also phosphate bonded & they
generally contain fine grained refractory fillers to
allow accurate reproduction of detail.
7) INVESTMENT OF TITANIUM & TITANIUM BASED
In 1993 a study was done by Togoxa T.& Maixazaki T
& Tamaki X. on the selection of investment for
improving fits of Ti castings & they said that the
castings of Ti should not be done with conventional
PBI or SBI, because, Ti is highly reactive with oxygen
& is capable of reducing some of the oxides commonly
found in these investments.
Ti can also dissolve residual oxygen, nitrogen &
carbon from the investment.
These elements can harden & embrittle Ti in the solid
Hence, modification in the existing refractory
formulation & binder or new refractory formulations &
binder systems are required.
- These investments can be classified as:-
1) Phosphate bonded
2) Ethyl silicate bonded
- Zirconia(ZrO2) www.indiandentalacademy.com
- The properties are at most same as conventional PBI or SBI.
- The main objective of the different refractory compositions &
binders is to reduce the breakdown of the investment & the
contamination of Ti.
- One approach of reducing the reaction of Ti with the investment
by employing models that have been expanded by the burnout
process & then cooled back to near ambient temp. prior to
- This reduces the time that the alloy is in contact with the mold
at elevated temp., & the overall reactivity is reduced. In order to
avoid the contamination of Ti by oxygen through the reduction
of refractory oxides of the investment, refractory materials that
are less easily reduced by Ti should be used.
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 of ethyl silicate bonded investments with
the liquid Ti has been reported to be somewhat less
than that of PBI.
- This is due to the use of highly refractory oxides in
the powder. But these investments require more
complex procedure for their use.
CEMENTED TITANIUM INVESTMENT:-
- This investment use magnesia bonded by an
aluminous cement(CaO-Al2O3) & 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 temp.
- The zirconia formed is highly stable & it does not
contaminate Ti. Ti castings from this investment had
smooth surface, free of contamination fron the mold
REVIEW OF LITERATURE
K. Asgars, D.B. Mahler & F.A. Peyton, 1955,
investigated a hygroscopic technique for inlay casting
using controlled water additions. A technique &
equipment for this particular technique was described
by them. They concluded that:-
- The av. Deviation of expansion values resulting from
controlled water additions was significantly less(0.1%)
than that for complete immersion(0.3%).
- The hygroscopic expansion for an av. mix was higher
than for a thinner mix & the expansion reduced with the
no. of spatulation turns within limits.
- The expansion for a new investment was significantly
higher than for an aged one.www.indiandentalacademy.com
A study conducted in our department under the able
guidance of DR. N.P.PATIL Sir, in 1998,
evaluated the influence of cast hardening agents on
surface abrasion, surface hardness & surface detail
reproduction properties of refractory investment
materials. It was concluded that:-
- Surface abrasion resistance improved significantly
using hardening agents.
- However the surface reproducibility was reduced with
both treated & untreated samples when compared
with a steel model.
Junzo Takahashi, Masayuki Okazaki,1999,
conducted a study with the purpose of measuring the
internal setting expansion of PBI & assessed the effect
that different pattern materials may have on internal
setting expansion. They concluded that vertical setting
expansion was higher than the horizontal setting
expansion & regardless of the type of pattern material,
a PBI caused non uniform setting expansion,
especially in horizontal direction, which lead to the
distortion of the pattern.
C.L. Chew, M.F. Land, 1999, conducted a
study with the purpose of evaluating & comparing
the compressive strength characteristics of
phosphate bonded v/s gypsum bonded investments.
They also investigated if these values changed as a
function of time & temp. It was concluded that at
elevated temp., all materials approximated peak
strength 2hrs after initial mixing.There was no
significant difference in their strengths at room temp.
However, the PBI exhibited significantly increased
compressive strength as a function of time & temp.
A study conducted in our department under the able
guidance of DR. N.P.PATIL Sir, in 2001
evaluated the fit of commercially pure titanium cast
copings using 3 different investment materials (Titec,
Rematitan plus, Tycast) . It was concluded that the
percentage thermal expansion was significantly
higher for Tycast resulting in a corresponding
increase in discrepancy in marginal fit & internal
surface adaptation. It was hence recommended that
Titec & Rematitan should be preffered over Tycast.
SUMMARY & CONCLUSION
Of the three main types of casting investment
materials, the phosphate bonded products are
becoming most widely used. Silica bonded materials
are rarely used now a days due to the fact that they
are less convenient to use than the other products &
that the ethanol produced in the liquid can
spontaneously ignite or explode at elevated
The investment which is best able to retain its
integrity at the casting temp. & able to provide the
necessary compensation for casting shrinkage is
1) Anusavice K.J. –“Phillips’ science of dental
2) Asgars K., Mahler D.B. – “Hygroscopic technique for
inlay casting using controlled water additions”
JPD Sept. 1955; 711-724
3) “A comparative evaluation of the fit of commercially
pure titanium cast copings using different investment
materials”. Sept. 2001
4) Chew C.L., Land M.F. – “Investment strength as a
function of time & temperature”
5) “Comparative evaluation of the influence of cast
hardening agents on surface abrasion, surface
hardness & surface detail reproduction properties of
refractory investment material”
6) Craig R.G. & Powers J.M.- “Restorative dental
7) O’Brien W.J.- “Dental materials & their selection”
8) Phillips R.W. – “Skinners science of dental materials”
9) Takahashi J.,Okazaki M. – “Non uniform Vertical &
horizontal setting expansion of a phosphate bonded
investment” JPD 1999;81: 386-391.www.indiandentalacademy.com