Investment materials / dental implant courses by Indian dental academy

INDIAN DENTAL ACADEMY
Leader in continuing Dental Education
www.indiandentalacademy.comwww.indiandentalacademy.com

INTRODUCTIONINTRODUCTION
Following the
production of wax pattern,
the next stage in many
dental procedures involves
the investment of the pattern
to form a mould.
.

A sprue former is attached to the
pattern and assemblage is
located in the casting ring.
Investment material is poured
around the wax pattern whilst
in a fluid stage.

When the investment sets hard
the wax and the sprue former
is removed by softening and/or
burning out to leave a mould
which can be filled with an
alloy or ceramic using a
casting technique.

(GPT-7)
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)
Refractory investment
An investment material that can withstand highwww.indiandentalacademy.comwww.indiandentalacademy.com

(CRAIG)
An investment may be described as a
ceramic material that is suitable for forming
a mold into which a metal or alloy is cast.

Requirements of investmentsRequirements of investments
for alloy casting proceduresfor alloy casting procedures
1.The investment material should be
capable of reproducing the shape, size
and detail recorded in the wax pattern.
2.The investment should be easily
manipulated . Not only should it be
possible to mix and manipulate the
mass readily and it paint the wax
pattern easily, but the investment also
should harden in a relatively short time.www.indiandentalacademy.comwww.indiandentalacademy.com

3.The investment should be able to
maintain the integrity at higher
temperatures (as the casting is carried
out in higher temperatures often as
higher than 1000c)
4.On being heated to higher
temperatures the investment should not
decompose to give off gases that would
damage the surface of the alloy.
5.The investment should have a
sufficiently high value of compressive
strength at the casting temperature so
that it can withstand the stresses set up
when the molten metal enters the mould

6.The investment material should expand
to compensate for the casting shrinkage
7. Casting temperature must not be
critical. Preferably thermal expansion
versus temperature curve should have
a plateau of thermal expansion over a
range of casting temperatures .
8. Investment should be porous enough
to permit the air or other gases in the
mold cavity to escape easily during the
casting www.indiandentalacademy.comwww.indiandentalacademy.com

9. Investment should produce a
smooth surface and fine detail and
margins on the casting.
10. After the casting is complete the
investment should break away
readily from the surface of the
metal and should not have reacted
chemically with it.
11.The investment material should
be inexpensive.

Factors involved in selectionFactors involved in selection
of investment materialof investment material
1. Type of alloy to be cast
2. Casting temperature to be
used.

Types of investment materialsTypes of investment materials
available for casting alloysavailable for casting alloys
• GYPSUM-BONDED INVSTMENTS
• PHOSPHATE BONDED
INVESTMENTS
• SILICA BONDED INVESTMENTS
• NEWER INVESTMENT MATERIALS
FOR TITANIUM AND TITANIUM
BASED ALLOYS

COMPOSITIONCOMPOSITION
Investment materials consist of a mixture of a
1. REFRACTORY MATERIAL
2. BINDER
3. MODIFIERS

REFRACTORY MATERIAL
SILICA (silicon dioxide) is
used as refractory material.
It is available in four allotropic forms such as
• Quartz
• Tridymite
• Cristobalite
• Fused quartz
Quartz and Cristobalite are used
extensively in dental investments.

Quartz is a common mineral . Cristobalite
occurs naturally as a rare mineral but is
normally manufactured by prolonged
heating of the quartz at high
temperatures to induce the appropriate
slow inversion.
Each form of silica exists in two phases.
1. Low temperature phase or alpha phase
2. High temperature phase or Beta phase

High temperature phase is less dense than
that of the Low temperature phase
On heating the change between the two
phases is rapid and readily reversible on
cooling .this change is known as high – low
inversion .
• When heated a change in the crystalline
form occurs at the transition temperature
characteristic of the particular form of silica

Quartz when heated inverts from alpha phase to
beta phase at a temperature of 575 c
Cristobalite when heated inverts from alpha
phase to beta phase at a temperature of 200
to 270 c
The beta allotropic forms are stable above the
transition temperature and an inversion to the
lower or alpha form occurs on cooling. In
powdered form the inversion occurs over a
range of temperature instantaneously.

• The density decreases when the alpha form
changes to beta form with a resulting increase
in the new volume. The increase in the volume
(or isothermal expansion) is probably due to
straightening of the chemical bonds to form a
less dense crystalline structure as illustrated in
the figure
•

• It is exhibited by a rapid increase in the
linear expansion as indicated in the figure.
• The graph shows that the over all thermal
expansion and inversion expansion of
materials containing cristobalite is greater
than that of quartz.

The isothermal expansion for the
Cristobalite is 1.3% at 250c
Quartz is 0.6% at 573c
Depending on type of silica used
the investment materials are
classified as
1. Quartz investments
2. Cristobalite investments

FUNCTIONS
1. It is added to provide refractory during
heating which is capable of
withstanding very high temperatures
during casting without degradation .
2. It regulates thermal expansion.
The wax pattern is eliminated from
the mold by heat. During heating the
Investment expands which is
necessary to compensate partially or
totally for the casting shrinkage of the
alloy .

The expansion is accomplished
by a combination of simple
thermal expansion coupled
with a crystalline inversion
(isothermal expansion) which
results in significant expansion.

It binds the refractory materials together.
The nature of the binder characterizes
the material
Ex:
• Alpha calcium hemi hydrate for
casting gold alloys
• Sodium silicate, ethyl silicate,
ammonium sulphate , sodium
phosphate for casting cobalt
chromium alloys
Binder

MODIFIERS
• Usually a mixture of refractory materials
and binder is not sufficient to produce
all the properties of the investment
materials
• Other chemicals such as sodium
chloride, boric acid, graphite, copper
powder, are often added in small
quantities to modify physical
properties,.

Gypsum bonded investmentsGypsum bonded investments
• They are the mold materials used in
the casting of dental gold alloys with
liquidus temperatures no more than
1080 c
ADA SPECIFICATION NO2 for casting
investments for dental gold alloys
encompasses three types of
investments .

They are
TYPE1 : THERMAL EXPANSION
TYPE employed in casting inlays
and crowns
TYPE 2 : HYGROSCOPIC
EXPANSION type employed in
casting inlays and crowns
TYPE 3: for casting complete and
partial denture bases

GYPSUM BONDED INVESTMENTS
INLAY
INVESTMENTS
DENTURE
INVESTMENT
THERMAL EXPANSION
(ISO Type 1)
HYGROSCOPIC EXPANSION
(ISO Type 2)
RAPID HEAT
SLOW HEAT

CompositionComposition
• REFRACTORY
MATERIAL –
Silica –(60-65%)
Increasing the
proportion of silica
in the investment
powder increases
the Manipulation
time , Initial setting
time ,

• setting expansion both in air and water and
thermal expansion and reduces compressive
strength

The increased manipulation and
setting time and reduced
compressive strength occur
because the particles of the
refractory filler interfere with the
interlocking of growing gypsum
crystals making this less effective
in developing a solid structure.

• Setting expansion is increased when
interlocking of growing gypsum
crystals is inhibited by refractory
particles because the crystal growth
is directed outward
• Thermal expansion is increased due
to summing of
Binder contraction + refractory
expansion

• BINDER – Alpha hemi hydrate form of
gypsum(30-35%)
it is used as binder for investments used
in casting gold containing alloys with
melting ranges below 1000 c
When this material is heated to the
temperature required for complete
dehydration and sufficiently high to
ensure complete castings, it shrinks
considerably and frequently fractures

All form s of gypsum shrink considerably
after dehydration between 200- 400 c
(due to loss of water of crystallization) a
slight expansion occurs between 400c
and approximately 700c, and then a
larger contraction occurs (due to
densification by sintering)
This later shrinkage is most likely due to
deposition and sulphur gases such as
sulphur dioxide are emitted.

They not only cause shrinkage but also
contaminates the castings with the
sulphides of the non noble alloying
metals such as silver and copper.
Thus, it is imperative not to heat the
gypsum products above 700 c for the
gypsum products containing carbon
the maximum temperature should be
650 c.

MODIFIER - (4-7%)
Used are
Reducing agents
Modifying chemicals
Coloring matter
Reducing agents : they reduce any
metal oxides formed on the metal by
providing a non oxidizing atmosphere
in the mold when the mold alloy
enters
• Ex– Copper

Modifying chemicals: They regulate
setting expansion and thermal
expansion and also prevent shrinkage
of gypsum when heated above 300 c .
• They act by reducing the two large
contractions of gypsum binder on
heating to temperatures above 300 c .
• Ex– Boric acid
Soluble salts of alkali or alkaline
earth metals

• Boric acid:
• when heated above 150 c forms a
viscous liquid which impedes
evaporation of last traces of water ,
delaying the gamma to beta
transformation of calcium sulphate.
• This viscous phase also reduces the
high temperature contraction that
results from sintering because it
stabilizes the original contact formed
between gypsum crystals and silica
during setting .www.indiandentalacademy.comwww.indiandentalacademy.com

• Investments containing this boric acid
when heated to 670-700 c shows
increases its compressive strength
ranging from 40-50%.

Salts of alkali and alkaline earth
metals : ex- sodium chloride
• Reduces first major shrinkage and
eliminates second shrinkage of
gypsum on heating
• The effect of halide ion is nullified
above 650 c and rapid contraction
occurs (probably the result of
accelerated sintering )
• A marked strength decreased on
heating to 700 c ranging from -50 to
-85 % www.indiandentalacademy.comwww.indiandentalacademy.com

• The large high temperature shrinkage
of the binder is not observed in
gypsum bonded investments
containing these modifiers because at
a concentration of 50% or more of
silica , the silica particles in the set
investment form a continuous
skeleton that resists over all
shrinkage .

1.Particle size of the powder
• Affects the smoothness of the mold
cavity surface
• Affects the inherent porosity of the mold
• Only the particle size of the refractory
filler is of practical importance as they
remain unchanged in the said investment
• The gypsum crystals formed during
setting of the binder are much smaller
than silica particles
PropertiesProperties

• Refractory with the fine particle size–
smooth mold surface and smooth
casting
• The venting of the mold cavity is
normally provided by porosity
inherent in set material (density)
• So the refractory powder used in the
investment should have uniform fine
particle, size no more than 75 m

2.Manipulation time2.Manipulation time
• Investing the wax pattern must be
completed while the mix is still fluid
• Loss of fluidity is indicated by
disappearance of glossy surface from the
mix.

3. Setting time3. Setting time
• According to ADA sp no 2 the setting time
for dental inlay casting investment should
not be shorter than 5 min and not more
than 25 min.
• The modern inlay investments set initially
in 9 – 18 min .

4.Expansion4.Expansion
• Inlay investments have total expansion in
the range of 1.5 – 2.5% .
• Purpose of setting expansion is to aid in
enlarging the mold to compensate partially
for casting shrinkage.
• Setting expansion of three types
1. normal setting expansion
2. hygroscopic expansion
3. thermal expansion

Normal setting expansionNormal setting expansion
• Mixture of silica and gypsum hemi
hydrate results in greater setting
expansion than that of gypsum
products when it is used alone.
• The silica particles probably
interfere with intermeshing and
inter locking of the crystals as
they form.

• Thus the thrust of crystals is outward
during the growth and they increase
expansion .
• ADA sp no 2 for type 1 investment
permits a maximum setting
expansion in air of 0.6% setting
expansion of modern investments is
0.4%which can be regulated by
accelerators and retarders .

Hygroscopic setting expansionHygroscopic setting expansion
• This is one of the methods for
expanding the casting mold to
compensate for casting shrinkage
• When the gypsum product is allowed
to set under or in contact with water
and the amount of expansion
exhibited is much greater than
normal setting expansion

• The hygroscopic setting expansion may be
6 or more times greater than the normal
setting expansion of a dental investment
• The increased amount of expansion is
because the water helps the outward
growth of crystals
• The investment should be immersed in
water before the initial set is complete.
• ADA sp no 2 for such type 2 investments
require minimum setting expansion in water
of 1.2% and maximum 2.2%.

Factors affecting hygroscopicFactors affecting hygroscopic
setting expansionsetting expansion
1.Effect of composition:
• Finer the particle size of silica greater the
hygroscopic expansion.
• Alpha hemi hydrate produces greater
hygroscopic expansion than beta hemi
hydrate in presence of silica.
• Higher the silica content greater the
expansion and at least 15%of binder is
necessary to prevent drying shrinkage

2.Effect of water powder ratio The
higher the water powder ratio of
original investment water mixture the
less the hygroscopic setting expansion
3.Effect of spatulation:
• The shorter the mixing time the less is
the hygroscopic expansion
4.Effect of shelf life:
• The older the investment the
lower is the hygroscopic
expansion.

5.Effect of time of immersion:
• The immersion before the initial set causes
greater expansion .
6.Effect of confinement:
• The confining effect of the opposing forces such
as the walls of the container in which the
investment is placed or the wall of the wax
pattern is much more pronounced on the
hygroscopic expansion than the normal setting
expansion
• The effective hygroscopic setting expansion is
likely to be less in proportion than in the normal
setting expansion.

7.Effect of amount of added water:
• Magnitude of hygroscopic expansion is
in direct proportion to the amount of
water added during the setting period
until a maximum expansion occurs
• No further expansion is evident
regardless of any amount of water
added
• Once the setting starts the later water
is added to the investment the less is
the hygroscopic setting expansion
because part of crystallization has
already started in normal fashion.

• Some of the crystals have intermesh and
inhibit further crystal growth after the
water is added
• On the same basis the less water that is
added the lower is the expansion

8.Effect of casting ring liner:
• Asbestos ring liner is used for lining the
casting ring
• This liner makes additional water available
to the setting investment and causes an
increased setting expansion.
• Even when the mold sets in the air as in
thermal expansion technique some
hygroscopic setting expansion occurs.

• The investments used in the thermal
expansion technique have relatively high
silica content, so increase in setting
expansion produced by exposure to water
is high .As this high setting expansion is
uncontrollable an isotopic a dry water proof
asbestos ring liner is used
• The combination of a low water powder
ratio and a wet liner considerably increases
the investment setting expansion

The hygroscopic setting expansion is a
continuation of ordinary setting
expansion because the immersion
water replaces water of hydration and
thus prevents the confinement of
growing crystals by surface tension of
the excess water. Because the diluent
effect of the quartz particle, the
hygroscopic setting expansion in
these investments is greater than that
of gypsum binder when used alone

• This phenomenon is purely physical .
• The water is drawn between the
refractory particles by the capillary
action and thus causes the particles to
separate creating an expansion
• The effect is not permanent after the
water is evaporated unless a binder is
present

• The term hygroscopic is an mis inomer
• This hygroscopic setting is as normal a
phenomenon as that which occurs during
normal setting expansion
• The water is drawn into setting material by
capillary action and not by hygroscopy

Thermal expansionThermal expansion
• The thermal expansion is directly related to the
amount and type of silica present.
• Type 1 investments should have thermal
expansion of not less than1% and not greater
than 1.6%.
• The desirable amount of thermal expansion
depends on whether thermal expansion will
compensate the casting shrinkage or it will be
compensated by hygroscopic setting expansion

• If hygroscopic setting expansion is used the
thermal expansion of 0.5- 0.6% is sufficient .
• If only a thermal expansion is used with normal
setting expansion then it should be 1 – 1.6%.
• The maximum thermal expansion should be
achieved at a temperature not greater than 700
c as the a breakdown of calcium sulphate binder
occurs in presence of carbon ( present as
graphite added to the investment as reducing
agent or a residue from the burn out the wax
pattern) liberating sulphur dioxide .

• The sulphur dioxide formed causes
sulphide alloy formation and gold alloy
casting resulting in discoloration and
embrittlement of the alloy
• Quartz has a low thermal expansion than
cristobalite hence additives are added to
investments containing quartz .

Factors affecting the thermal expansion :
1.Effect of water powder ratio:
• More the amount of the water used for mixing less
is the thermal expansion
2.Effect of chemical modifiers:
• The addition of small amounts of sodium
potassium or lithium chlorides to the investments
eliminates the contraction caused by gypsum and
increases the expansion without the presence of
excessive amounts of silica . Silcas donot
prevent gypsum shrinkage but counter balance it
where as chlorides reduce gypsum shrinkage

5.Strength5.Strength
• According to ADA sp no 2 the
compressive strength should not be less
than 2.5MPA
• Alpha hemihydrate increases the
compressive strength
• Chemical modifiers increase the strength
• More water during mixing less is the
strength

• Heating the investments above to 700 c
increase or decrease strength as much as
65% depending on composition
• Greatest reduction in strength is found
upon heating in investments containing
sodium chloride
• As the investment sets to room
temperature strength decreases
considerably because of fine cracks that
formed during cooling

6.Fineness6.Fineness
Fineness affects
• Setting time
• Surface roughness of the casting
• Hygroscopic expansion
Finer silica is preferrable

7.Porosity7.Porosity
• During the casting process, the molten
metal is forced into the mold under
pressure . As the molten metal enters the
air must be forced out ahead of it. If the air
is not completely eliminated a back
pressure builds up to prevent the gold alloy
from completely filling the mold.
• Common method for venting the mold is
through the pores of the investment.

• More gypsum crystals ->less is the
porosity
• Lower the hemi hydrate -> greater the
amount of water used to mix the
investment ->more the porous is the
investment
• Uniform particle size ->greater is its
porosity

StorageStorage
• Should be stored in air tight and moisture
proof containers
• Should be purchased in small quantities
• as the investment materials are
composed of different ingredients each of
which posses a different specific gravity ,
these components settle , under a normal
vibration that occurs in dental laboratory.

• This separation influences on the setting
time and other properties of the investment
• For this reason and as well as to avoid
accidental moisture contamination the
investment should be purchased in small
quantities

DivestmentDivestment
• It is a gypsum bonded material mixed with
colloidal silica
• Setting expansion is 0.9%
• Thermal expansion is 0.6% when it is heated to
677 c
• As it is a gypsum bonded material it is not
recommended for high fusing alloys.
• Divestment phosphate is a phosphate bonded
investment used as a divestment for fusing
alloys.

Rapid heat investmentsRapid heat investments
• Investments based on a cristobalite
refractory require slow heating while the
alpha to beta inversion is occurring some
rapid heat investments have been
introduced which are placed immediately
after setting into a furnace pre heated to
700 c .

• Cristobalite a form of silica is used as
refractory material.
Technique :
• Place the mold 30 min after the pattern is
invested into the preheated furnace for a n
additional 30 min, the casting is then
made .

Setting and thermal expansionSetting and thermal expansion
• Setting expansion measured under
ordinary conditions is still occurring rapidly
at 30 min it is not complete until 2 hours
after mixing and measures 1%.
• The rapid rate of expansion at 30 min
means the precise timing of placement of
the mold in the furnace is critical if
reproducible mold expansion is to occur

• This drastic heating program could be
expected to cause severe thermal cracking
in an ordinary cristobalite investment.
• Measurements on a mold in a lined inlay
ring showed that the periphery of the
investment mass reached 250 c within 6
min of entering the hot furnace while the
centre was at only 110 c until 10 min.
• Both periphery and centre reached a
maximum of 690 c within 30 min heating
period.

• The expansion
caused by the
inversion of
cristobalite shown on
the graph beginning
at 110 c and finishing
at 170 c took place
then enough of the
outer parts of the
specimen reached
250 c to produce a
volume change.

AdvantagesAdvantages
• They save the laboratory time as the
furnace is maintained at 700 c instead of
being repeatedly heated and cooled
• The investment total expansion under
these conditions was 1.95% , more than
enough to compensate casting shrinkage
of ordinary dental alloys

Gypsum bonded investments
cannot withstand temperatures greater than
700c
A large contraction occurs when gypsum
bonded investments are heated above
700c.
The later shrinkage is due to decomposition
by interaction of silica with calcium sulphate
to liberate sulphur trioxide gas.
CaSO4+SiO2 -> CaSiO3+SO3

Another reaction which may take place is on
heating is that of between calcium sulphate and
carbon (May be derived from the residue left after
burning out wax pattern or may be present as
graphite in the investment):
CaSO4+4C -> CaS+4CO
further reaction can occur liberating
sulphurdioxide
3CaSO4+CaS -> 4CaO +4SO2
This decomposition not only causes shrinkage but
also contaminates the casting with the sulphides
of the non noble alloying elements .
So gypsum should not be heated above 700c

Most palladium and base metal alloys used
for partial dentures and porcelain fused to
metal restorations have high melting
temperatures. They should be cast

As suggested by skinner (1963)
“ the definitive advantage of this type of investment is
that there is less chance for the contamination of
the gold alloy during casting…. So far as is known
at present such contamination is avoided with
phosphate bonded investments . On this basis , I
am inclined to predict that the dental investment of
the future may be phosphate bonded not gypsum
bonded”.
As predicted the phosphate bonded investments
are widely used.
PhosphatePhosphate bondedbonded
investmentsinvestments

APPLICATIONS
• They are used in construction of high
melting temperature dental alloys .
• Soldering and porcelain veneering
• To make soldering fixtures that hold
prosthetic components in alignment while
they are being joined with solders brazing
alloys or welding alloys

ClassificationClassification
Type 1
• For casting of inlays crowns and other
restorations especially for alloys like gold,
platinum ,palladium cobalt chromium and
nickel chromium
Type 2
• For casting of removable partial dentures

Refractory materials – (concentration of
approximately 80%)silica in quartz ,
cristobalite or a mixture of two .
Purpose
• To provide high temperature thermal
shock resistance
• High thermal expansion
• To control thermal stresses related to
thermal phase transformation of cristobalite
and along with glasses and other metal
oxides to provide bulk and help to control
the surface finishing of castingwww.indiandentalacademy.comwww.indiandentalacademy.com

Binder (<20%)
• Magnesium oxide (acid) and a
phosphate (base)
• Originally phosphoric acid was used but
mono ammonium phosphate has
replaced it (as it can be incorporated in
powder form
• Mono ammonium phosphate which in
reaction with water in the presence of
calcined magnesium oxide powder
provides for binding of particles at
ambient temperatures

Modifiers
• Carbon is often added .
• It produces clean casting.
• Facilitates easy divesting of casting and mold .
• Generally added when casting alloy is gold.
• 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.

• The basic binding reactions is the same
for all phosphate bonded investments,
there are important differences in
properties due to composition.
Those used for:
• Casting of high temperature alloys and
• Making dies used in fabrication of
porcelain veneers
Contain quartz and cristobalite to achieve
expansion to compensate shrinkage

Soldering investments do not require fine
powders and are designed without high
expansion fillers
It is to keep parts that are to be joined from
shifting while they and the surrounding
investment is heated to the joining
temperature
Graphite is found in some of the investments
to render them more permeable after burn
out to provide a reducing atmosphere.

• It is available as two component systems
• 1- It is a Powder which contains refractory
materials and binders and modifiers
• 2- Aqueous solution stabilized with colloidal
silica
Because the newer gold containing
alloys and other alloys used for metal ceramic
restorations have higher melting temperatures
their contraction during solidification is also
greater . Colloidal silica suspension facilitate
greater expansion of the investment which can
compensate the casting shrinkage.

Setting reactionsSetting reactions

Setting and thermal expansionSetting and thermal expansion
• In practice there is a
slight expansion and
this can be increased
by using colloidal
silica solution instead
of water .

• When phosphate investments were
mixed with water they exhibited a
shrinkage with in a range of 200 -400 c
• This contraction is eliminated when
colloidal silica solution replaces water
• The early thermal shrinkage of
phosphate investments is associated with
a of binder magnesium ammonium
phosphate and by evolution of ammonia .

• Expansion can be varied by the
proportions of silica and water.
1.More silica and less water – more
expansion .
2.Less silica and more water- less
expansion.
Liquid can be used as full strength or
diluted with water to provide some
degree of control over setting or
thermal expansion.

• High temperature mold is achieved by
formation of complex
silicophosphates( from the reaction of
some of the silica with the excess of
dihydrogen phosphate)

• Casting investments
1 Compressive strength
• Type 1- 2.5 mpa
• Type 2- 3 mpa
• Setting expansion within 15% of manufacture
stated value
• Full strength liquid .4%
2 Thermal expansion within 15% of manufacture
stated value
• 0.8% when 50:50 mixture of liquid and water
3 Modulus of rupture- .1to .5 mpa

• Refractory die stones
1. Compressive strength 13 Mpa
2.Setting expansion within 30%of
manufacture stated value
3.Thermal expansion within 15% of
manufacture stated value

Working and setting timeWorking and setting time
Affected by
1.Temperature
Warmer the mix faster it sets
The setting reaction liberate the heat and
accelerates rate of setting
2.Mixing time
increased mixing time and mixing
efficiency result in faster set and
greater rise in temperature

• The more the efficient the the better the
casting in smoothness and accuracy
• Mechanical mixing under vaccum is
preferred
3.Liquid:powder ratio
• Increase in the liquid:powder ratio
increases the setting time.

1.They have high fired strength. This make
them handle without breaking before they
are placed in a furnace for the wax burn out
process and strong enough to with stand
the impact and the pressure of centrifugally
cast molten alloy
2.They also provide high setting and thermal
expansion enough to compensate cast
metal prosthesis or porcelain veneers
during cooling

3.They have ability to with stand the
burn out process with temperatures
that reach 900 c and also 1000 c for
short period of time (for fabricating
porcelain veneers or performing
metal joining operations

DisadvantagesDisadvantages
1. When used with higher melting
alloys those with casting
temperatures higher than 1375 c
they result in mold breakdown and
rougher surfaces on casting
2. Their higher strength although an
advantage make divesting a difficult
and tedious task

3. When higher expansion is required more
of silica liquid is used with the result that
more dense and less porous mold is
produced this results in incomplete
casting if a release for trapped gases is
not provided
4.When the powder is supplied in bulk form
rather than in sealed pre measured
packages it can react over time with
moisture in air and result in lower
expansion during setting or loss of ability
to set to a strong mass

ETHYL SILICATE BONDEDETHYL SILICATE BONDED
INVESTMENTSINVESTMENTS
APPLICATIONS
They are used in construction of
high fusing base metal partial
denture alloys

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

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 the calcined
magnesium oxide and some other
refractory oxides in minor
amounts
• Liquid contains stabilized alcohol
solution of silica gel

If supplied as 2 liquids
• One is ethyl silicate
Certain types of amines are
added for hydrolysis and
gelation to occur
simultaneously
• Other is acidified solution of
denatured alcohol

SETTING REACTION
When binder silica gel is used
• Silicic aid gel is formed when sodium
silicate is added to a acid or acid salt
• MgO added strengthen the gel
When ethyl silicate is used as a binder
• Colloidal silicic is first formed by
hydrolyzing ethyl silicate in presence
of Hcl, ethyl alcohol, and water.

• Si(OC2H5)4 + 4H2O _
Si(OH)4+4C2H5OH
• This silicic acid (sol) is mixed with
the silica
• to which MgO is added to render the
mixture alkaline
• A coherent gel of polysilicic acids is
formed
• The soft gel is dried at a temperature
below 168 c www.indiandentalacademy.comwww.indiandentalacademy.com

• During the drying process the gel
loses alcohol and water to form a
concentrated ,hard gel .
• The volumetric contraction
accompanies drying which reduces
the size of the mold .
• This contraction is known as “GREEN
SHRINKAGE” occurs in addition to
setting shrinkage

So the mold enlargement with
this type of investment must
compensate
• Casting shrinkage
• Setting shrinkage and
• Green shrinkage

ManipulationManipulation
These investments have a special
particle size gradation and are
handled in a different manner
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 .
The top of the mold is prone to
cracking due to greater drying
shrinkage from evaporation of the
ethyl alcohol. www.indiandentalacademy.comwww.indiandentalacademy.com

The cracks must be removed before
the firing process
Otherwise ,when the mold is heated
to burn out a pattern and achieve
thermally induced expansion the
cracks will grow and result in faulty
casting.
To overcome this problem a sufficient
header of the investment is provided
to allow for the removal of the
cracked portion by grinding.(or)www.indiandentalacademy.comwww.indiandentalacademy.com

In about 30 minutes the accelerator in the powder
hardens the settled part, and the excess is
poured off (to avoid crack formation).
The liquid powder ratio in settled part is greatly
reduced and the setting shrinkage is reduced to
0.1%
Thus distortion is minimized and these
investments are well suited for producing
large ,precise castings.
The expansion of the investment is all due to
thermal expansion.

1.Compressive strength -1.5Mpa
2.Thermal expansion( linear) –
Within 15% of manufacturers value.
About 1.5% to 1.8% can be attained
between room temperature and 1000 c to
1177 c

• High temperature cobalt chromium and
nickel chromium alloys can be casted
• Good surface finish is obtained
• Low distortion
• High thermal expansion
• Thin sections with fine detail can be
reproduced (as they are less dense)
• Divesture is easier as they have low fired
strength

DisadvantagesDisadvantages
• Extra precaution needed in
handling the low strength fired
molds
• Low strength and high thermal
expansion require a more precise
burn out process (flammable
alcohol is released) and firing
schedule to avoid cracking and
hence destruction of mold.www.indiandentalacademy.comwww.indiandentalacademy.com

Newer investments for castingNewer investments for casting
titanium based alloystitanium based alloys
• Newer investments have been aimed at the
casting of titanium or titanium based alloys .
• Conventional phosphate bonded or ethyl
silicate bonded investments are deficient
for this purpose.
• Molten titanium is highly reactive with the
oxygen and is capable of reducing some of
the oxides commonly found in the
investment.

• Titanium can also dissolve residual
oxygen, nitrogen, and carbon from the
investments.
• These elements can also harden and
embrittle titanium in the solid state
• As a result a modification of the existing
refractory formulations and binders or new
refractory formulations and binder
systems are required.

• According to the source of binder they
can be classified as
1. Phosphate bonded
2. silicate bonded
3. Cemented
Refractories that can be used are
1. Silica
2. Alumina
3. Magnesia
4. Zirconia

• Standards do not exist for investments for
the casting of titanium and its alloys so
mechanical properties for the conventional
phosphate and ethyl silicate based binders
would apply here as well

Objectives for a titaniumObjectives for a titanium
investment should beinvestment should be
1. To reduce breakdown of the
investment
2. To reduce contamination of
thetitanium.

To reduce breakdown of the investment
• Reducing the reaction with investment is
to employ molds that have been expanded
by burn out process and then cooled back
to near ambient temperature prior to
casting process
• This reduces the time that the alloy is in
contact with mold at elevated
temperatures and over all reactivity is
reduced

• Lowering of the mold temperature
requires that either non reversible
expanders such as metals that expand by
oxidation at elevated temperatures be
used or that the temperature of the mold
be kept just above the temperature where
a reversal of expansion due to crystalline
phase changes take place

• To avoid contamination of titanium by
oxygen through the reduction of refractory
oxides of the investment ,refractory
materials that are less easily reduced by
titanium should be used

• The GIBBS free energy of formation per
mole of oxygen
• Titanium oxide at 1727 c is -716 kJ/mol of
oxygen
• Titanium dioxide at 1727 c is -580 kJ/mol
of oxygen
• Cristobalite and quartz are -550 and
-549kJ/mol respectively
• From this it is clear that titanium may be
expected to be oxidized by silicon dioxide
which is reduced

• Some modifications of phosphate
bonded investments have been
explored for the purpose of rendering
them more compatible with molten
titanium alloys
• One investment consisting of
phosphate binder ,magnesia and
quartz was developed under the
hypothesis that quartz would not be as
reactive as silica

• This investment was
recommended for use of room
temperature mold to reduce
reaction with titanium but the
contamination of castings by
reaction with the investment was
still encountered

• To make the use of setting expansion of
phosphate binder , alumina and magnesia,
both of which are good heat resistance,
can be used as refractories: however the
thermal expansion is low .
• If either is of the powders are mixed with
silica to raise expansion some
contamination with silica again becomes
as a problem

• To achieve expansion with out the use
reactive powders a phosphate investments
that contains both magnesia and alumina
as refractories was developed .
• This investments can attain large
expansion by the spinel reaction of alumina
and magnesia.

• Reaction of ethyl silicate bonded
investments with liquid titanium have been
reported to be some what less than that of
phosphate bonded investments this is
most likely due to use of highly refractory
oxides in the powder. Regardless these
investments require a more complex
procedure for their use

• A more recent development is an
investment using magnesia bonded by an
aluminous cement which contains a mass
fraction of 5% zirconium powder.
• The aluminous cement serves as a binder
for the magnesia as a refractory .
• It sets by mixing with water.

• Oxidation of the zirconium powder to
zirconia during the burn out process
provides irreversible expansion to
compensate for shrinkage of the casting
during cooling from the solidification
temperature.
• The zirconia formed is highly stable as it
has an FEFof -728kJ /mol of oxygen and it
should not contaminate titanium
• Titanium casting from this investments
were reported to have smooth surfaces
free of contamination from mold reaction

SUMMARYSUMMARY
Of the various types of investments
described the most commonly used
investment used is phosphate bonded
investment material. The increase in the
use of the higher melting alloys resulted in
the increase in the use of the phosphate
bonded investments.

Gypsum bonded investments cannot
withstand temperature higher than 700c
and it can be used only with conventional
gold alloys .
The processing attention and extra care
needed in burn out procedures limited the
use of silicate bonded investments
Newer investments are developed for
titanium based alloys . Titanium is highly
reactive with the oxygen and is capable of
reducing some of the oxides commonly
found in the investment.

Modification of the existing refractory
formulations and binders is done or new
refractory formulations and binder systems
are developed
The applications of the investments available
are summarized below in the table:

GYPSUM BONDED
INVESTMENTS
(700c)
PHOSPHATE
BONDED
INVESTMENTS
(900-1000C)
SILICA BONDED
INVESTMENTS
(1090 -1180c)
NEWER
INVESTMENTS FOR
TITANIUM
Mold for gold casting alloys
Mold for base metal and gold
casting alloys ; mold for cast
ceramics and glasses
Refractory die for ceramic
build up
Mould for base metal casting
alloys
Mould for titanium containing
alloys

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