Investing: the process of covering or enveloping,
wholly or in part, an object such as denture, tooth,
wax form, crown, etc. with a suitable investment
material before processing, soldering or casting.
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).
1. should be capable of reproducing the shape, size and
detail in the wax pattern.
2. easily manipulated.
3. setting time should be less.
4. should maintain the integrity at higher temperatures and
should not decompose to give off gases.
5. Possess sufficiently high value of compressive strength at the
casting temperature to withstand the stresses set up when the
molten metal enters the mould.
6. should expand to compensate for the casting shrinkage
7. Investment should be porous enough to permit the
air or other gases in the mold cavity to escape easily
during the casting
8. Investment should produce a smooth
surface and fine detail and margins on
9. should be inexpensive.
Wax shrinkage- 0.3-0.4%.
Alloy shrinkage- 1.3- 1.6% ( noble
alloys), 2.0- 2.3%( Predominantly base
Investment materials are basically
1. A refractory material
2. A binder material
3. Additives used to improve the
SILICA (silicon dioxide) is used as refractory material & to
regulate the thermal expansion.
It is available in four allotropic forms such as
Quartz and Cristobalite are used extensively in dental
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.
Low temperature phase or alpha phase
High temperature phase or Beta phase
On heating the change between the two phases is rapid
and readily reversible on cooling .this change is
known as high – low inversion .
Depending on type of silica used the
investment materials are classified as
It binds the refractory materials together.
According to the binder used investment materials are
of three groups.
1. GYPSUM-BONDED INVESTMENTS
2. PHOSPHATE BONDED INVESTMENTS
3. SILICA BONDED INVESTMENTS
Other chemicals such as sodium chloride, boric acid,
graphite, copper powder, are often added in small
quantities to modify physical properties.
boric acid and sodium chloride, not only regulate
the setting expansion and the setting time, but also
prevent most of the shrinkage of gypsum when it is
heated above 300 ºC (572° F).
are the mold materials used in the
casting of dental gold alloys with liquidus
temperatures no more than 1080 ºc.
Used for conventional casting of gold alloys
inlays, onlays, crowns and Fpd.
of silica (quartz or
Silica is added to
provide a refractory
component during the
heating of the investment
and to regulate the
– Alpha hemi hydrate form of
Strength of investment depends on amount of
MODIFIER - (4-7%)
Reducing agents : they reduce any metal oxides formed
on the metal by providing a non oxidizing atmosphere
in the mold when the alloy enters mold.
Modifying chemicals: They regulate setting expansion
and thermal expansion and also prevent shrinkage of
gypsum when heated above 300 ºc .
They act by reducing the two large contractions of
gypsum binder on heating to temperatures above 300
Ex– Boric acid
Soluble salts of alkali or alkaline earth metals
According to ANSI/ADA Specification No. 2 for
dental inlay casting investment, the setting time should
not be shorter than 5 min or longer than 25 min.
Usually, the modern inlay investments set initially in 9
to 18 min.
Should allow sufficient time for mixing and investing
1. normal setting expansion: the expansion takes
place when the material sets in air
2. hygroscopic setting expansion: when setting occurs
3. thermal expansion: when it is affected by heat
Purpose: to enlarge the mold to compensate for the
casting shrinkage of the gold alloy
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 .
It is enlarging the mould containing the wax pattern
and it depends on the thermal expansion of pattern
caused by heat of reaction.
High gypsum content- high expansion.
Other variables – lower w/p ratio, drier mix of
investment- high NSE.
Thin wall pattern- more expansion than thick
Soft wax- more NSE.
Wax softer than type II inlay wax- NSE causes
serious distortion of the pattern.
Greater in magnitude than NSE.
the gypsum product is allowed to set under or in
contact with water for mould expansion.
The hygroscopic setting expansion may be 6 or more times
greater than the normal setting expansion of a dental
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%.
Composition: more finer silica particles -more HSE.
α hemihydrate greater HSE than β hemihydrate.
W:P ratio: less water, more powder in mix.
Spatulation: more mixing time- more HSE.
Time of immersion: immerse in water before initial
Confinement: less opposing force from walls of
casting ring (wet cellulose), immersion in water bath at
37.7°C- expansion of wax pattern.
Water: more immersion water.
Shelf life: fresher investment.
The thermal expansion is directly related to the amount
and type of silica present.
Contraction of gypsum is balanced- quartz 75%
More Expansion in the presence of cristobalite and
initial contraction of gypsum is eliminated.
Type 1 investments should have thermal expansion of
not less than1% and not greater than 1.6%.
Type 1I investments should have thermal expansion of
between 0% to 0.6% at 500°C.
CONTRACTION: When an
investment is cooled from 700°C, it contracts.
On reheating it expand again but not
recommended as investment may crack on
According to ADA sp no 2 the compressive strength
should not be less than 2.5MPA to withstand force of
FINENESS: Surface roughness of the casting and
setting time depend upon the fineness of the material.
Fine silica- more hygroscopic expansion.
POROSITY: The material should be porous to
allow escape of air from mold space while casting.
Lower content of hemihydrate- more water gaugingmore porous.
It is a gypsum bonded material mixed with colloidal
Setting expansion is 0.9%
Thermal expansion is 0.6% when it is heated to 677 ºc
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 high fusing alloys.
Easier to manipulate.
Amount of dimensional
change easier to control.
Thermally unstable at
Powder is hygroscopic.
Improper wax burnout &
high casting force may
produce cracks in
investment leading to
Most palladium and base metal alloys used for partial
dentures and porcelain fused to metal restorations have
high melting temperatures. They should be cast
a mold temperature higher than 700 ºc.
To withstand these high temperatures ,molds require
different types of binders such as phosphate
They are used in construction of high melting
temperature dental alloys
Soldering and porcelain veneering
For casting of inlays crowns and other restorations
especially for alloys like gold, platinum ,palladium
cobalt chromium and nickel chromium
For casting of removable partial dentures
Refractory materials – (concentration of
approximately 80%)silica in quartz , cristobalite or a
mixture of two .
To provide high temperature thermal shock resistance
To provide high thermal expansion.
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
Carbon is often added -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.
Palladium reacts with carbon even at above temp
1500°C, so carbon free phosphate bonded invst used
for higher temp.
It is available as two component
1- It is a Powder which contains
refractory materials and binders
2- Aqueous solution stabilized
with colloidal silica
Colloidal silica suspension
facilitate greater expansion of the
investment which can compensate
the greater casting shrinkage of
alloys used in MC & newer gold
Expansion can be varied by the proportions of silica
and water. More silica and less water – more
When mixed with water- initial shrinkage is seen same
as that of GBI. Completely eliminated with colloidal
Liquid can be used as full strength or diluted with water
to provide some degree of control over setting or
thermal expansion. But if more liquid is used it caused
dense nonporous investment which can effect casting.
Early thermal expansion is associated withdecomposition of binder, magnesium ammonium
phosphate and evolution of ammonia gas.
Some shrinkage is masked by expansion of
refractory filler ( crystobalite)
1 Compressive strength
Type 1- 2.5 mpa
Type 2- 3 mpa
2 Thermal expansion
0.8% when 50:50 mixture of liquid and water
Warmer the mix faster it sets
The setting reaction liberate the heat and
accelerates rate of setting
increased mixing time and mixing efficiency
result in faster set.
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 can withstand temp more than 900°C
These are used for high fusing base metal alloy
Refractory material – Silica
Binder –Silica gel or ethyl silicate
Magnesium oxide (strengthen the gel)
Ammonium chloride - accelerator
Silica gel: pH of sodium silicate lowered by
addition of an acid or acid salt.
Colloidal silica converted to gel by adding
Ethyl silicate: colloidal polysilicic acid is formed by
hydrolizing ethyl silicate in presence of Hcl, ethyl
alcohol & water. It is then mixed with quartz or
cristobalite and magnesium oxide ( alkaline). Coherent
gel of polysilicic acid formed, dried at temp 168°C,
loses alcohol & water to form concentrated hard gel .
Volumeric contraction during drying is green
Amines added to ethyl silicate- hydrolysis and gelation
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
Liquid contains stabilized alcohol solution of silica
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
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
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
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
Can be used for higher temp castings ( 1090°C1180°C) and compactible with higher fusing
Care should be taken while handling and burnout
as inflammable alcohol is given off.
Investment can be mixed in 2 ways
1. Hand mix: The liquid is added to a clean, dry mixing
bowl, and the powder is gradually added to the
liquid, using the care and caution to minimize air
entrapment. Mixing is formed gently until all the
powder has been wet.
2. Vacuum mixing: this is a type of mechanical mixing
done under vacuum created by a vacuum mix
Burnout in furnace
Flow of molten metal