dental investments / academy of fixed orthodontics

INDIAN DENTAL ACADEMY
Leader in continuing dental education
www.indiandentalacademy.com

The principal laboratory technique of making
metal inlays, onlays, crowns and bridges, is
based on casting practice. This application of
casting practice is one of the major advances in
restorative dentistry, which is mainly based on
“Lost wax technique”.

This process of casting involve some basic steps:
1. Preparation of a wax pattern.
2. Preparation of mold - pour the mixed investment
material around the wax pattern and allow it to set.
Burn out: wax is eliminated from the investment by
boiling (or) burning it in oven.
3. Then casting is done by melting the alloy and forcing
the molten metal into the mold cavity.

Investing:
The process of covering, enveloping, wholly (or) in part an
object such as denture, tooth, wax form, crown, etc. with a
suitable material before processing, soldering, or casting.
Dental casting Investment:
A material consisting principally of an allotrope of silica and a
bonding agent. The bonding substance may be gypsum (for use
in lower casting temperature) or phosphates and silica (for use in
higher casting temperatures).
Refractory:
Difficult to fuse/corrode, capable of enduring high temperatures.
Refractory investment:
An investment that can withstand high temperature using a
soldering or casting.

Allotropic phase:
Phases of similar composition but different
crystallographic structures, with different properties.
Casting:
The act of forming an object in a mold.
Something that has been cast in a mold; an object
formed by the solidification of a fluid that has been
poured or injected into a mold.

Easy manipulation.
Good flow.
Detailed reproduction.
Fine particle size to ensure a smooth surface on the casting.
The mixed unset material should have a smooth consistency.
Should have sufficient strength:
Should exhibit sufficient strength at high temperatures.
Inner surface of the mold should not break at a high
temperature.
Should exhibit sufficient strength, to withstand the force of
molten alloy entering the mold.

At higher temperatures:
It should be stable without any decomposition of
investment.
Should show sufficient expansion enough to compensate for
shrinkage of wax pattern and solidification of molten metal.
The material should be sufficiently porous (permeability)
enough to permit escape of air/other gases from the mold
cavity during casting of molten metal.
It should show ease of divestment:
It should not react with metal.
It should easily break away (separable) from the surface of
casting.

Should wet the surface of the wax pattern properly.
Should be compatible with wax pattern, casting alloys or
castable ceramics.
Should not discolor the casting.
Should not be bio-hazardous.
Long shelf life.
It should be economical.
No single material is known that can fulfill all the ideal
requirements. So various ingredients/modifiers are added
to get the desired properties!

INVESTMENTS USED IN MAKING MOLD FOR
ACRYLIC DENTURES
PLASTER OF PARIS
DENTAL STONE
HYDROCOLLOID (GLYCERIN MODIFIED AGAR)
INVESTMENTS USED FOR CASTING GOLD ALLOYS
GYPSUM BONDED INVESTMENT
TYPE I (THERMAL EXPANSION)
TYPE II (HYGROSCOPIC EXPANSION)
TYPE III (FOR RPD FRAMEWORKS)
PHOSPHATE BONDED INVESTMENT
TYPE I (INLAYS & CROWNS)
TYPE II (RPD FRAMEWORKS)

INVESTMENTS USED FOR CASTING BASE METAL
ALLOYS
PHOSPHATE BONDED INVESTMENT
TYPE I (INLAYS & CROWNS)
TYPE II (RPD FRAMEWORKS)
SILICATE-BONDED INVESTMENT
SODIUM SILICATE BASED
ETHYL SILICATE BASED

INVESTMENTS USED FOR CASTING CPTI AND
TITANIUM ALLOYS
Phosphate-bonded investments
Ammonia free PBI
Zircon (ZrO2.SiO2) coated PBI
Silicate bonded investments
Cemented investments
Silica (SiO2) based investments
Magnesia (MgO) based investments
Zirconia (ZrO2) based investments
Alumina (Al2O3) based investments
Resin based calcia investment
INVESTMENTS USED FOR SOLDERING AND
BRAZING PROCEDURES
Gypsum-bonded investments
Phosphate-bonded investments

BASIC STRUCTURE OF INVESTMENTS:
INVESTMENT MATERIALS CONSIST OF A
REFRACTORY AND A BINDER.
REFRACTORY:
Resists high temperature
Provides thermal expansion

The refractory is one or more of the forms of silica,
usually quartz or cristobalite in powdered form. When
heated the forms of silica undergo displacive
transformations, which cause them to expand
significantly. This displacive transformation coupled with
the setting expansion accounts for the total expansion
required, to compensate for the shrinkage of the alloy as it
solidifies.

The amount of expansion and the temperature
at which it occurs depends on the type and
amount of silica used in the investment.

Binder:
Binds the refractory particles
Provides strength and rigidity to investment

Gypsum-bonded investments consist of gypsum
products (autoclaved calcium sulphate
hemihydrate, α-form) as the binder, which
reacts with water to form calcium sulphate
dihydrate. There is no interaction between the
refractory (silica) and the binder.

Phosphate-bonded investment materials consist of
ammonium phosphate and magnesium oxide as the
binder. On setting magnesium oxide reacts with the
phosphate ions to produce magnesium ammonium
phosphate. In these investments the binder does
interact with the surface of the silica particles,
which gives, rise to a stronger set material.

Additives:
Modifiers such as boric acid and sodium chloride are
added to regulate the setting time and expansion.
Reducing agents like carbon (graphite) and powdered
copper are present in the investment to produce a
reducing atmosphere in the investment mould and thus
minimize the oxidation of the alloy.
Coloring agents may be added to the investments.

Liquid:
Some investment materials are marketed with an
accompanying high-expansion liquid, which
either replaces water or is used in addition to
water to control expansion on heating. Such
liquids contain alkaline colloidal silica giving
them improved strength when set.

Setting time:
The setting time of these investments is dependent
upon the gypsum content and upon the type of
gypsum employed. It may also be varied by the
conditions of mixing. Initial setting times vary
between 8 and 15 minutes and final setting times
between 12 and 25 minutes.

According to ADA specification no.2, the setting time
should neither be shorter than 5 minutes nor longer than 25
minutes.
Finer the particle size faster is the set.
More the w/p ratio, fewer nuclei per unit volume and hence
setting time is prolonged.
The longer and the rapidly the material is mixed, the
shorter is the setting time.

No much change in setting time when water between 0 C (32F) and
50 C (120 F) is used, but increased more than 50 C (120 F), a gradual
retardation occurs. As the temperature reaches 100 C (212 F), no
reaction takes place.
Accelerators such as potassium sulphate in concentrations higher than
2% produces syngenite [K2Ca(SO4)2.H2O], which crystallizes rapidly.
Sodium chloride <2% acts as accelerator but when used >2% it retards
the setting reaction. Sodium sulfate < 3.4% acts as accelerator.
Retarders such as citrates, acetates and borates are used to prolong the
setting time.

Compressive strength:
In practice it is the compressive strength of the
investment material at the casting temperature that is
significant in resisting the possible distortion caused
by the inrush of molten alloy. If the material is
mixed with a low water/powder ratio (thick mix)
then the compressive strength will be increased.

It appears both from laboratory testing and practical
experience, that the majority of the materials on the
market are satisfactory if used in accordance with the
recommended proportions.
Compressive strength of PBI type I is 2.5 Mpa and that of
type II is 10 Mpa.
Compressive strength of ESBI is 1.5 Mpa.

Expansion:
The linear casting shrinkage of gold alloys
will vary from one alloy to another, but it will
not be less than 1.5 per cent and may be as
high as 2 per cent. In order that dental
castings shall not be that much too small that
it is necessary to expand the mould into which
they are cast.

There are three possible ways by which this may be
done:
a) By setting expansion
b) By hygroscopic expansion
c) By thermal expansion

NORMAL SETTING EXPANSION IS THE
EXPANSION THAT OCCURS DIRECTLY
AFTER MIXING AND PRIOR TO IMMERSION
IN WATER OR HEATING. IT OCCURS DURING
THE SETTING OF THE INVESTMENT AND
HENCE THE NAME.

Hygroscopic expansion occurs if the investment is allowed
to set in contact with water. It is of two types:
That achieved by immersing the casting ring in water.
That achieved by placing measured amount of water on
the investment within the ring.

Thermal expansion occurs on heating the
investment through burnout of the wax pattern and
to the temperature at which molten metal is cast
into the mould. The refractory is responsible for
this type of expansion.

Permeability:
This is necessary to allow the air to pass out as
the molten metal enters the mould space. The
greater the proportion of gypsum the less will be
the permeability, but the most significant factor in
this respect is the uniformity of the particle sizes
of the investment.

If all the particles are of equal sizes, the
permeability will be greater than that of a
mixture of large and small sizes. But greater
the permeability of the mould the rougher will
be the casting.

Phosphate and gypsum-bonded investments are
more porous as compared to the silicate-bonded
investments. [As the silicate-bonded investments
contain admix of small and large particles and also
they are condensed sufficiently to obtain good
strength, the porosity decreases.]
Graphite in some investments renders more
permeability after burnout.

Expansion of investments:
This is an anomalous use of the word
expansion because gypsum products, without
refractory, contract or shrink on heating.
Between 200°C and 400°C shrinkage occurs
as a result of dehydration.

Between 400°c and 700°c a slight expansion
occurs and above 700°c the materials shrinks
severely as it decomposes. Gypsum bonded
investment can be made to expand sufficiently on
heating with the addition of either quartz or
cristobalite by counterbalancing the shrinkage of
the gypsum binder.

Normal Setting Expansion:
Normal setting expansions are seen in Gypsum-
bonded (0.06-0.6%) and Phosphate-bonded (0.5%)
investments. The casting ring may restrict the
expansion, but when the ring is lined with wet
absorbent kaolin impregnated paper, the
expansion markedly increases.

• Lower the Water/Powder ratio greater is the
expansion
• Longer the Mixing time within the practical
limits, greater is the expansion
• Chemical accelerators and Retarders
generally reduce the setting expansion

No setting expansion will occur with any of
the investment materials bonded with ethyl
silicate. In certain materials of this type a
contraction of about 0.2% to 0.4% may take
place.

Hygroscopic Setting Expansion:
If the material is allowed to set in contact with
water a greater expansion occurs. This is
known as hygroscopic expansion. The extent
to which the investment expands
hygroscopically varies according to their
composition.

Hygroscopic expansion is inversely
proportional to the percentage of silica in the
investment.
Finer the silica particle size of silica greater
the expansion.
Higher the water content in the original mix,
lesser is the hygroscopic expansion.

•The magnitude of hygroscopic expansion is directly
proportional to the amount of water added during the
setting.
•Reduced mixing time decreases the hygroscopic
expansion.
•Older materials show less expansion.
•Greatest amount of expansion is observed if the
investment is placed in water before its initial setting.
•Less expansion seen when confined to the casting
ring.

The common method of obtaining hygroscopic
expansion is to immerse the ring filled with
investment into water at 37 C, at the initial setting
time of the material and allows it to remain there for
30 minutes. A controlled expansion can be obtained
by adding measured amount of water on to the
surface of the investment, within the casting ring.
Hygroscopic expansion of about 0.6 to 0.8 % can be
seen in PBI.

Thermal expansion:
Thermal expansion occurs during the heating
period, first when the wax is being eliminated,
and subsequently during the time necessary to
reach the temperature suitable for casting to
be carried out. The percentage thermal
expansion for any material depends upon:

The water/powder ratio of the mix. Greater the
water/powder ratio lesser is the expansion.
The proportion of silica in the material. When
silica content increases the expansion increases.
The variety of silica, either quartz or cristobalite.
Cristobalite gives more thermal expansion than
quartz even at a lower temperature (200 C).

When Gypsum alone is heated it expands slightly up to
118C but then contracts markedly, as much as 2 % at
600C. If the silica content is increased the expansion of
silica counterbalances the thermal contraction of
gypsum. But there may be weakening of the investment.
The addition of small amounts of modifiers such as
sodium, potassium, or lithium chlorides may eliminate
the contraction caused by the gypsum.

With quartz owing to its higher inversion
temperature, the optimum thermal expansion does
not occur until approximately 650-700 C. This
means that casting must not be carried out bellow
that temperature when using quartz-type
investments.

If cristobalite is used the expansion takes place at a
lower temperature and to a greater degree.
Expansions of 1.2 % may be obtained at 500 C,
remaining constant up to 900 C if necessary. This
means that the casting temperature range is much less
critical if cristobalite is used, than in case of quartz.

Whatever type of investment is used, the heating of the
ring must be gradual to obtain the maximum thermal
expansion.
Thermal expansion of 0.8% is seen in PBI if
50:50 mixtures of liquid and water are used. Thermal
expansion may increase to 1.0-1.2% if undiluted liquid is
used.
In ESBI the thermal expansion may be 1.5 to 1.8%
when heated from room temperature to 1000-1177C.

Hydrocolloids:
Fluid resin technique (Shepard and Winkler 1967). Makes
use of hydrocolloid as investment material. The waxed- up
denture is sealed and positioned in a specially designed
flask. Which is then filled with reversible hydrocolloid
investment medium. After gelation of hydrocolloid, the
cast with the attached waxed up denture is removed then
vents and sprues are cut from outside the flask into the
mold space. The wax and base plate are eliminated, then
the teeth and the cast are replaced in the vented flask.

Fluid resin is mixed and poured into the mold
via the sprue openings (28 ml powder of 13 ml
of liquid). Then filled flask is held in a
pressurized (0.14 Mpa) chamber at room
temperature until the resin cures completely .
The set resin is removed and trimmed.

Advantages:
1) Better tissue fit
2) Fewer open bites
3) Less fracture of porcelain teeth during deflasking
4) Decreased material cost
5) Simple processing method.

Disadvantages :
1) Air Inclusions
2) Shifting of teeth
3) Decrease in occlusal vertical dimension
4) Occlusal imbalances
5) Incomplete flow of denture base material
6) Technique sensitive

Gypsum Investment:
They are used in investment of conventional heat and cold cure
dentures, and also in fluid resin denture preparation
Basic reaction of gypsum products
Gypsum products used for processing dentures are : Type
II, III plaster

INVESTMENTS FOR GOLD ALLOYS:
The materials are supplied in the form of powder,
which is mixed with water. The binding agent is
usually the α-hemihydrate of gypsum. Since it gives a
greater crushing strength to the investment; the binder
content is usually between 25 and 40 percent by
weight.

In addition to silica there may be incorporated
small amounts of reducing agents such as
carbon, which produce a reducing atmosphere
in the investment mould and thus minimize
the oxidation of the alloy. Silica may be
present in of its forms, quartz or cristobalite.

When these substances are heated to
temperature known as their “inversion points”,
they undergo an inversion from the α-form to
the β-form. This change is accompanied by a
marked expansion. The inversion of quartz
occurs at 575 c, and of cristobalite at between
200 c and 270 c.

ADA 2
Type I - For inlay and crown (thermal expansion)
Type II - For inlay and crown (hygroscopic)
Type III – RPD framework
Cannot be used in conjunction with cobalt chromium alloys
whose melting point is approximately 1425 c; decomposition of
gypsum takes place in the presence of silica. This causes rapid
evolution of oxides of sulphur and consequent porosity and
embitterment of casting occurs.

However cobalt-chromium alloys with lower melting
points around 1260oc are used, then gypsum bonded
investments of the type used for gold casting can be
employed.
WHEN MIXED WITH WATER,
(CaSO4)2.H2O + 3 H2O  2CaSO4.2H2O + HEAT
• WHEN HEATED TO 110-130 C,
CaSO4.2H2O  (CaSO4)2.H2O

WHEN HEATED TO 130-200 C,
(CaSO4)2.H2O  CaSO4
WHEN HEATED TO 200-1000 C,
Hexagonal Anhydrate  Orthorhombic Anhydrate

INVESTMENTS FOR BASE-METAL ALLOYS:
They must withstand temperatures of the order of
1000 C without cracking or distortion.
They must have sufficient expansion to compensate
for the thermal contraction of the cobalt-chromium
alloy, which has been established to be about 2.2 %.

PHOSPHATE-BONDED:
This investment material is essentially a powder containing
the silica refractory, a soluble acid phosphate and a
metallic oxide. When this powder is mixed with water, the
following reaction occurs:
NH4H2PO4 + MgO  NH4MgPO4+ H2O
OR
NH4H2PO4+ MgO +H2O NH4MgPO4.6H2O
This chemical reaction is accompanied by a physical
reaction in which the slurry changes into a solid; this gives
the initial or green strength to the investment.

On heating:
Undergoes dehydration at 160 C,
NH4MgPO4.6H2O  NH4MgPO4.H2O + 5H2O
And decomposition between 300 C and 650 C,
2NH4MgPO4.H2O  Mg2P2O7 + 3H2O + 2NH3
Further reaction with excess mgo if heated above 1040 C,
Mg2p2o7 + mgo  mg3(p2o4)2
When ammonium magnesium phosphate
and silica react to form complex silico-phosphates, it
gives greater strength to the investment at higherwww.indiandentalacademy.com

SILICATE-BONDED:
1.Binder based on sodium silicate:
This can be used for dip or spray coating for wax
patterns. An aqueous solution of sodium silicate is
acidified by the addition of hydrochloric acid and
abounding silicic acid gel is formed. However such
investments are not generally used.
2.Binder based on ethyl silicate:
Once it was widely used but now used mainly in large
laboratories, where its inconvenience of preparation is
offset by its lower cost.

Ethyl silicate is a colorless liquid, of specific
gravity 1.06, insoluble in water but soluble in
alcohol and other organic solvents. Its important
chemical property is its ability to undergo
hydrolysis to give silicic acid as follows:
Si(C2H5O)4 + 4H2O  Si(OH)4 + 4C2H5OH
nSi(OH)4+ MgO  MgO[Si(OH)4]n

This can be brought about by addition of a small
amount of dilute hydrochloric acid to a mixture of
ethyl silicate, industrial spirit and water.
500 ml ethyl silicate
1500 ml industrial spirit
50 ml water
10 ml dil. Hydrochloric acid

The addition of hydrochloric acid is necessary in order to
accelerate the hydrolysis. Solution, such as that above, are
prepared and allowed to stand for 12 to 24 hours before
use.
The silica investment powder, which may be quartz or
cristobalite or a mixture of the two, is then mixed with the
solution and setting takes place, usually within hour after
mixing.

THE MECHANISM OF THE SET IS AS FOLLOWS:
Hydrolysis of the solution is commenced by the addition
of the acid and if the solution is allowed to stand at room
temperature for about a week, hydrolysis proceeds to
complete gelation and the solution turns into a jelly. If
silica powder is added to the solution before this occurs,
say 24 hours after adding the acid, then this silica speeds
up the hydrolysis, produces gelation and hence the
investment sets.

The end products are soft silica gel and ethyl alcohol; on
subsequent heating of the ring, water and alcohol are
driven out and a hard gel of amorphous silica remains.
It has been found that the particle size and shape
of the silica powder are important factors in the bonding
process. If particles are of uniform size, bonding with the
hydrolyzed ethyl silicate solution will not take place.
Consequently varying grades of silica powder must be
used and their structure should be subangular and porous.

However in order to obtain good surfaces on the castings
some fine grade must be used and it is recommended that
70% coarse and 30% fine grade is employed. Above this
percentage the mould is liable for crack on heating.

INVESTMENTS FOR CASTING TITANIUM:
There are various investments on the market for casting
titanium. Phosphate-bonded investments have been used
but are usually modified forms that give sufficient
expansion to compensate for the shrinkage of the metal
at lower mould temperatures. To avoid contamination
and surface degradation of the casting the mould is
heated to a much lower temperatures prior to casting.

In some cases the metal is cast into a mould at
room temperature. In this case, all the expansion
of the mould must be achieved by setting through
the use of high expansion liquids. Other types of
materials for casting titanium include alumina-
based, spinel-based, zirconia-based and
magnesia-based investments. One such
magnesia-based investment utilizes magnesia as
the refractory and aluminous cement as the
binder.

Recently a new investment material for
titanium casting has been developed, which
contains calcia (cao2) as refractory and cold-
cure acrylic resin (PMMA) as binder, known
as “resin-bonded calcia investment”.

This is a the recently developed investment
for casting titanium inlay, crown and bridge.
Binder — calcia
Refractory —Zirconia

There are 2 types of CaO and mixing liquid.
1. Saturation type (total expansion 2 ± 3%)
2. Delayed expansion type
Properties
1. Total thermal and setting expansion found was -1 .5 -
2.5%
2. The maximum thermal expansion is found at - 900 -
1200°c

Composition:
Powder:
Zirconia (Zr sio4) Mixture
Zirconia flower
Properties:
1. High refractoriness
2. High thermal conductivity
3. Low co-efficient of thermal expansion
4. Low reactivity with molten titanium

Advantages:
1. Quick casting procedure
2. Increased accuracy of casting

They usually contain phosphate binder and
Mgo /quartz refratories.
1. Magnesia bonded investment
Phosphate binder
Alumina /Mgo refractories
good heat resistance
Low thermal expansion

2. Phosphate bonded investment:
Phosphate binder
Mgo + Al2O3 →Mgo –Al2O5 (spinel→ highly refractory.)
Show large expansion due to spinel reaction.
3. Spondumen (H2O - A12O5 - Sio2) Expand
irreversible on heating at 900 - 1100°c

4. Aluminous cement :
(CaO - A12O5 + Mgo (refractory) + 5% Zirconia
Burnout (o)
ZrSio4 ---------------------› Zr↑Expansion

(whipmix corporation)
It is a combination of Die stone and gypsum
bonded investment material. The powder is mixed
with colloidal silica.
Properties:
Setting expansion - 0.9%
Thermal expansion - 0.6% (at 977°c)

Advantages:
The wax pattern and die are invested simultaneously
with out removal of pattern. Useful with gold alloys
Divestment phosphate(DVP):
Similar to divestment, but used for casting post and
core, crowns of base metal alloys without any need
of removal of wax pattern.

ADA Sp No 93
Type I - Gypsum bonded dental brazing investment
Type II - Phosphate bonded.

As Prosthodontists, our aim is to make a restoration
as accurate as possible. Most of the restorations
what we are making are cast restorations and that is
why we should have knowledge about various
materials and techniques used in casting a dental
restoration.
Investment materials are to be selected based on
the type of restoration, the type of metal or alloy to
be casted, as well as technique used for casting and
that is how we can achieve a better restoration.

Thank you
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