Investment Materials

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INVESTMENT MATERIALS
DR. ARATI
DEPT. OF PROSTHODONTICS,CROWN &
BRIDGE AND IMPLANTOLOGY
S B Patil Dental College And Hospital

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Contents
• Introduction
• Definition
• Ideal requirements
• Classifications
• Composition
• Gypsum bonded investments
– Compositions
– Properties
– Expansion types
• Phosphate bonded investments
– Composition
– Properties
– Advantages and disadvantages

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• Silica bonded investments
– Composition
– Properties
– Advantages and disadvantages
• Soldering investments
• Recent advances in investment's for Ti
• Manipulations
• Methods of investing
• References

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INTRODUCTION
• When the wax pattern is to be reproduced
in an alloy a mould is made in a material
which will withstand the temperature at
which these alloys melts.
• The procedure of making such a mould is
called investing and the material used is the
investment material.

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Metal Casting
• Lost wax technique (Taggart, 1907)
A  crucible former
B  sprue
C  wax pattern
D  investment material
E  ring liner
G  thickness of investment
at the top

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Definition {GPT-8}
• Invest : to surround, envelop, or embed in an
investment material.
• Investing : the process of covering or enveloping,
wholly or in part, an object such as a denture,
tooth, wax form, crown, etc. with a suitable
investment material before processing, soldering,
or casting
• Investment : DENTAL CASTING , REFRACTORY .
• Investment Cast : A cast made of a material that
will withstand high temperature without
disintegration.

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• An INVESTMENT can be described as a
ceramic material that is suitable for forming
a mould into which a metal or alloy is cast.
( Craig)

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Ideal requisites of investment
• Easily manipulated and capable of reproducing
the size, shape and details recorded in the wax
pattern.
• Sufficient strength at room temperature to
withstand the forces of molten metal.
• Stability at high temperature and must not
decompose to give off gases that could damage the
surface of alloy.

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• It should have sufficient setting, hygroscropic,
thermal expansion to compensate for the
shrinkage of cast metal on cooling.
• Should be permeable to permit escape of gasses
from the mould cavity.
• Should not segeregate in the container during
transition nor when they are mixed.
• Ease of divestment
• Should be inexpensive

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CLASSIFICATIONS

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1According toANSI /ADA specification no -2
for casting investment for dental gold alloy
encompasses three types of investments.
Type-1
Type-2
Type-3

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• The type of investment used depends on –
• Appliance to be fabricated
• Expansion required to compensate the
casting shrinkage of metal.

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TYPE-1
• When the casting shrinkage is compensated
by the thermal expansion of the investment.
• Used for inlays and crown.

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TYPE-2
• When the casting shrinkage of the metal is
compensated by the hygroscopic expansion
of the investment by immersing the
investment ring in water bath.
• Also used for inlays, onlays and crown.

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TYPE-3
• Used in the construction of partial dentures
with gold alloys.

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2 Depending on the melting range of alloy
and the type of binder
• Gypsum bonded- traditional type & used
for conventional casting of gold alloy inlays ,
onlays and FPD’s.
• Phosphate bonded – designed primarily for
alloy to produce coping or framework for
metal ceramic prosthesis and for some base
metal alloys, also for some press able
ceramics.
• Silicate bonded- used for casting of RPD’s
with base metal alloys.

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COMPOSITION
• In general, an investment is a mixture of 3
distinct types of materials.
Refractory(65%-75%)
Binder(25%-45%)
Other chemicals.(2%-15%)

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Refractory Material
• Usually in the form of silicondioxide such as
Quartz, Tridimite, or Cristobalite or
mixture of these.
• These are contained in all dental investment
weather for casting gold or high melting
point alloy.

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Binder
• They are used to hold the other ingredients
together and to provide rigidity.
• Strength of investment depends on the
amount of binder present.
• Ex:
– Alpha CaSo4 hemi hydrate (25-45%).
– Phosphates
– Ethyl silicate

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Other chemicals
• Usually a mixture of refractory materials and a
binder alone cannot produce all the desirable
properties, required for an abutment.
• NaCl, Boric acid, K2So4, graphite, Cu powder or
Mg o are often added in small quantities to modify
various physical properties, such as boric acid and
chlorides enhances thermal expansion of gypsum
bonded investment.

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Gypsum-bonded investment

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Gypsum bonded investments
(below 10000C)
• Composition:
– Silica is added as a refractory component to
provide refractory during heating of
investment and to regulate the thermal
expansion.(55-75%)
– Binder: Alpha Caso4 hemi hydrate.
– Modifying agent: coloring agent and reducing
agent like C / Cu powder. They produce non
oxidizing atmosphere in the mould when gold
alloy is cast.
– Additives: boric acid and Nacl to regulate
setting time and expansion, and to prevent
shrinkage of gypsum when heated above 3000 c

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Effect of temperature on refractory and
binder.
• In casting with lost wax process the wax
pattern after being invested , is melted and
removed from the investment leaving the
mold behind.
• In this way the investment is heated to an
elevated temperature which varies from
one technique to another.

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• Silica is added to provide a refractory component
during the heating of the investment and to
regulate the thermal expansion.
• Investment is expected to expand thermally to
compensate partially or totally for the casting
shrinkage of gold alloy during heating.
• Gypsum shrinks considerably when it is heated. If
proper form of silica are employed in the
investment this contraction can be eliminated and
changed to an expansion.

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• The transition temperature for quartz is 575C and for
Cristoballite is 200 C.
• Two inversions of Tridymite occurs at 117 C and 163 C.
• All forms of silica are in the alpha form in the investment
and during heating process to eliminate the wax process
they are converted completely or partially to their
corresponding beta form. Such a transition involves an
expansion of the mass which helps to compensate for the
casting shrinkage.

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Effect on binder
• During the investing process some of the water
mixed with the investment reacts with the
hemihydrate and is converted to CaSo4 di
hydrate. Where as the remaining water exist
uniformly distributed in the mix as excess water.
• This excess water is evaporated during the early
stage of heating but a t temperature above 220 F
the water of crystallization of CaSo4 .2H2o is
attacked and driven out of the investment, and is
converted into anhydrous CaSo4.

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Thermal Expansion (1.0-1.5%)
• Directly related to the
amount of SiO2
present and to the
type of SiO2 employed
(quartz vs.
crystobalite).
• The effect of SiO2 will
balance the
contraction of the
gypsum during
heating.
• The maximum
thermal expansion is
attained at a
temperature < 700oC.
SiO2

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Factors to Control Thermal
Expansion
• Thinner mixed  less thermal expansion
• The sufficient amount of SiO2 to prevent
any contraction during heating can
weaken the investment.
– Chemical modifiers, e.g., sodium, potassium
and lithium chlorides, are added to eliminates
the contraction caused by the gypsum and
increases the expansion without the presence of
an excessive amount of SiO2.

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• Finer the size greater will be the expansion.
• Type of binder:
– Alpha or Beta?

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Cooling of investment
• As we have seen the conversion of alpha
form of silica to the beta form of silica and
the CaSo4 dihydrate to Anhydrous CaSo4,
both of these changes involves some
dimensional changes in the mould so it
becomes important to know that by cooling
the investment mould and again reheating ,
the same value of thermal expansion cannot
be obtained.

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• So upon cooling these materials cannot be
changed to their original form which results
in some contraction so that heating should
be continued to the recommended casting
temperature and the casting operation
should be completed promptly.
• Cooling and reheating of the investment can
produce an inaccurate casting.

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Setting expansion and hygroscopic
expansion
• Setting expansion is the linear expansion
which takes place during the normal setting
of the investment in air.
• Hygroscopic setting expansion is the linear
expansion which takes place if the
investment is in contact with water from
any source during setting process.

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Setting Expansion
(normal vs. hygroscopic)
• Stage I: initial mix
• Stage II: crystals of dihydrate are
formed
– Lt: water around the particles is
reduced by the hydration and the
particles are drawn more closely
together by the surface tension
action of the water
– Rt: hydrated water is replaced
• Stage III: crystals grow
– Lt: water is decreased, particles are
drawn together
– Rt: water is replaced, crystals grow
freely
• Stage IV and V

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Hygroscopic Setting Expansion
• Occurs when the
gypsum product is
allowed to set under or
in contact with water
• Can be obtained from
– Water immersion
technique
– Water added
– Wet ring liner
• Greater magnitude
than the normal setting
expansion (> 6 times)
Normal setting expansion
Hygroscopic setting expansion

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Factors to Control Normal and
Hygroscopic Setting Expansion
• Composition
– Proportional to the SiO2 content
– The finer the particle size of the SiO2, the
greater the hygroscopic expansion.
• Water:Powder Ratio
– Higher W:P  less expansion
• Spatulation
– Insufficient spatulation  decrease expansion
• Shelf life of the investment
– Older investment  lower expansion

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• Time of Immersion
– Delayed immersion  decrease expansion
• Confinement
– the walls of the investment container, or the walls
of a wax pattern
• Water bath / Amount of Added Water
– The magnitude of the expansion is in direct
proportion to the amount of water added during
the setting period until a maximum expansion
occurs.

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Role of added water
• HSE is a continuation of normal setting
expansion, bcz of the immersion water
replaces the water of hydration thus
preventing the confinement of growing
crystals by the surface tension of excess
water.

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Setting time Setting
expansion
Hygroscopic
setting
expansion
Thermal
expansion
Not < 5mins
Nor > 25mins
Modern : 9-
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Type 1:
0.6%
in air
Modern:
0.4%
Type 2:
minimal:
1.2%
Max: 2.2%
Type 1: 1-
1.6%
Type 2:
0.6% at
500C
According to ADA specification no 2

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Storage
• Under high relative humidity the setting time, HSE,
SE may be altered. Therefore they should be stored
in air tight or moisture proof container.
• All investment composed of diff ingredients with
diff specific gravity and having tendency for these
components to separate as they settle, according to
their SG under normal vibration in lab.

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Phosphate-bonded investment

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• The rapid growth in use of metal ceramic
and hot pressed ceramic prosthesis has
resulted in an increased use of phosphate or
silicate bonded investments.
• They are used for metal ceramic prosthesis
using high melting gold alloy and for base
metal alloys. (above than 13000 C)
• Ex: AgPd, AgPt & NiCr.

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Composition
• Filler (refractory) = quartz or cristobalite or both
(80%)
• Binder = magnesium oxide and acid phosphate
• Liquid = colloidal silica suspension

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• Setting Reactions:
– NH4H2PO4 + MgO  NH4MgPO4 +H2O
– The reaction is not simple and changed on
heating.

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Advantages
• They have high strength.
• They have high thermal and setting
expansion.
• They can withstand temp upto 9000 C.

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Disadvantages
• Casting made with this investments are more
rougher than casting made with gypsum bonded
investment.
• They are difficult to remove from the investment.
• The casting are more likely to have surface
nodules.
• When powder is supplied it will react with air
borne moisture and gradually deteriorate.

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Silica bonded investments
• These investments materials are used for
casting alloys which have high casting
temperature.
• They are used in the construction of the
high fusing base metal partial denture alloy.
• This type of investment is losing popularity
bcz of more complicated and time
consuming procedures involved.

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composition
• These investments are mixture of powder
and liquid.
• Powder consists of refractory particles of
silica in various forms, plus MgO.
• Several liquids

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Advantages
• They have high refractory nature.
• Small dimensional changes on seting.
• High degree of thermal expansion.
• High permeability to achieve very fine
surface details.
• Divesting is easy.

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Disadvantages
• Complicated and time consuming.
• Low shelf life.
• More care required in handling during the
casting procedures.
• They are more prone to cracking during the
burn out and firing process due to the low
strength with high thermal expansion.

Divestment
• This is gypsum bonded investment which is
used both as a die and investment material
• To make the working cast and die , it is
mixed with colloidal silica
• The wax pattern is fabricated on this and
along with it the cast is invested in same
material
• This eliminates the need to remove the wax
pattern for investing and prevents distortion
• It cannot be used for high fusing alloys(base
metal and metal ceramic) and can be used
only for conventional gold alloys
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Manipulation of investments:
• The technique of investing the wax pattern should
not be done in haste.
• All the materials and the instruments required
should be in hand.
• The manufacturers direction for any given
investment should be followed quickly.
• A variation of only 1 ml of water can significantly
alter the setting expansion ad also affect the
manipulation of the investment and the character
of the casting surface.

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Procedure:
• The appropriate amount of distilled water ( gypsum
investment) or colloidal silica special liquid ( Po4
investment) is measured out.
• The liquid is added to a clean dry mixing bowl and
the powder is weighed and sifted into the liquid with
care and caution to minimize air entrapment.
• Mixing is started gently until all the powder particles
have been wet.

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Methods of investing
• Double investment technique
• Hand painting
• Vacuum mixing/ investing.

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Double investment
• In this method a thin layer of fine investment
material is painted on the pattern by means of a
brush. Before setting of this layer , investment
powder is dusted until porous mass of investment
surrounds the pattern.
• By this means, any excess water in the 1st mix is
soaked up by the dry powder and resultant thick
consistancy of investment is said to ensure a high
setting expansion.

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Hand painting
• In this measured mix of investment is used to
paint the pattern to ensure the elimination of
air bubbles.

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Vacuum investing
• The alternate method of avoiding air blows
is to remove the air by subjecting the
investment to reduced atmospheric pressure
either before or after it has been poured
around the pattern.

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Hazards
• Three diseases associated with asbestos exposure are
• Asbestosis {fibrotic thickning of lung tissue}
• Brochogenic carcinoma
• Asbestos are group of fibrous mineral that differ in
physical form, chemistry ad toxicity
• Three major kind of asbestos are
• White ,blue and brown

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