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1. INVESTMENT MATERIALS
Introduction
When a restoration or appliance is being made by a lost wax
process, the wax pattern is embedded in an investment material. The
wax is then removed from this mould, and the space which it occupied is
filled by the material of which the restoration or appliance is to be made
for example.-
The wax pattern of an inlay or other cast restoration is embedded
in a heat resistant investment material which is capable of setting to a
hard mass. The wax is removed from such a mould usually by burning
out, before casting the molten alloy.
Definition:
An investment can be described as a ceramic material which is
suitable for forming a mold into which a metal or alloy is appropriately
cast.
The procedure for forming the mold is described as “investing”.
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2. Ideal properties:
1. The investment should be easily manipulated. Not only should it
be possible to mix and manipulate the mass readily and to paint
the wax pattern easily, but the investment also should harden
within a relatively short time.
2. The investment mold must have sufficient strength of room
temperature to permit ease in handling and enough strength at
higher temperatures to withstand the impact force of the molten
metal.
3. On being heated to high temperatures, the investment must not
decompose to give off gases that could damage the surface of the
alloy.
4. Investment should have enough expansion to compensate for
shrinkage of the wax pattern and the metal that takes place during
the casting procedure.
5. Should be porous enough to permit the air or gases in the mold
cavity to escape easily during the casting procedure.
6. Should produce a smooth surface and fine details and margins on
the casting.
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3. 7. Investment material should be comparatively inexpensive.
Composition:
In general, an investment material is a mixture of three different
types of materials.
Refractory material: Usually form of silicon dioxide, such as
quartz, tridymite, cristobalite or a mixture of these.
Binder material: Common binder used for dental casting gold
alloy is α calcium sulphate hemihydrate, phosphates and ethyl silicate.
Other Chemicals:
Such as sodium chloride, boric acid potassium sulphate, graphite,
copper powder or magnesium oxide.
Classification
Investment materials are classified into:
1. Gypsum bonded investment
- Type I
- Type II
- Type III
2. Phosphate bonded investments
3. Ethyl silicate bonded investments
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4. Gypsum Bonded Investments
ADA specification No. 2 for casting investments for dental gold
alloys encompasses three types of investments.
The types are determined by whether the appliance to be
fabricated is fixed or removable, and the method of obtaining the
expansion required to compensate for the contraction of the molten gold
alloy during solidification.
Type I: Investments are those employed for the casting of inlays or
crowns when the alloy casting shrinkage compensation is
accomplished principally by thermal expansion of the
investment.
Type II: Investments are also used for the casting of inlays or crowns,
but the major mode of compensation is by the hygroscopic
expansion of the investment.
Type III: Used in the fabrication of partial dentures with gold alloys.
Composition:
The essential ingredients of the dental inlay investment employed
with the conventional gold casting alloys are α-hemihydrate of gypsum
and a form of silica.
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5. Gypsum:
The α-hemihydrate form of gypsum is generally the 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 forms shrink considerably after dehydration between 200°C
and 400°C. A slight expansion then occurs between 400°C and
approximately 700° and then a large contraction occurs. This latter
shrinkage is most likely caused by decomposition, and sulfur gases, such
as sulphur dioxide are emitted. This decomposition not only causes
shrinkage but also contaminates the castings with the sulfides of the
non-noble alloying elements such as silver and copper.
Thus, it is imperative that gypsum investments not to be heated
above 700°C. In this alloy proper fit as well as uncontaminated alloys
are obtained.
Silica:
Silica (SiO2) is added to provide a refractory during the heating
of the investment and to regulate the thermal expansion. During the
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6. heating, the investment is expected to expand thermally to compensate
partially or totally for the casting shrinkage of the gold alloy.
If the proper form of silica is employed in the investment, this
contraction during heating can be eliminated and changed to an
expansion.
Silica exists in atleast 4 allotrophic forms:
1) Quartz
2) Tridymite
3) Cristobalite and
4) Fused quartz.
Quartz and cristobalite are of particular dental interest.
When quartz, tridymite or cristobalite is heated, a change in
crystalline form occurs at a transition temperature chracteristic of the
particular form of silica.
For example, when quartz is heated it ensures from a “low” form,
known as α-quartz to a “high” form, called β-quartz, at a temperature of
575°C.
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7. In a similar manner, cristobalite undergoes an analogous
transition between 200°C and 270°C from “low” called α-cristobalite to
a “high” called β-cristobalite.
The α-allotropic forms are stable only at the transition
temperature noted, and an increase to the lower or α form occurs on
cooling in each case.
The density decreases as the α-form changes to the β-form, with
a resulting increase in volume that is exhibited by a rapid increase in the
linear expansion, consequently, the shrinkage of gypsum can be
counterbalanced by the inclusion of one or more of the crystalline silica.
Modifiers:
In addition to silica, certain modifying agents, cooling matter,
and reducing agents such as carbon and powdered copper are present.
The reducing agents are used in some investments to provide a non-
oxidizing atmosphere in the mold when the gold alloy is cast.
Some of the added modifiers, such as boric acid, and sodium
chloride, not only regulate the setting expansion and the setting time, but
they also prevent most of the shrinkage of gypsum when it is heated
above 300°C.
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8. Setting time:
According to ADA specification No. 2 for dental inlay casting
investment, the setting time should not be shorter than 5 minutes nor
longer than 25 minutes. Usually, the modern inlay investments set
initially in 9 to 18 minutes. Sufficient time should be allowed for mixing
and investing the pattern before the investment sets.
Normal Setting Expansion
The purpose of setting expansion is to aid in enlarging the mold
to compensate partially for the casting shrinkage of the gold.
ADA specification No. 2 for Type I investment permits a
maximum setting expansion ‘in air’ of only 0.6%.
The setting expansion of such modern investment is
approximately 0.4%. It can be regulated by retarders and accelerators.
A mixture of silica and gypsum hemihydrate results in setting
expansion greater than that of the gypsum products when it is used
alone. The silica particles probably interfere with the intermeshing and
interlocking of the crystals as they form. Thus the thrust of the crystals
is outward during growth and they increase expansion.
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9. Variables other than the exothermic heat of reaction also
influence the effective setting expansion. As the investment sets, it
essentially gains sufficient strength to produce a dimensional change in
the maximum pattern as setting expansion occurs.
Also, the softer the wax, the greater the effective setting
expansion, because the softer wax is more readily moved by the
expanding investment.
Hygroscopic Setting Expansion
If the setting process of gypsum is allowed to occur under water,
the setting expansion will be more than doubled in magnitude. This is
because to hemihydrate allowed to react under water is related to the
additional crystal growth permitted.
ADA specification No. 2 for Type II investments requires a
minimum setting expansion in water of 1.2% while the maximum
allowed is 2.2%.
A number of factors are important in the control of the
hygroscopic expansion.
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10. 1. Effect of composition
The magnitude of setting expansion of a dental investment is
generally proportional to the silica content of the investment.
- Finer the particle size of silica greater the expansion.
- α-hemihydrate will produce a greater expansion than β-
hemihydrate.
Effect of water:powder ratio:
The highest the W:P ratio of the original investment water
mixture, the less the hygroscopic expansion.
Effect of spatulation:
Mixing time and hygroscopic expansion as well.
Effect of time of immersion:
The greatest amount of hygroscopic setting expansion is observed
if the immersion takes place before the initial set. The longer the
impression of the investment in the water bath is delayed beyond the
time of the initial set of the investment, the lower is the hygroscopic
expansion.
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11. Effect of the amount of water added:
The 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.
Expansion can be detected when water is poured into a vessel
containing only small, smooth quartz particles. The water is drawn
between the particles by capillary action and thus causes the particle to
separate, creating an expansion.
The effect is not permanent after the water is evaporated, unless a
binder is present.
The greater the amount of the silica or the inert filler, the more
easily the added water can diffuse through the setting material and the
greater is the expansion.
Thermal Expansion
The thermal expansion of a gypsum bonded investment is directly
related to the amount of silica present and to the type of silica employed.
A considerable amount of quartz is necessary to counterbalance the
contraction of gypsum during heating.
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12. The contraction of the gypsum is entirely balanced when the
quartz content is increased to 75%.
The investments containing cristobalite expand earlier and to a
greater extent than those containing quartz.
The desirable magnitude of the thermal expansion of a dental
investment depends on its use.
If hygroscopic expansion is to be used to compensate for the
contraction of the gold alloy, as for the Type II investment. ADA
specification No. 2 requires that the thermal expansion be between 0%
and 0.6% at 500°C.
However, for Type I investment, which rely principally on
thermal expansion for compensation, the thermal expansion must be not
less than 1% nor greater than 1.6%.
Another desirable feature of an inlay investment is that its
maximum thermal expansion be attained at a temperature not higher
than 700°C. Thus when a thermal expansion technique is employed, the
maximum mold temperature for casting of gold alloy should be less than
700°C.
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13. Effect of Water:Powder ratio
The magnitude of thermal expansion is related to the amount of
solids present. Therefore it is apparent that the more water that is used in
mixing the investments, the less is the thermal expansion that is
achieved during subsequent heating.
Effect of chemical modifiers:
The addition of small amounts of sodium, potassium, or lithium
chlorides to the investment eliminates the contraction caused by the
gypsum and increases the expansion without the presence of an
excessive amount of silica.
Strength:
According to ADA specification No. 2, the compressive strength
for an inlay investment should not be less than 2.4Mpa tested 2 hours
after setting.
Heating the investment to 700°C may increase or decrease the
strength as much as 65%, depending on the composition. The greatest
reduction in strength on heating is found in investments containing
sodium chloride.
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14. Other gypsum investment considerations:
Fineness:
The fineness of the investment may affect the setting time, the
surface roughness of the casting and other properties, fine particle size is
preferable to a coarse one, the finer the investment, the smaller are the
surface irregularities on the casting.
Porosity:
During the casting process, the molten metal is forced into the
mold under pressure, as the molten metal enters the mold, the air must
be forced out ahead of it. If the air is not completely eliminated, a back
pressure builds upto prevent the gold alloy form completely filling the
mold. The common method for venting the mold is through the pores of
the investment.
Generally, the more gypsum crystals that are present in the set
investments, the less is its porosity.
The particle size of the investment is also a factor. The more
uniform the particle size, the greater is its porosity.
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15. Phosphate Bonded Investments
The rapid growth in use of metal ceramic restorations and
increased use of a higher melting alloys have resulted in a increased use
of phosphate bonded investment.
As suggested by Skinner (1963) “The definite advantage of this
type of investment is that there is less chance for contamination of gold
alloy during casting and hence could be the investment of the future.
The present trend is towards the use of less expensive base metal alloys,
most of which require phosphate investments.
Composition:
These investments, like the gypsum investments consist of
refractory fillers and a binder.
The filler is silica, in the form of cristobalite, quartz, or a mixture
of the two and in the concentration of approximately 80%.
The purpose of this filler is to provide high temperature thermal
shock resistance (refractoriness) and a high thermal expansion.
The binder consists of magnesium oxide and a phosphate
(Monoammonium phosphate).
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16. Colloidal silica liquid suspensions are available for use with the
phosphate bonded investments in place of water 33% dilution of
colloidal silica is required.
Carbon is often added to the powder to produce clean castings,
and facilitate the ‘devesting’ of the casting from the mold.
Setting Reaction
The chemical reaction for the binder system that causes the
investment to set and harden is
NH4H2PO4 + MgO + 5H2O  NH4 MgPO4 6H2O
Setting and Thermal Expansion
Substitution of colloidal silica solution instead of water
considerably increases the expansion.
When phosphate bonded investments are mixed with water they
exhibit the same shrinkage as gypsum bonded investments.
This contraction is practically eliminated when a colloidal silica
solution replaces the water.
The early thermal shrinkage of phosphate investments is
associated with the decomposition of the binder, magnesium ammonium
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17. phosphate and is accompanied by the evolution of ammonia, which is
readily apparent by its odor.
Working and Setting Time
Phosphate investments are markedly affected by temperature.
The warmer the mix, the faster it sets. The setting reaction itself gives
off heat, and this further accelerates the rate of setting. The more
efficient the mixing better the casting in terms of smoothness and
accuracy.
The ideal technique is to mix, as long as possible, yet have
enough time for investing. Mechanical mixing under vacuum is
preferred.
Ethyl Silicate Bonded Investments
ETHYL SILICATE bonded investments are being used in the
construction of the high fusing base metal partial denture alloys. These
investments are losing popularity because of the more complicated and
time consuming procedures involved.
The silica is the binder which may be derived from ethyl silicate
or sodium silicate.
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18. The REACTION
The silica is bonded by the hydrolysis of ethyl silicate in the
presence of hydrochloric acid. The product of the hydrolysis is the
formation of a colloidal solution of silica acid and ethyl alcohol.
Si (O2C5H4) + 4H2O
HCl
Si(OH)4 + 4C2H5OH
Ethyl silicate has the disadvantage of containing inflammable
components because sodium silicate and colloidal silica are more
common binders used.
These investments are supplied with two bottles of special liquid
to be mixed with the investments. One bottle contain diluted water
soluble silicate solution such as sodium silicate, the other bottle usually
contains diluted acid solution such as solution of HCl.
Before use of the equal volume of each bottle is mixed so that
hydrolysis can take place and freshly prepared silicic acid is formed.
The Powder: liquid ratio is used according to manufacturers instruction.
This type of investment can be heated to 1090°C and 1180°C and is
compatible with higher fusing alloys.
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