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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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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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 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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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 -hemihydrate of gypsum

and a form of silica.

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Gypsum:

The -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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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 -quartz to a “high” form, called -quartz, at a temperature of

575°C.

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In a similar manner, cristobalite undergoes an analogous

transition between 200°C and 270°C from “low” called -cristobalite to

a “high” called -cristobalite.

The -allotropic forms are stable only at the transition

temperature noted, and an increase to the lower or form occurs on

cooling in each case.

The density decreases as the -form changes to the -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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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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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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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.

- -hemihydrate will produce a greater expansion than -

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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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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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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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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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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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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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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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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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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