COLLEGE OF DENTAL SCIENCES

DEPARTMENT OF CONSERVATIVE DENTISTRY AND ENDODONTICS

Seminar On

DENTAL INVESTMENTS

Presented by : -

Dr. Niju Aelias

DENTAL INVESTMENTS

INTRODUCTION :

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. This is mainly

based on “Lost Wax Technique”.

This process of casting involves some basic steps.

1. Preparation of a wax pattern.

2. Preparation of mold – It is done by pouring the mixed investment material

around the wax pattern and allow it to set.

Burnout : Wax is eliminated from he 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.

HISTORY :

This meticulous procedure of casting was used by various craftsmen to

produce jewelary and ornaments. Its history can be traced back around 3000

B.C. But origin of lost wax technique, when viewed history makes its presence

in the writings of Theophilus (11th century).

11th Century Theophilus described lost wax technique, which was a

common practice prevailed in 11th century.

1558 Benvenuto Cellini claimed to have attempted, use of wax and clay

for preparation of castings.

1884 Aguilhon de saran used 24K gold to form inlay.

1887 J.R. Knapp invented Blowpipe.

1897 Phillibrook described a method of casting metal filling.

1907 Taggart devised a practically useful casting machine.

Various studies conducted on the properties of investment materials and casting

alloys have led to a path for better, practical and useful processing methods.

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DEFINITIONS :

1. 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, casting.

2. Dental Casting Investment : 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).

3. Refractory : Difficult to fuse / corrode, capable of enduring high

temperatures.

4. Refractory investment : An investment that can withstand high temperature

using a soldiering /casting.

5. Allotropic phase : Phases of similar composition but different

crystallographic structures, with different properties.

6. Casting :

Noun : Something that has been cast in a mold, an object formed by the

solidification of a fluid that has been formed / injected into a mold.

Verb : the art of forming an objet in a mold.

IDEAL REQUIREMENTS OF AN INVESTMENT MATERIAL :

1. The powder should have a fine particle size to ensure a smooth surface on

the casting.

2. The mixed unset material should have a smooth consistency.

3. It should be easy to manipulate – easy to mix and also harden within a

relatively short time.

4. It should have sufficient strength at room temperature

- Should exhibit sufficient strength at high temperature.

- 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.

5. Inner surface of mold should be smooth.

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6. 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.

7. The material should be sufficiently porous enough to permit escape of

air/other gases from the mold cavity during casting of molten metal.

8. It should show ease of divestment.

- It should not react with metal

- It should easily break away from the surface of casting.

9. 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.

CLASSIFICATION OF DENTAL INVESTMENT :

I. Based on Processing Temperatures :

A. High temperature casting investments

- Phosphate bonded investments

- Silicate bonded investments

B. Low temperature casting investments

- Gypsum bonded investments (for low temperature gold alloy)

II. Depending on type of refractory used (Silica)

- Quartz investment

- Cristoballite investment

III. Based on type of binder used

A. Gypsum bonded investments : According to ADA Specification 2

Uses Shrinkage compensation

Type I Inlay, Crown Purely thermal expansion

Type II Inlay and Crowns Purely hygroscopic expansion

Type III R.P.D. Frame work Thermal

B. Phosphate bonded investments

C. Silicate bonded investments

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BASIC COMPOSITION OF INVESTMENTS :

In general, an investment is a mixture of the following 3 distinct types of

materials,

A. Refractory Materials : A material that will withstand high temperatures

without decomposing on disintegrating. The most commonly used

refractory material is silicon dioxide such as quartz, tridymite, or

cristoballite or a mixture of these.

Function :

- Resist the heat and forces of casting.

- To expand and compensate for casting shrinkage

B. Binder : The refractory material alone do not form a coherent solid mass,

so some kind of binder is needed. Commonly used binders are ;

- - Calcium sulfate hemihydrate

- Others are – Sodium silicate, Ethyl silicate, Ammonium sulfate, Sodium

phosphate.

C. Other Chemical Modifiers : Usually a mixture of refractory materials and a

binder alone is not enough to produce all the desirable properties required

of an investment. Other chemicals such as sodium chloride, boric acid,

potassium sulfate, graphite, copper powder or magnesium oxide are often

added in small quantities to modify various physical properties.

Eg. Small amounts of chlorides or boric acid enhance the thermal expansion

of investment bonded by calcium sulfate.

INVESTMENTS FOR LOW CASTING TEMPERATURE :

Gypsum Bonded Investments : The gypsum based material represent the type traditionally used for

conventional gold alloys.

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ADA Specification No.2 for casting investment for dental gold alloys

comes in three types.

Type I : For inlay and crown – Use mainly thermal expansion

Type II : For inlay and crown – Use mainly hygroscopic expansion

Type III : For R.P.D. frame work – Use mainly thermal expansion

Common Brands :

Baker’s Sterling, Begocast, Cristobalite, Inlay – Vest, Luster cast.

Composition :

1. Binder :

- Calcium sulfate hemihydrate (25 – 45%)

2. Refractory Material

Silica – Quartz or Cristoballite or a blend of two in varying proportions (65

– 75%)

3. Chemical modifiers

a) Carbon / Copper powder (Reducing agents) (2-3%)

b) Boric acid and sodium chloride (Balancing agents)

Functions of Each Constituent :

1. Gypsum Hemihydrate :

- The hemihydrate form of gypsum is the binder for investments used in

casting gold containing alloys with melting ranges below 1000oC.

- Hold other ingredients together and provide rigidity.

- Gypsum after heating undergoes dehydration with shrinkage and

fracture of mold.

Effect Of Temperature On Calcium Sulfate Binders :

The binder used for gold investments in dentistry is - calcium sulfate

hemihydrate.

During the investing process some of the water mixed with the

investment reacts with the hemihydrate and in converted to calcium sulfate

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dihydrate, whereas the remainder of excess water is uniformly distributed in the

mix.

During the heating process the excess water is evaporated. As the

temperature rise about 105oC, calcium sulfate dihydrate starts losing water and

expands, then shrink / contract considerably after dehydration between 200oC

and 400oC. A slight expansion then occurs between 400oC and approximately

at 700oC a large contraction occurs.

This latter shrinkage in most likely caused by decomposition, and sulfur

gases such as sulfurdioxide are emitted. This decomposition not only causes

shrinkage but also contaminate the castings with the sulfides of the non noble

alloying elements such as silver and copper. So gypsum bonded investment

should not be heated above 700oC.

These properties are explained as follows ; Upto about 105oC, ordinary

thermal expansion occurs. Above 105oC, the calcium sulfate dihydrate is

converted to anhydrous calcium sulfate.

Dehydration of the dihydrate and a phase change of the calcium sulfate

anhydrate cause a contraction. The form of tridymite (which might be

present as an impurity) is expanding and sufficiently compensates for the

contraction of the calcium sulfate to prevent the investments from registering a

serious degree of contraction.

At elevated temperature the forms of silica present in the investment

are converted to the forms, which cause some additional expansion.

2. Silica :

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Silica (SiO2) is added to provide a refractory during the heating of the

investment and to regulate the thermal expansion and also counter shrinkage of

gypsum.

Usually the wax pattern is eliminated from the mold by heat. During

the heating, the investment is expected to expand thermally, to compensate

partially or totally for the casting shrinkage of the gold alloy.

Gypsum shrinks considerably when it is heated. If the proper form of

silica is employed in the investment, this contraction during heating can be

eliminated and changed to an expansion.

Silica exist in four allotropic forms :

- Quartz

- Tridymite

- Cristobalite and

- Fused quartz

Effect Of Temperature On Silicon Dioxide Refractories :

The most commonly used refractory material is silica (SiO2).

Each of the polymorphic forms of silica, quartz, tridymite and

cristoballite expands when heated, but the percentage of expansion varies from

one type to another.

Pure crystoballite expands to 1.6% at 400oC, whereas quartz expands

about 1.4% at 600oC and the thermal expansion of tridymite at 600oC is less

than 1%.

The percentage of expansion of the 3 types of silica versus temperature

shows, none of the three forms of silica expands uniformly, instead they all

show a break (non linearity) in their thermal expansion.

In case of crystoballite the expansion is somewhat uniform to about

200oC. At this temperature its expansion increases sharply from 0.5% to 1.2%,

and then above 250oC it again becomes more uniform.

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At 573o C quartz also shows a break in the expansion and tridymite

shows a similar break at a much lower temperature.

The breaks on the expansion versus temperature indicate that

cristoballite and quartz, each exist in two polymorphic forms. One of which is

more stable at a higher temperature and the other at a lower temperature.

The form that is more stable at room temperature is called the form,

and the more stable form at higher temperature is designated as the from.

Tridymite has three stable polymorphic forms. Thus the temperatures of

220oC for cristobalite, 573o C for quartz and 105o and 160o C for tridymite are

displacive transition temperatures.

A displacive change involves expansion or contraction in the volume of

the mass without breaking any bonds. In changing from the form (which is

the more stable form at room temperature) to the form (which is stable at

higher temperatures), all three forms of silica expand.

The amount of expansion is highest for cristoballite and lowest for

tridymite.

The quartz form of silica is found abundantly in nature, and it can be

converted to cristobalite and tridymite by being treated through a reconstructive

transition during which bonds are broken and a new crystal structure is formed.

The quartz is converted to quartz at a temperature of 573oC.

If the quartz is heated to 870oC and maintained at that temperature, it is

converted to tridymite.

From tridymite obtaining either tridymite or cristoballite is possible.

If tridymite is cooled rapidly to 120oC and hold at that temperature, it is

changed to - tridymite, which is stable at room temperature.

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On the other hand, if tridymite is heated to 1475o C and hold at that

temperature, it is converted to - cristoballite. Further heating of

cristoballite produces fused silica, but if it is cooled to 220oC and held at that

temperature, cristoballite is formed.

870o C 1475oC 1700oC quartz tridymite cristobalite Fused Silica

160oC

573oC Middle tridymite 220oC

105oC-Quartz -tridymite -cristobalite

All forms of silica are in either forms in the investment and during the

heating process they are converted completely or in part to their corresponding

forms. This transition involves an expansion of the mass, which helps to

compensate for casting shrinkage.

Fused quartz is amorphous and glass like in character and it exhibits no

inversion at any temperature below its fusion point. It has an extremely low

linear coefficient of thermal expansion and is of little use in dental

investments.

Quartz, cristobalite or a combination of the two forms may be used in a

dental investment. Both are now available in pure from.

Tridymite is no longer an expected impurity in cristobalite.

3. Modifiers :

a) Reducing agents : Carbon and powdered copper provide a non oxidizing

atmosphere in the mold when the gold alloy is cast.

b) Balancing agent : Boric acid and sodium chloride

Regulate

- Setting time

- Setting expansion

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- Prevent shrinkage of gypsum when it is heated above 300oC

Setting Reaction of Gypsum Bonded Investments :

Same as Dental stone.

CaSO4 ½ H2O + ½ H2O CaSO42H2O + 3900 Cal / gmol.

(Ca. Sulfate hemihydrate) (Ca. Sulfate dihydrate)

When calcium sulfate hemihydrate is mixed with water, calcium sulfate

hemihydrate is converted back to calcium sulfate dihydrate which sets to form

a solid mass which binds the silica particles together.

The reaction is exothermic and whenever 1 gm mol of calcium sulfate

hemihydrate is reacted with 1.5 gm mol of water. 1 gm mol of calcium sulfate

dihydrate is formed and 3900 calories of heat are developed.

The microstructure of set material shows; rod like particles of gypsum

intermeshed with large irregular particles of silica refractory.

Setting Time :

It is dependent on the gypsum content and upon the type of gypsum

employed.

Initial setting time : 8 – 15 min

Final setting time : 12-25 min.

This can be altered by addition of K2SO4, NaCl (Increase setting time),

Borax and potassium citrate (decrease setting time).

ADA Specification No.2 ; States that setting time should not be less

than 5 minutes and nor longer than 25 minutes. Modern inlay investments set

initially in 9-18 minutes. This provides sufficient time for mixing and

investing the pattern.

Factors Controlling Setting Time :

Manufacturing process – finer the particle size – faster setting

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Mixing time and rate - mixing time - setting time.

Water / powder ratio - water : powder ratio - setting time

Temperature - temperature - setting time

Modifier – Accelerator and retarders – Accelerates setting time, Retarders

setting time

Properties of Gypsum Bonded Investment :

1. Expansion

2. Contraction

3. Strength

4. Other consideration

1. Expansion : Expansion aids in enlarging the mold. This property of investment is

needed for compensation of casting shrinkage of alloy.

Expansions are of 3 types :

1. Normal setting expansion.

2. Hygroscopic setting expansion

3. Thermal expansion

Normal Setting Expansion :

The setting expansion of an investment, is the linear expansion that take

place during the normal setting of the investment in air. A mixture of silica and

gypsum hemihydrate results in setting expansion greater than that of the

gypsum product when it is used alone. It is because of interference of silica

with the growing crystals.

The silica particles interfere with the intermeshing and interlocking of

the crystals as they form. Thus, the thrust of the crystals is outward during

growth and therefore more effective in the production of expansion.

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ADA Specification No.2 for Type I investment permits a maximum setting

expansion in air of only 0.6%. Modern investments show setting expansion of

approximately 0.4%. It is regulated by retarders and accelerators.

Effect of Wax Pattern on Normal Setting Expansion :

The setting reaction of gypsum bonded investment is exothermic in

nature (3900 cal/gmol). The heat causes expansion of wax pattern leading to

expansion of the mold.

The amount of heat liberated depends on

The ratio of gypsum – gypsum heat - S.E.

W:P Ratio W:P ratio Heat S.E.

B. Hygroscopic Setting Expansion :

Hygroscopic expansion is the linear expansion of the investment that

occurs if the investment is in contact with water from any source during the

setting process, after investing the wax pattern.

Contact with water can be achieved by placing the casting ring in a

water bath, before initial set is complete or by putting some water on the

surface of the investment in the ring or by using a wet liner inside the casting

ring.

Distinguishing between a setting expansion and hygroscopic expansion

is difficult because both take place almost at the same time and end at the same

time. In practice a sum of the hygroscopic and setting expansion of the

investment is obtained, which is six times more than the normal setting

expansion.

Hygroscopic expansion may occur because of

Gypsum

Refractory

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Gypsum :

According to some theories,

Addition of water during the setting of an investment increases the surface

film thickness on the inert particles and gypsum crystals, thereby forming

them apart.

Added water during the setting process permits further hydration of calcium

sulfate, thus causing expansion of the investment.

Added water may force gypsum gel to swell.

Addition of water provides additional volume in to which gypsum crystals

can grow.

Refractory :

The water physically seperates the fine parities of silica by capillary

action leading to expansion of mass. This is reversible in case of absence of

binder. But if binder is present and set, the expansion is retained.

The decreased expansion is affected by :

Amount of silica amount of silica - expansion

W:P ratio W:P ratio Expansion

Time of insertion – Investment immersed in water after initial setting cause

decreased expansion.

ADA Specification No.2 for Type II Investments requires a

Minimum expansion of 1.2%

Maximum expansion of 2.2%

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FACTORS AFFECTING HYGROSCOPIC EXPANSION :

1. Composition : HSE is directly proportional to the amount of silica present.

Finer the particle size of silica : Increased hygroscopic expansion

HSE is greater in hemihydrate than hemihdyrate.

A dental investment should have enough hemihdyrate binder with the

silica to provide sufficient strength after hygroscopic expansion. Otherwise, a

shrinkage occurs during the subsequent drying of the set investment. At least

15% of binder is necessary to prevent a drying shrinkage.

2. Water Powder Ratio : Increased water powder ratio Decreased

Hygroscopic exposure.

3. Spatulation : Increased missing time : Increased hygroscopic expansion.

4. Ratio of Spatulation : Increased spatulation : Increased hygroscopic

expansion.

5. Shelf life of investment : Old investment : Decreased hygroscopic

expansion.

6. Time of investment : Immersion of setting investment.

Before initial setting : Increased hygroscopic expansion

After initial setting : Decreased hygroscopic expansion

7. Effect of confinement : Both the normal and the hygroscopic setting

expansion are confined by opposing forces, such as the walls of the

container in which the investment is placed or the walls of the wax pattern.

The confining effect in the hygroscopic expansion is more pronounced than

the similar effect on the normal setting expansion.

8. Water bath temperature : Although the water bath temperature has little

effect on the hygroscopic expansion of the investment, it has a definite

effect on the wax pattern. At higher water bath temperature, the wax

pattern expands, requiring less expansion of the investment to compensate

for the total casting shrinkage.

In addition higher water bath temperature soften the wax. The softened

wax then gives less resistance to the expansion of the investment, thus

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making the setting and hygroscopic expansion more effective. The net

effect is higher expansion of the mold with higher water bath temperature.

During the setting process, dental casting investments actually absorb water

from their surroundings and expand. If during setting more water, an

investment is permitted to take up from any source, the higher the

hygroscopic expansion, upto a point where further addition of wax do not

create any additional expansion. This degree of expansion or its

corresponding quantity of water is called the critical point.

For an investment to reach its maximum hygroscopic expansion, sufficient

water should be available.

If hygroscopically expanding investments are in contact with less water

than they are able to absorb, but they will not exhibit their maximum

hygroscopic expansion.

9. Amount of water added : More amount of water added during setting period,

more is the expansion.

Thermal Expansion :

In case of gypsum investments, thermal expansion is achieved by

placing the mould in a furnace at a temperature not greater than 700oC. When

the investment ring with the investment is heated two events take place.

a. Gypsum undergoes shrinkage, as it becomes calcium sulfate anhydrate

losing water.

b. Silica undergoes a thermal expansion. Cristobalite contributes more to such

expansion.

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.

When the quartz content of the investment is increased to 60% with the

balance being hemihydrate binder, the initial contraction of gypsum is not

eliminated.

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The contraction of gypsum is entirely balanced when the quartz content is

increased to 75%.

If a sufficient amount of setting expansion had been present, the casting

made at 700oC would have fit the die reasonably well.

Quartz expands to 1.4% at 575oC

Cristobalite expands to 1.6% at 250oC

Cristobalite produce adequate mold expansion at lower temperature

because of the lower inversion temperature of the cristobalite in comparison

with that of quartz. Thus the normal contraction of the gypsum during heating

is easily eliminated.

A reasonably good fit of the casting is obtained when the gold alloy is

cast into the mold at temperature of 500oC and higher.

The amount of thermal expansion of a dental investment depend on it use.

If the hygroscopic expansion is used to compensate for the contraction

of the gold alloy for Type II investments, ADA specification No.2 requires that

the thermal expansion be between 0% and 0.6% at 500oC.

However, for the Type I investments 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 maximal

thermal expansion be attained at a temperature not higher than 700oC. Thus

when a thermal expansion technique is used, the maximum mold temperature

for the casting of gold alloy should be less than 700oC. The gold alloys can

become contaminated at a mold temperature higher than this.

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FACTORS AFFECTING THERMAL EXPANSION :

1. Water Powder Ratio : The magnitude of thermal expansion is related to the

amount of solids present.

W:P ratio thermal expansion.

2. Effect of chemical modifiers : Disadvantage of an investment that contains

sufficient silica to prevent any contraction during heating is that the

weakening effect of silica in such quantities is likely to be too great.

Addition of small amounts of sodium, potassium or lithium chlorides to the

investment.

- Eliminate the contraction caused by the gypsum.

- Increases expansion without the presence of an excessive amount of

silica.

Boric acid has a similar effect

- Increases expansion

- Hardens the set investment

- It apparently disintegrates during the heating of the investment and a

roughened surface on the casting may result.

Silica do not prevent gypsum shrinkage but counter balance it.

Chlorides actually reduce gypsum shrinkage below temperatures of

approximately 700oC.

2. Thermal Contraction : When the investment is allowed to cool from 700oC the refractory and

binder contact (This contraction is because of gypsum when it is first heated).

On cooling to room temperature, the investment exhibits an overall

contraction compared with its dimension before heating.

On reheating to the temperature previously attained, the investment does

not expand thermally to the previous level. Moreover the process of cooling

and reheating causes internal cracks in the investment that can affect the quality

of the casting.

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3. Strength : The strength of the investment must be adequate to prevent fracture or

chipping of the mold during heating and casting of the gold alloy.

The total thermal contraction of the investment is similar to that of the

gold alloy from the casting temperature to room temperature, the contraction of

the investment is fairly constant, until it cools to below 550oC. Thus, when the

alloy is still quite hot and weak, the investment can resist alloy shrinkage by

virtue of its strength and constant dimension.

This can cause distortion and even fracture in the casting if the hot

strength of the alloy is low. Although this is rarely a factor with gypsum-

bonded investments, it can be important with other types of investments.

The compressive strength of the investment mold is a primary factor to

be considered in addition to the expansion when evaluating the dimensional

accuracy of dental castings. Ideally, the investment should have sufficient

expansion to compensate for all of the thermal contraction of the alloy.

However, after burnout of the mold, the strength need be no greater than that

required to resist the impact of the metal entering the mold.

According to ADA Specification No.2, the compressive strength for the

inlay investment should not be less than 2.4 MPa (350 psi) when tested 2 hours

after setting.

Factors Affecting Strength :

1. Increased W:P ratio :decreased compressive strength.

2. Heating the investment to 700oC may increase or decrease strength by 65%.

Greatest reduction in strength on heating is found in investments containing

NaCl.

3. After the investment has cooled to room temperature, its strength decreases

considerably, because of fine cracks that form during cooling.

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4. The use of hemihydrate instead of plaster increases the compressive

strength.

5. The use of chemical modifiers increases the strength because more of

binder can be used without a marked reduction in the thermal expansion.

4. OTHER CONSIDERATIONS :a. Fineness : Fineness of the investment affects its setting time, surface

roughness of the casting.

Fine the silica particle – greater the hygroscopic expansion than a coarse

silica.

Finer the investment – Smaller are the surface irregularities on the casting.

b. 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

backpressure builds up, which prevent the gold alloy from completely

filling the mold. The common method for venting the mold is through the

pores of the investment.

The amount of porosity depends on :

Gypsum crystals. More gypsum crystals are present in the set investment

less porosity.

Therefore lower the amount of hemihdyrate and greater the amount of water

used to mix the investment more porous it becomes.

More uniform the particle size Greater is the porosity.

A mixture of coarse and fine particles Less porosity than an investment

composed of a uniform particle size.

c. Storage : Under conditions of high relative humidity, the setting time may

change. Under such conditions, the setting expansion and the hygroscopic

expansion may be altered so that the entire casting procedure may be

adversely affected. Therefore the investments should be stored in airtight

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and moisture proof containers. During use, the containers should be opened

for short time as possible.

All investments are composed of a number of ingredients each of which

possess a different specific gravity. Therefore there is tendency for these

components to separate according to the specific gravity. This separation may

sometimes influence the setting expansion and other properties of the

investment so it is advisable to purchase the investment in relatively small

quantities.

The investment should be weighed and the water should be measured

according to the proportion of the investment mix. Only in this manner can

one expect to control the setting or the thermal expansion in relation to the

compensation needed for the casting shrinkage and other important properties.

Some manufacturers supply their investment in preweighed packages so that

one needs only to measure the gauging water.

Uses : For casting of inlays, bridges, removable partial denture frame works

using gold alloys and other low fusing alloys.

MODIFIED TYPES OF GYPSUM – BONDED INVESTMENTS :

Hygroscopic Thermal Inlay Casting Investment :

A new inlay casting investment that can be used as a hygroscopic or

thermal type has became available. This investment contains a blend of quartz

and cristobalite as the refractory. When hygroscopic casting technique is used,

the investment is heated to 482oC after setting in accordance with the normal

water immersion technique. When used in the thermal casting technique, the

investment is not immersed in water, but after setting it is heated to 644oC, then

the appropriate expansion is achieved.

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INVESTMENTS USED FOR HIGH TEMPERATURE CASTING PROCEDURE :

Most palladium and base metal alloys used for partial dentures and

porcelain fused-to-metal restoration have high melting temperatures. They are

cast in moulds at 850 to 1100oC.

So, gypsum bonded investments cannot be used, because it disintegrates

at such high temperature. To withstand these high temperatures, the molds

require different types of binders such as silicate and phosphate compound.

The investment used for this purpose are ;

- Phosphate bonded investments

- Silica bonded investments

Phosphate Bonded Investments : The most common type of investment for casting high melting alloys is

the phosphate bonded investment.

Common Brands :

Aurobond, Calsite, Cerafine, Deguvest, DVP, Eurocent, Nirobond,

Roma exalet etc.

Composition :

1. Binder : 20%

It consists of 2 components.

- Acidic part Ammonium diacid phosphate (NH4H2PO4)

- Basic part Magnesium oxide (mgo)

Which react at room temperature to form a phosphate binder

Ammonium magnesium phosphate (NH4MgPO4.6H2O) which gives the

investment green strength / room temperature strength.

2. Refractory : Silica. Either cristoballite / quartz or a mixture of two in a

concentration of approximately 80%. They function as refractory (i.e. to

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provide high temperature thermal shock resistance) and provide thermal

expansion at high temperature.

3. Modifiers :

Carbon – Act as a reducing agent to produce clean casting and facilitate

devesting of the casting from the mould. It is used for high temperature gold

casting alloys. But with silver palladium or base metal alloy, carbon embrittles

the alloy even though the investment is heated to temperatures that burnout the

carbon.

Palladium reacts with carbon at temperatures above 1504oC. Thus if the

casting temperature of a high palladium alloy exceeds this critical point, a

phosphate investment without carbon should be used. s

Mode of Supply :

Powder in packets with special Liquid :

1. Powder contain NH4H2PO4, MgO, Silica, Traces of carbon. This powder

may be mixed with water to form the investment.

2. Special liquid Contain colloidal silica.

Colloidal silica suspensions are available for use with phosphate

investments in place of water. This liquid shows increased setting expansion,

produce significant amount of hygroscopic expansion (as with pure water the

amount of hygroscopic expansion is less) and increases its strength.

For base metal alloys a 33% dilution of colloidal silica is required.

Setting Reaction :

At room temperature ammonium diacid phosphate reacts with

magnesium oxide to give the investment green strength or room temperature

strength.

1. NH4H2PO4 + MgO + 5H2O NH4 MgPO4. 6H2O

(Ammonium diacid phosphate) (Magnesium ammonium phosphate)

The ammonium diacid phosphate is used in a greater amount than is

necessary for this reaction, so that the additional amount can react with silica at

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an elevated temperature. At higher temperature there is probably a superficial

reaction between P2O5 and SiO2 to form silica phosphate, which increases

strength of investment at higher temperature.

Phosphates are quite complex and the reaction is not simple. The

stoichometric (determination of the relative proportion of the compounds

involved in a chemical reaction) quantities are equal molecules of magnesia

and ammonium diacid phosphate, an excess of magnesia is usually present and

some of it is never fully reacted. The product formed is predominantly

colloidal multimolecular (NH4MgPO4.6H2O)n which aggregate around excess

MgO and fillers. After the initial setting reaction, the set colloid undergoes

various thermal reactions on heating.

2. MgO + NH4 H2PO4 + H2O

(NH4MgPO4. 6H2O)n

MgO Colloidal type particles

NH4H2PO4

H2O

Prolonged setting at room temperature ordehydration at 50oC

(NH4MgPO4.6H2O)n

H2O Dehydrated at 160oC

(NH4 MgPO4.H2O)n

Heated from 300 – 650o C

(Mg2P2O7)n Non crystalline polymeric phase

Above 690o C

Mg2P2O7 Crystalline in nature

Heated above 1040oCMg3(P2O4)2

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The final products are crystalline Mg2P2O7 and some excess MgO, along

with essentially unchanged quartz, cristobalite or both. Some Mg3(PO4)2 may

be formed if the investment is grossly overheated or when the molten metal

contacts the mold cavity surface.

Manipulation :

The powder is mixed with a measured amount of liquid using a bowl

and spatula. Hand mixing or mechanical mixing under vacuum can be done.

The mixed material is vibrated into the casting ring.

Setting and Thermal Expansion :

Theoretically, the setting reaction should show shrinkage but in practice

there is slight expansion when colloidal silica solution is used instead of water.

When phosphate investments are mixed with water, they exhibit a shrinkage at

the same temperature range as gypsum bonded investments (200oC to 400oC).

This contraction is practically eliminated when a colloidal silica solution

replaces the water.

The shrinkage is due to decomposition of the binder, magnesium

ammonium phosphate with evolution of ammonia, which is readily apparent by

its odor, but this shrinkage is masked by expansion of cristoballite.

According to ADA specification No.42 there are 2 types of phosphate bonded

investments.

Type I : Inlays, crown and other fixed restorations.

Type II : Partial dentures and other cast removable restorations

Working and Setting Time :

Working time : 2 minutes

Setting time : 1 hour

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Factors Affecting Setting Time :

1. Increased Temperature Fast set

The setting reaction itself is exothermic, and this further accelerates the rate

of setting.

2. Increased mixing time Fast set

Generally mechanical mixing under vacuum is preferred.

3. Increased L : P ratio Increased working time

Miscellaneous Properties :

Compressive Strengths Type I Should not be less than 2.5 MPa

Type II 3.0 MPa

Setting expansion 0.4%

Hygroscopic expansion 0.6 – 0.8%

Thermal expansion With water 0.8%

With special liquid 1.2%

Advantages :

1. High green strength

2. High fired strength – Less mold cracking and few fins on casting.

3. They can withstand temperatures upto 1000oC for short periods of time.

Disadvantages :

1. At temperatures greater than 1375oC Cause mold breakdown and

roughen the surface of casting.

2. Due to high strength devesting is defect.

3. To increase expansion, with use of special liquid Cause less porous mold

Incomplete casting

4. High tendency for reaction with non-precious alloy producing oxides which

is difficult to remove from castings.

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Surfactant Containing Phosphate Bonded Investment :

Addition of surfactants to phosphate bonded investment can increase the

hygroscopic setting expansion. The surfactant also makes the unset investment

more viscous and reduces the compressive strength.

Ethyl-Silicate Bonded Investments :Another type of binding material for investments used with casting high

melting alloys is silicate bonded investments. This investment material are

being used since 1930. But now it is loosing popularity due to complicated and

time consuming procedures involved.

Common Brands :

- Nobilium rapid set (low iron and sodium investment)

- Saddle lock (a ferruginous investment)

- Howmet Vary rapid (intermediate iron and sodium)

Composition :

A. Binder Silica gel that reverts to silica (cristobalite) on heating.

B. Refractory Silica (cristobalite)

C. Additive Magnesium oxide – Make it alkaline, strengthen the gel

D. Wetting agent To reduce accumulation of air bubbles on surface of wax

pattern.

Various methods of producing silica or silicic acid gel binder.

1. pH of sodium silicate is lowered by the addition of an acid or acid salt

Silicic acid gel forms.

2. Aqueous suspension of colloidal silica can be converted to gel by addition

of an accelerator such as ammonium chloride Silicic acid gel.

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Another system for binder formation is based on ethyl silicate.

3 Stages :

Stage I : Hydrolysis :

A colloidal silicic acid is first formed by hydrolyzing ethyl silicate in the

presence of hydrochloric acid, ethyl alcohol and water.

HCl, C2H5OHSi(OC2H5)4 + 4H2O Si(OH4) + 4C2H5OH (Ethyl alcohol) Ethyl silicate Sol of polysilicic acid

Because of the use of polymerized form of ethyl silicate, a colloidal sol

of polysilicic acid is expected instead of simpler silicic acid sol.

Stage II : Gelation

The sol is then mixed with quartz or cristoballite, to which is added a

small amount of finally powdered magnesium oxide to render the mixture

alkaline.

nSi (OH)4 + Mgo Mgo [Si(OH)]n

A coherent gel of polysilicic acid forms, accompanied by a setting

shrinkage.

Stage III : Drying (<168oC)

This soft gel is dried at a temperature below 168oC. During the drying

process, the gel loses alcohol and water to form a concentrated hard gel. A

volumetric contraction accompanies the drying, which reduces the size of the

mold. This contraction is known as “Green Shrinkage” and it occurs in

addition to the setting shrinkage.

This gelatin process is slow and time consuming. An alternative and

faster method for the production of silica gel is employed. Certain types of

amines can be added to the solution of ethyl silicate so that hydrolysis and

gelation occur simultaneously.

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Simultaneous hydrolysis Ethyl silicate + Piperidine Silica gel

(amine) Gelation With an investment of this type, the mold enlargement before casting

must compensate not only for the casting shrinkage of the metal but also for the

green shrinkage and setting shrinkage of the investment.

Mode of Supply :

1. Powder Refractory particles of silica and glass

Calcined MgO

Other refractory oxide

2. Liquid Single liquid of stabilized alcohol solution of silica gel.

or

With 2 liquids

- One bottle contains a properly diluted water soluble silicate solution

- Other bottle contains a properly diluted acid solution such as solution of

hydrochloric acid.

Before use equal volume of each bottle should be mixed and allowed to

stand for a prescribed time according to the manufacturers instruction. So that

hydrolysis can take place and freshly prepared silicic acid formed.

Manipulation :

Powder is added to the hydrolyzed ethyl silicate liquid, mixed quickly

and vibrated into a mold. This allows the heavier particles to settle quickly

while the excess liquid and some of the fine particles rise to the top.

In about 30 minutes The accelerator (NH4Cl) in the powder hardens

the settled part, and the top excess is poured off. Thus the liquid: powder ratio

in the settled part is greatly reduced and the setting shrinkage is reduced to

0.1%.

It is little more complicated than phosphate type in that care must be

exercised in handling and burnout, because flammable alcohol is given off. If

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it is heated high enough, some silica converts to quartz and provides added

expansion.

This type of investment can be heated to 1090oC to 1180oC and is

compatible with the higher fusing alloys. Its low setting expansion minimizes

distortion.

Properties :

Compressive strength Not less than 1.5 MPa

Setting contraction 0 – 0.4%

Thermal expansion 1.5 – 1.8% (This material has only thermal

expansion and no other expansion)

Can withstand high temperature 1090oC to 1180oC

Porosity The particles in the set material are very closely packed leading to

low porosity. Air space / vents must be left in investment to permit escape of

air from the mould.

Advantages :

1. High permeability, yields sharply defined castings.

2. Low setting expansion

3. The investment has more refractory Form smooth castings

4. Low burnout strength results in easy removal of castings and cleaning of

oxides from the castings.

Disadvantages :

1. Limited shelf life of liquid

2. Must wait for substantial period of time, prior to using freshly mixed liquid.

3. Potential of cracking exits during burnout, owing to high thermal

expansion.

4. Very expensive

5. Gives off flammable components during processing.

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Uses :

1. They are mainly used for casting Co-Cr R.P.D. frame work.

2. Accurate casting of Nickel based alloys.

Divestments (By Whipmix Corporation) :It is a combination of die stone and gypsum bounded investment

material. The powder is mixed with colloidal silica.

Properties:

Setting expansion : 0.9%

Thermal expansion : 0.6% (at 977oC)

Advantages : The wax pattern and die are invested simultaneously without

removal of pattern. Useful with gold alloys. (Used for casting minute patterns)

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.

Brazing Investment :ADA Sp. No.93 : Type I – Gypsum bonded dental brazing investment

Type II – Phosphate bonded

Steps :

1) Broken parts are stabilized by sticky wax.

2) The broken parts are then embedded in investment with portion to be solder

is left exposed and free of investment.

They should have low setting and thermal expansion. Particle size is

usually not fine. They possess usually a compressive strength of 2-10 MPa.

CONCLUSION :

It should be emphasized that several investment techniques can produce

comparable results. The dentist or technician should become familiar with

different method and different investment materials. In any technique the

fundamentals of that techniques should be applied.

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