DENTAL INVESTMENTS

INTRODUCTION:

The production of the wax pattern by either direct or indirect method is

followed by the next stage in many dental procedures involves the investment of

the pattern to form a mould. A sprue former is attached to the pattern and the

assemblage is located in a casting ring. Investment material is poured around the

wax pattern whilst still in a fluid state. When the investment sets hard, the wax and

sprue former are by softening and/or by burning out to leave a mould space, which

can be filled with an alloy or ceramic using a casting technique.

In the case of acrylic denture production the baseplate wax is invested in

two-part split mould using dental plaster or stone as investment. But the

investment mould used for casting alloys and some castable ceramics need to be

constructed from a material, which retains its integrity at the casting temperature.

Unmodified dental plasters and stones are not suitable for this purpose.

To obtain a good cast restoration, care should be taken at each step of fabrication

namely impression making, wax pattern fabrication, spruing and investing and

casting. Investing the wax pattern and casting procedure is the most critical step

where there are more chances for making an accurate casting. Therefore it is very

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much essential to know about properties and behaviors of the materials used in

investing and casting procedures for a range of dental appliances and also the

interaction that can occur with casting alloys, when cast into an appropriate mould.

In Lost wax casting molten metal is cast into moulds made of investment

materials. The type of investment material depends on the alloy to be cast. Metal

casting alloys have different melting ranges - only pure metals and alloys of

eutectic composition have a melting point. The melting range of gold casting

alloys (approx 900°C) is lower than that of Co-Cr alloys (approx 1350°C).

Therefore, investment materials used for gold casting alloys are sometimes

different from those used for Co-Cr alloys.

DEFINITION:

Dental casting investment is defined as a material consisting principally of an

allotrope of silica and a bonding agent. The bonding substance may be gypsum

(for use in lower casting temperatures) or phosphates and silica (for use in higher

casting temperatures).

Requirements of investments:

1. The investment should be manipulated easily.

2. The investment should have a good flow after mixing so that it will fill all

the minute regions of the wax pattern within the casting ring.

3. The investment should be capable of reproducing the shape, size and details

recorded in the wax pattern. The accuracy of the casting can be no better

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than the accuracy of the mould. [The investment material should be of a

suitable consistency for adaptation to the wax model and have a reasonable

setting time.]

4. The investment should properly wet the surface of the wax pattern.

5. The investment should be compatible with wax pattern and the casting

alloys or castable ceramics.

6. The investment should form a smooth mould surface. The fine particles of

material should be used.

7. The investment should have a reasonable setting time between 5 to 25

minutes.

8. Since casting is carried out at high temperatures, often excess of 1000oC,

the investment mould should be capable of maintaining its shape and

integrity at these elevated temperatures. It must not crack upon heating. [To

withstand the temperatures required for the casting process there should be

no distortion; no decomposition; the investment should not fragment or

disintegrate under the impact of the molten metal.]

9. The investment should have a sufficiently high value of compressive

strength at the casting temperature so that it can withstand the stresses set

up when the molten metal enters the mould.

10. The investment should have good expansion to compensate for the casting

shrinkage. The alloy casting undergoes considerable contraction when

cooled from the casting temperature to room temperature. Such

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contractions may result in a casting with poor fit, for example loose fitting

or short of the margins. This generally achieved by a combination of setting

expansion during the hardening of the investment and the thermal

expansion during the heating of the mould to the casting temperature. [An

investment material to be used for a casting mould should expand on setting

and heating to compensate for the shrinkage of molten metal as it

solidifies.]

11. The investment should have sufficient permeability to allow air to be

displaced from the mould while casting. Uniform sized powder particles are

preferred.

12. It should be easily separable from the casting after the completion of the

casting procedure.

13. It should not contain any chemicals that may react with the metal.

14. It should not discolor the casting.

15. It should not be bio-hazardous.

16. It should have a long shelf life.

17. It should be economical.

According to ADA specification No.2 for casting investments for dental

gold alloys are of three types of determined by whether the appliance is to be

fabricated is fixed or removable and the method of obtaining the expansion

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required to compensate for the contraction of the molten gold alloy during

solidification.

Type I – are used for the casting of inlays or crowns, and the compensation

of casting shrinkage is principally by thermal expansion of the investment.

Type II – are also used for the casting of inlays or crowns but major mode

of compensation is by the hygroscopic expansion of the investment.

Type III – are used in the construction of partial dentures with gold-alloys.

Gypsum-Bonded Investment

The essential ingredients are :

1. α-hemihydrate of gypsum and

2. A form of silica.

α-hemihydrate gives greater strength to the material and acts as a binder to

hold the other ingredients together and provide rigidity. Although depends on

amount of binder – may contain 25% - 45% and is used for alloy with melting

ranges below 1000°C (i.e., gold-containing).

When heated to the required temperatures it shrinks considerably and

frequently fractures all form shrink considerably after dehydration between 200°C

and 400°C. A slight expansion then occur between 400°C and approximately

700°C, and then a large contraction occur. This is most likely carried by

decomposition and sulfur gases such as sulfur dioxide are emitted which

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contaminates the castings (with the sulfides of non habit allogest elements such as

silver and copper). Thus not to be heated above 700°C.

α-hemihydrate requires less mixing water and shrinks less.

Silica

Added to provide a refractory during the heating of the investment and to

regulate the thermal expansion.

It exists in an allotrophic form.

1. Quartz

2. Tridymite

3. Cristobalite and

4. Fused quartz.

When heated a change in crystalline form occurs at a transition

temperatures, characteristics of the particular form of silica.

- When heated quartz invasion from a ‘low’ form -quit to high form to quartz

at 570°C.

- Cristobalite undergoes – between 200°C-270°C from –cristobalite.

- Tridymite – 117°C to 163°C.

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- -allotropic form are stable only above the transition temperature, inversion to

the lower form occur on cooking in each case. The density decrease as the -

changes to -form resulting increase in volume.

Fused quartz is amorphous and glucobites in character exhibits inversion at

any temperature below its fusion points has an extremely low coefficient of

thermal expansion and is of little use in dental expansion.

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

dental investment.

Modifiers

Such as coloring matter reducing agents such as carbon powdered copper to

provide a non-oxidizing atmosphere in the mold when the gold alloy is cast.

Some of the added modifiers such as toxic acid and sodium chloride not

only regulate setting expansion and the setting time, but also prevent most of the

shrinkage of gypsum when it is heated above 300°C.

Setting Time

According ADA specifications No.2 for dental alloy casting, S.T. should

not be shorter than 5 minutes nor longer than 25 minutes the modern inlay

investments set initially in 9-18 minutes.

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Normal Setting Expansion

A mixture of silica and gypsum hemihydrate results in setting expansion

greater than that of the gypsum product when it is used alone. The silica particles

probably interfere with the inert washing and interlocking of the crystals as they

form. Thus the thrust of the crystals is outward during growth and they increase

expansion.

ADA specification No.2 for type I invest permits maximum setting

expansion in air of only 0.6%, that of modern invest is approximately 0.4%.

The purpose of setting expansion is to aid in enlarging the mold to

compensate partially for the casting shrinkage of the gold.

The effectiveness of the setting expansion in enlarging the mold containing

the wax pattern may be related to the thermal expansion of the pattern caused by

the heat of reaction that occurs coincidentally with the setting of the investment. It

follows from such a theory that the setting expansion and effective only to the

extent that the exothermic heat is transmitted to the pattern.

The amount of heat present depends on the gypsum content of the

investment ; therefore the setting expansion of the invest with comparably high

content of gypsum more effective in enlarging the mold than is a product with a

lower gypsum content. Likewise manipulative conditions that increase the

exothermic heat increase the effective setting expansion, (eg, the lower the water

powder ratio for the investment, the greater is the effective setting expansion).

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Other variables are:

As the investment sets, it eventually gains sufficient strength to produce a

dimensional change in the wax pattern as setting expansion occurs.

The inner wall of the investment within a MOD wax pattern can actually

force the proximal walls outward to a certain extent. If the pattern has a thin wall

then the effective setting expansion, is somewhat greater than for a pattern with

thicker walls because the investment can move the thinner wall more readily. Also

the softer wax, the greater the effective setting expansion because the softer wax is

more readily moved by the expanding investment.

Hygroscopic Setting Expansion

The hygroscopic setting expansion differs from the normal setting

expansion in that it occurs when the gypsum product is allowed to set under or in

contact with water and that it is greater in magnitude than the normal setting

expansion.

This is related to the additional crystal growth permitted and not to any

differences in chemical reaction.

In normal setting condition, the water around the particle is reduced by the

hydration and the particle are brought more closely together by the surface tension

action of the water.

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In hygroscopic reaction the setting is taking place under water, the water of

hydration is replaced and the distance between the particles remain same.

As the crystals of dehydrate grow they contact each other and the setting

expansion begins in normal setting reaction the crystals being inhibited become

intermershed and entangled much sooner than those on hygroscopic reaction

which grow much more freely during the early stage before the intermeshing

finally prevents with further expansion, the hygroscopic setting expansion is one

of the methods for expanding the casting mold to compensate for the casting

shrinkage of the gold alloys.

Commercial investments exhibit different amounts of wax expansion. ADA

specification No.2 for such type II investments requires a minimum setting

expansion in water of 1.2%, the wax expansion permitted is 2.2%. the factors

controlling hygroscopic expansion.

Effect of Composition

Proportional to the silica content of the investment the fine the particle size

of silica the greater hygroscopic expansion -hemihydrate produce more with

silica than pH. Should have enough binder with silica, at least 15% of binder is

necessary to prevent and drying shrinkage.

Effect of the Water / Powder Ratio (W:P)

Higher the W:P ratio less the hygroscopic expansion.

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Effect of Spatulation

- Mixing time is reduced hygroscopic expansion decreased.

- Older the inert lower is the setting expansion.

Effect of Time of Investment

The greater amount of hygroscopic expansion is observed if the immersion

take place before the initial set, the longer the immersion of the investment in the

water both is delayed beyond the time of the initial set of the invest. The lower is

the hygroscopic expansion.

The effect of confinement

Both the normal and hygroscopic setting expansion are confined by

exposing forces such as walls of the container in which the investment is placed on

the walls of a wax pattern, the confining effect on the hygroscopic expansion is

much more pronounced than the normal setting expansion.

The increase in the effective setting expansion when the investment is

immersed in a 38°C water bath is caused mainly by the softening of the wax

pattern at the water bath temperature permitting an increase in effective setting

expansion, softened conditions of wax reduces its confining effect on the

expansion of the setting expansion.

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Effect of the Amount of Added Water

The magnitude of the hygroscopic setting expansion can be controlled by

the amount of water than is added to the investment.

Magnitude is in direct proportion to the amount of water added during the

period until a maximum expansion occurs also further expansions to evident

regardless of any amount of water added, the hygroscopic setting expansion is a

continuation of the ordinary setting expansion because the immersion water

replaces the water of hydration and thus prevents the confinement of the growing

crystals by the surface tension of the excess water. Because of the diluent effect of

the quartz particles the hygroscopic expansion in these invest is greater than that of

the gypsum binder when used alone.

The phenomenon is purely physical, the hemihydrate binder is not

necessary for the hygroscopic expansion. Investment with other binder exhibit

similar expansion when allowed to set under water. 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

particles to separate, creating an expansion. Any water insoluble powder that is

wettable can be mixed with hemihydrate and hygroscopic expansion results. The

greater the amount of silica or inert filler the more easily the added water can

diffuse thus the setting material and the greater is the expansion.

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The term hygroscopic is a misnomer, although the added water may be

drawn into the setting material by capillary action, the effect is not related to

hygroscopy.

On the basis of theory the hygroscopic expansion is a normal phenomenon

as that which occurs during normal set expansions the terms have gained general

acceptance by usage.

Thermal Expansion

The thermal expansions of a gypsum bonded investment is directly related

to the amount of silica present and to the type of silica employed, the contraction

of the gypsum is entirely balanced when the quartz content is immersed to 75%.

The thermal expansion curves of the quartz is influenced by particle size of the

quartz, the type of the gypsum binder and the resultant water powder ratio

necessary to provide a workable mix.

Much greater expansion occurs during the inversion of cristobalite, the

normal contraction of the gypsum during heating is easily eliminated. The

expansions occurs at a lower temperature because of the lower inversion

temperature.

Investments containing cristobalite expand earlier and to a greater extent

than those containing quartz.

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ADA specifications no.2 requires that the thermal expansion must be not

(0-66%) less than 1% nor greater than 1.6%.

Maximum thermal expansion is obtained at a temperature not higher than

700°C.

W:P Ratio

More water that is used in mixing the investment 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 libuim chlorides to

the investments eliminates the contraction caused by the gypsum and increase the

expansion without the presence of excessive silica.

Strength

The strength of the investment must be adequate to prevent fracture or

chipping of the mold during heating and casting gold alloy. When the alloy is still

quite hot and weak the investment and resist alloy shrinkage by strong and

constant dimension. After burnout of the pattern (mold), the strength need be no

greater than that required to resist the impact of the metals containing the mold.

ADA specifications no.2 the compressive strength for the inlay investments

should not be less than 2.4 Mpa for gypsum.

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Other Gypsum Considerations

Investments fineness affect the setting time, the surface roughness of the

casting, a fine silica results in higher hygroscopic expansion.

Porosity

As the molten metal enters the mold, the air must be forced out ahead of it.

If not a back pressure builds up to prevent the gold alloy from completely filling

the mold, the common method for venting the mold is though pores of investment,

the more gypsum crystals, the less is its porosity lower the hemihydrates content

and the greater the amount of gauging water used to mix, the more porous it

becomes.

More uniform the particles size, the greater the porosity.

Storage

Phosphate Bonded Investment

The rapid growth of use of metal ceramic restorations and the increased use

of higher melting alloys have resulted in an increased use of phosphate or silica

bonded investment.

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Composition

Consists of refractory fillers and binder, the filler is silica, in the form of

cristobalite, quartz or a mixture of two – 80% concentration approximately. The

purpose of silica is to provide high temperature thermal shock resistance and a

high thermal expansion.

The binder consists of magnesium oxide (basic) and a phosphate that is acid

in nature.

Originally phosphoric acid was used, but mono ammonium phosphate has

replaced it, because it can be incorporated into the powdered investment.

Newer gold-containing alloys and other alloys used for metal ceramic have

higher melting temperature ranges and then contraction during solidification is

also greater. This necessitate greater expansion, can be achieved by using colloidal

silica suspensions with the phosphate investments, in place of water colloidal

silica liquid suspension freeze, should be assessed before winter, freeze solid act

low temperature.

Some are made to be mixed with water, for predominantly base metal

alloys, a 23% dilution of the colloidal silica is required.

Carbon is often added to the powder to produce clear castings and

facilitates the divesting of the casting from the mold, appropriate when the casting

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alloys is gold not with silver containing and base metal alloys. It is believed

carbon embrittles the alloys.

Latest evidence palladium reacts with carbon if heated above 1504°C in

this case investment without carbon should be used.

Setting and Thermal Expansion

There is a slight expansion during the reaction compared to gypsum

products, and this can be increased considerably by using a colloidal silica

solution instead of water.

When phosphate investments are mixed with water this exhibit a shrinkage

within essentially the same temperature range as gypsum inert (200°C-400°C). this

contraction is practically eliminated when a colloidal silica solution replaces.

(Some users of phosphate bonded) expansion can be decreased by the

increasingly the liquid : powder reaction ratio also by decreasing the

concentration of the special liquid or by they may use a combination of these

methods.

Working and Setting Time

Phosphate investments are markedly affected by temperature. The normal

the mix, the faster it sets the setting reaction itself gives off heat (this itself gives

heat) and this further accelerates the rate of setting.

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Increased mixing time and mixing efficiency results in a faster set and a

greater rise in temperature. The ideal technique is to mix as long as possible yet

have just enough time for investing. Mechanical mixing under vacuum is

preferred.

Ethyl-Silicate – Bonded Investments

Involves more complicated and time consuming procedures involved. Used

in the construction of the high fusing base metal palladium alloys.

The binder is a silica gel, that reverts to silica cristobalite on heating.

Several method may be used to produce the silica or silicic acid gel binder.

When the pH of sodium silicate is lowered by the addition of an acid salt, a

bonding silicic acid gel forms. The condition of magnesium oxide strengthen the

gel. An aqueous suspension of colloidal silica can be converted to a gel by the

addition of an accelerator, such as ammonium chloride.

Another system for binder formation is based on ethyl silicate. A colloidal

silicic acid is first formed by hydrolyzing ethyl silicate in the presence of

hydrochloric acid, ethyl alcohol and water. The solution is then mixed with the

quartz or cristobalite to which is added a small amount of finely powdered

magnesium oxide to render the mixture alkaline. A coherent gel of polysilicic acid

then forms accompanied by shrinkage. The soft gel is dried at a temperature below

168°C. During the drying process, the gel losses alcohol and water to form a

concentrated hard gel, a volumetric contraction accompanies the drying which

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reduces the size of the mold. This contraction is known as “green shrinkage”, and

it occurs in addition to the setting shrinkage.

The gelation process is slow and time consuming certain types of amines

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

simultaneously.

Methods of Altering Die Dimensions

To reduce the setting expansion of the type-IV die stone to less than 0.1%

there by reducing diameter additional accelerator (potassium sulfate) and retarder

(borax) can be added to the gauging water.

To produce relief space for cement, die spacer can be used with a stone die,

the most common die spacers are resins. Although proprietary point on liquids are

sold for this purpose, model paint, colored nail polish or thermoplastic polymers

dissolved in volatile solvents enjoy wide spread popularity.

These spacers are applied in several coats to within 0.5mm of the

preparation finish line to provide relief for the cement luting agent and to ensure

complete seating of an otherwise precisely fitting casting.

Casting Ring Liners

With the use of solid metal rings or casting flasks, the mold may actually

become smaller rather than larger because of the reverse pressure resulting from

the confinement of the setting expansion.

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This effect can be overcome by using a split ring on flexible rubber ring

that permits the setting expansion of the investment.

The most commonly used technique to provide investment expansion is to

line the walls of the ring with ring liner.

Traditionally, (earlier) asbestos was the material of choice, no longer be

used because of its carcinogenic potential.

Two types of non-asbestos ring liner used are aluminium silicate ceramic

liner and a cellulose (paper) liner.

To ensure uniform expansion, the liner is cut to fit the inside diameter of

the casting ring with no overlap. The cut liner is added in position with sticky wax

and then is used with a dry or wet, with a wet liner technique the liner ring is

immersed in water for a time and the excess water is shaken away.

Squeezing the liner should be avoided because this leads to variable

amounts of water removal and uneven expansion.

Ceramic liner doesn’t absorb water like a cellulose liner, its network of

fibres can retain water on the surface.

In the liner the absorbed water causes a semihygroscopic expansion as it is

drawn into the investment during setting. A thicker liner material or two layers of

liner provide even greater semihygroscopic expansion and also affect a more

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unrestricted normal setting expansion of the investment in any case, the thickness

of the liner should not be less than approximately 1mm.

The length of the liner remains a matter of controversy. If the liner is

shorter than ring, the investment is confined at one or both end of the ring, the

longitudinal setting and hygroscopic expansion are thereby restricted as compared

with the end where the liner is flush with the ends of the ring.

The expansion of the investment is always greater in the unrestricted

longitudinal direction than in the lateral direction that is toward the ring itself.

Therefore it is desirable to reduce the expansion in the longitudinal direction.

Placing the liner somewhat shorter of the end of the ring tends to provide a

more uniform expansion; thus there is less chance for distortion of the wax pattern

and the mold.

Investing Procedure

The wax pattern should be cleaned of any debris, grease or oils. A

commercial wax pattern cleaner or a diluted synthetic detergent is used. Any

excess liquid is shaken off and the pattern is left to air dry while the investment is

being prepared. The thin film of cleaner left on the pattern reduces the surface

tension of the wax and permits better “wetting” of the investment to ensure

complete coverage of the intricate portions of the pattern.

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While the wax pattern cleaner is air drying, the approximate amount of

distilled water (gypsum investment) or colloidal silica special liquid (phosphate

investment) is measured. The liquid is added to a clean dry mixing bowl, and the

powder is gradually added to the liquid care should be taken to minimize air

entrapment, mixing be started gently until all the powder has been wet, or the

unmixed powder may inadvertantly be ejected from the bowl. Hand mixing is an

option.

It is far more common place to mechanically mix all casting investments

under vacuum.

Vacuum Mixing

Mechanical mixing under vacuum removes air bubbles created during

mixing and eliminates potentially harmful gases produced during chemical

reaction of the high heat investment.

Once the mixing is completed, the pattern may be hand invested or vacuum

invested. For investing by hand, the entire pattern is painted (inside and out) with a

thin layer of investment. The casting ring is positioned on the crucible former, and

the remainder of the investment is vibrated slowly into the ring, with vacuum

investing, the same equipment used to mix the investment is employed to invest

the pattern under vacuum.

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Amount of porosity in vacuum investment is reduced the texture of the cast

surface is smoother with better detail reproduction and tensile strength also

increases.

In one study it has found 95% of vacuum invested castings were free of

nodules where as 17% castings made in hand investment molds were entirely free

of defects.

Air bubbles that are remain in the mix, can be entraped on flat or concave

surfaces that are not orientated suitably for air evacuation tilting the ring slightly

aids in releasing these bubbles so they can rise to the surface.

Excessive vibration is to be avoided it can cause solids in investments to

settle and may lead to free water illumination adjacent the wax pattern. Resulting

surface roughness. Excessive vibration may also dislodge small pattern from the

sprue former with miscast.

If the hygroscopic technique is employed, the filled casting ring is

immediately placed as 37°C water bath with crucible former side down. For high

heat expansion, the invested ring is allowed to bench cool undisturbed for the time

recommended by the manufacturers.

Compensation for Shrinkage

A number of factors influence the mold size:

1. Two liners allows a greater setting and thermal expansion than does a single

liner.

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2. Setting thermal and hygroscopic expansion can be controlled to a certain extent

by varying the liquid : powder ratio of the investment.

3. Lower the L:P ratio greater the potential for expansion, thinner mixes reduces

the expansion.

With some investment minor adjustments with L:P ratio is insignificant.

There is a limit to which L:P can be altered if it is too thick, it can’t be

applied to the pattern without distorting the pattern and producing air voids. If the

mixture is too thin, a rough surface on the casting may result.

In controlling hygroscopic expansion along with L:P ratio can also be

regulated either by reducing the time of immersion of the setting investment or by

controlling the amount of water to be added during the setting process.

The longer the delay before immersion in the water bath, the less the

hygroscopic expansion that occurs.

Increasing the burnout temperature and the water bath temperature

increases the expansion and vice versa.

CONCLUSION:

As Prosthodontists, our aim is to make a restoration as accurate as possible.

Most of the restorations what we are making are cast restoration and that is why

we should have knowledge about various materials and techniques used in casting

a dental restoration. Investment materials are to be selected based on the type of

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restoration, the type of metal or alloy to be casted, as well as technique used for

casting and that is how we can achieve a better restoration.

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