CASTING PROCEDURES

CASTING: casting is the process by which the wax pattern of a restoration is

converted to a replicate in a dental alloy. The casting process is used to make dental

restorations such as inlays, onlays, crowns, bridges and removable partial dentures.

CASTING PROCEDURE: It involves both the clinical and the laboratory steps:

Step 1: Mouth preparation

Step 2: Direct wax pattern is done on the tooth or the impression of the prepared tooth

is taken and the die is made for indirect wax pattern.

Step 3: Preparing the wax pattern for investing.

Spruing the pattern:

The sprue former or the sprue pin acts as the channel or passage for the entry of the

liquid metal into the mold in an investing ring after wax elimination. The diameter

and length of the sprue former depend to a large extent on the dimensions of the flask

or ring in which the casting is to be made.

Sprue former gauge selection is often empirical, is yet based on the following five

general principles:

1. Select the gauge sprue former with a diameter that is approximately the

same size as the thickest area of the wax pattern. If the pattern is small, the sprue

former must also be small because a large sprue former attached to a thin delicate

pattern could cause distortion. However if the sprue former diameter is too small this

area will solidify before the casting itself and localized shrinkage porosity may result.

2. If possible the sprue former should be attached to the portion of the

pattern with the largest cross-sectional area. It is best for the molten alloy to flow

from the thick section to the surrounding thin areas. This design minimizes the risk of

turbulence.

3. The length of the sprue former should be long enough to properly

position the pattern in the casting ring within 6mm of the trailing end and yet short

enough so the molten alloy does not solidify before it fills the mold.

4. The type of sprue former selected influences the burnout technique used.

It is advisable to use a two-stage burnout technique whenever plastic sprue formers or

patterns are involved to ensure complete carbon elimination, because plastic sprues

soften at temperatures above the melting point of the inlay waxes.

5. Patterns may be sprued directly or indirectly. For direct sprueing the

sprue former provides the direct connection between the pattern area and the sprue

base or crucible former area. With indirect spruing a connector or reservoir bar is

positioned between the pattern and the crucible former. It is common to use indirect

spruing for multiple stage units and fixed partial dentures.

Reservoir:

Reservoir is the piece of wax that is attached to the sprue former approximately 1mm

from the pattern as an added precaution to prevent localized shrinkage porosity. When

the liquid metal in the mold solidifies first and shrinks the liquid metal in the reservoir

will flow into the mold and thus overcomes that shrinkage. Reservoir is necessary

only with sprue formers of very small diameter.

Wax pattern removal:

Sprue former can be used to remove the pattern. If not the pattern is removed with a

sharp probe. Then the sprue former is attached to it. The pattern should be removed

directly in line with the principle axis of the tooth or the prepared cavity. Any rotation

of the pattern will distort it. Hollow sprue pin is advisable because of its greater

retention to the pattern.

Crucible former:

It is also known as the sprue base. It is like a stand to hold the sprue former along with

the pattern within the casting ring while the pattern is being invested with the

investment material. The shape of the crucible former is such that when it is removed

after the investment is set it forms a funnel like shape which is most suitable to pour

liquid metal into it. Crucible former can be made of metal, rubber or resin.

Casting ring: It is a hollow tube fitted over the crucible former encircling the wax

pattern to a height of ¼” or so above the edge of the pattern. The ring and the crucible

former provide a seal and so the investment material can be poured inside the ring to

surround the wax pattern and sprue former.

Casting ring liners:

For the setting and hygroscopic expansion of an investment to take place more

uniformly, some allowance must be made for the lateral expansion of the investment.

Solid rings do not permit the investment to expand laterally during the setting,

hygroscopic and the thermal expansions of the mold. To overcome this lateral

restriction a liner is placed inside the ring. With the metal casting ring is used it must

be lined with a liner of moistened paper made of glass fibre. This liner provides a

cushion for the hardening investment material to expand into, during the setting

reaction. The ceramic paper liner is cut to fit the inside of the metal ring and is and

held in place with the fingers. The ring containing the liner is then soaked into the

water until it is completely wet. The liner is moistened because a dry liner would

absorb water from the investment and minimize the setting expansion.

The liner is done in two layers inside the ring, and the thickness must be not less than

1mm, so that ring can accommodate more expansion. The liner is placed somewhat

short of ends of the ring to enable the investment to obtain a grip and provide a seal.

And this also restricts the longitudinal expansion, so that a more uniform expansion

takes place and less distortion of the wax pattern.

Step 4: INVESTING

Mixing investment with distilled water is done according to the manufacturers ratio in

a clean dry bowl without entrapment of the air into the mix.

Mixing methods:

a. Hand mixing and the use of the vibrator to remove air bubbles.

b. Vacuum mixing- This is the better method because it removes air bubbles as

well as gases that are produced and thus produces a smoother mix.

Methods of investing:

a. Hand investing

b. Vacuum investing

Hand investing:

First the mixed investment is applied on all the surfaces of the pattern with a soft

brush. Blow off any excess investment gently, thus leaving a thin film of investment

over the pattern, then apply again.

Then the coated pattern can be invested by two methods;

1. Placing the pattern in the ring first and then filling the ring full with investment.

2. Filling the ring with the investment first and then force the pattern through into it.

Vacuum investing :

Vacuum investing unit: This consists of the chamber of small cubic capacity from

which air can be evacuated quickly and in which casting ring can be placed.

Evacuation of air can be done by electrically or water driven vacuum pump.

Procedure:

The ring filled with investment is placed in the vacuum chamber. Air entry tube is

closed. Then the vacuum is applied. The investment will rise with froth vigorously for

about 10-15 sec and then settles back. This indicates that air has been extracted from

the ring. The pressure is now restored to atmospheric by opening the air entry tap

gradually at first and then more rapidly as the investment settles back around the

pattern. Then the ring is removed from the chamber and the investment is allowed to

set. Modern investing unit does both mixing and investing under vacuum and is

considered better than hand mixing and pouring.

Then there are two alternatives to be followed depending upon what type of

expansion is to be achieved in order to compensate for metal shrinkage. They are:

1. If hygroscopic expansion of the investment is to be achieved then immediately

immerse the filled ring in water at the temperature of 37C.

Or “under controlled water adding technique”. A soft flexible rubber ring is used

instead of usual lined metal ring. Pattern is invested as usual. Then specified amount

of water is added on top of the investment in the rubber ring and the investment is

allowed to set at room temperature. In this way only enough water is added to the

investment to provide the desired expansion.

2. If thermal expansion of the investment is to be achieved, then investment is

allowed to set by placing the ring on the bench for 1 hour or as recommended by the

manufacturer.

Step 5: WAX BURNOUT AND HEATING THE RING

After the investment has set hard, the crucible former and the metal sprue former is

removed carefully, and any loose particles at the opening of the sprue hole are

removed with small brush.

The purpose of the wax burnout is to make room for the liquid metal. The ring is

placed in the oven at 250C with the sprue end down, thus allowing the melted wax to

flow, out for 30min or even up to 60min may be a good procedure to ensure complete

elimination of the wax and the carbon.

Heating the ring: The object is to create a mold of such dimension, condition and

temperature so that it is best suited to receive the metal.

Hygroscopic Low-Heat Technique.

After the wax elimination the temperature of the same furnace can be set to a higher

temperature for heating or else, the ring can be transferred to another furnace, which

has already set to the higher temperature. In any case accurate temperature control is

essential and therefore these furnaces have pyrometer and thermocouple arrangement.

The ring is placed in the furnace with the sprue hole down and heated to 500C and

kept at this temperature for 1 hour. In this low heat technique the thermal expansion

obtained is less but together with the previously obtained hygroscopic expansion the

total expansion amounts to 2.2 percent, which is slightly higher than what is required

for gold alloys.

So this technique obtains its compensation expansion from three sources:

(1) The 37º C water bath expands the wax pattern

(2) The warm water entering the investment mold from the top adds some

hygroscopic expansion

(3) The thermal expansion at 500' C provides the needed thermal expansion.

High-Heat Thermal Expansion Technique.

After the wax elimination, the ring should be placed in the furnace which is at room

temperature and then the temperature is gradually raised, until it comes to 700C in 1

hour. Then the ring is heat soaked at this temperature for ½ hour. This slow rise in

temperature is necessary to prevent

This approach depends almost entirely on high-heat burnout to obtain the required

expansion, while at the same time eliminating the wax pattern. Additional expansion

results from the slight heating of gypsum investments on setting, thus expanding the

wax pattern, and the water entering the investment from the wet liner, which adds a

small amount of hygroscopic expansion to the normal setting expansion.

Step 6: CASTING THE METAL

Casting Machines:

Alloys are melted in one of the three following ways, depending on the available'

types of casting machines:

Centrifugal Casting Machine.

This method makes use of centrifugal force to thrust the liquid metal into the mold.

The aim is to force the liquid metal under sufficient pressure, so that the pressure can

be maintained for at least four seconds after the metal has been cast. Pressure is

necessary because the liquid with high surface tension will not enter the mold on its

own.

Centrifugal casting machine also known as broken arm casting machine has an arm

which is supported in the middle by a rotating spindle. One side of the arm has the

weights to balance the machine. Other side of the arm has crucible to melt the metal

and an arrangement to hold the casting ring. The spindle is spring loaded.

Procedure:

1. The force exerted by the machine is adjusted by turning 3-4 turns of the arm to

wound the spring and kept in that wounded position with the help of a stop

rod.

2. Balancing the machine should have been done before the ring is heated by

placing the ring on the casting machine so that the arm is balanced to

compensate for the weight of the ring and the investment.

3. Preheating the alloy to its melting point is done by using the reducing zone of

the torch flame in ceramic crucible attached to the broken arm of the casting

machine. Use of reducing zone only is

necessary to avoid carburization of the metal and because it is the hottest part

of the flame. Reducing zone is blue in color.

During the heating of the alloy reducing flux such as borax is sprinkled over the

alloy as soon as it is hot enough for the flux to adhere to it. Applying flux removes

the oxide skin on the surface of the alloy and reduces its surface tension so that the

liquid metal becomes fluidy.

4. Then the ring is immediately taken out of the heating furnace and place firmly

against the back plate of the machine. Then the crucible is moved up against

the sprue hole end of the ring. The crucible also has a hole in it. Thus both the

holes are up against each other.

5. The alloy is reheated again until it spins, and looks bright red hot (1100C)

with shiny mirror like surface. This indicates its proper fusion.

6. At this stage torch flame is removed and arm of the machine is released by

dropping the stop rod simultaneously.

The machine begins to spin and stops on its own. This act will throw the metal

through the hole and directly through the sprue hole into the mold cavity, in the

investment material.

Two things are important during this final step—one is metal must be in full liquid

state- that means flame must be held at the metal until the arm of the machine is

released.

Secondly, there must be enough rotational force to fill the mold cavity quickly

before the metal solidifies in the sprue area.

As the metal fills the mold there is a hydrostatic pressure gradient develops along the

length of the casting. The pressure gradient from the tip of the casting to the bottom

surface is quite sharp and parabolic in form, reaching zero at the button surface.

Ordinarily, the pressure gradient at the moment before solidification begins reaches

about 9.21 to 0.28 MPa (30 to 40 psi) at the tip of the casting. Because of this

pressure gradient, there is also-a gradient in the heat transfer rate such that the greatest

rate of heat transfer to the mold is at the high pressure end of the gradient (i.e., the tip

of the casting). Because this end also is frequently the sharp edge of the margin of a

crown, there is further assurance that the solidification progresses from the thin

margin edge to the button surface.

Electrical Resistance-heated Casting Machine.

In this instance there is an auto-6iatic melting of the metal in a graphite crucible

within a furnace rather than by use of a torch flame. This is an advantage, especially

for alloys such as those used for metal -ceramic restorations, which are alloyed with

base metals in trace amounts that tend to oxidize on overheating.

Another advantage is that the crucible in the furnace is located flush against the

casting ring. Therefore, the metal button remains molten slightly longer, again

ensuring that solidification progresses completely from the tip of the casting to the

button surface. A carbon crucible should not be used in the melting of high palladium

or palladium-silver alloys, where the temperature exceeds 1504' C or with nickel-

chromium or Cobalt-Chromium base metal alloys.

Induction Melting Machine.

With this unit, the metal is melted by an induction field that develops within a

crucible surrounded by water-cooled metal tubing. Once the metal reaches the casting

temperature, it is forced into the mold by air pressure, vacuum, or both, at the other

end of the ring. The device has become popular in the casting of jewelry but has not

been used as much as the other two techniques for noble alloy castings. It is more

commonly used for melting base metal alloys.

There is little practical difference in the properties or accuracy of castings made with

any of the three types of casting machines. The choice is a matter of access and

personal preference

Casting Crucibles.

Generally, three types of casting crucibles are available: clay, carbon, and quartz

(including zircon-alumina). Clay crucibles are appropriate for many of the crown and

bridge alloys, such as the high noble and noble types. Carbon crucibles can be used

not only for high noble crown and bridge alloys but also for the higher-fusing, gold-

based metal-ceramic alloys.

Quartz crucibles are recommended for high-fusing alloys of any type. They are

especially suited for alloys that have a high melting range and are sensitive to carbon

contamination. Crown and bridge alloys with a high palladium content, such as

palladium-silver alloys for metal-ceramic copings, and any of the nickel-based or

cobalt-based alloys are included in this category.

Step 7: CLEANING THE CASTING

After the casting has been completed, the ring is removed and quenched in water as

soon as the button exhibits a dull-red glow. Two advantages are gained in quenching:

(1) the noble metal alloy is left in an annealed condition for burnishing, polishing, and

similar procedures, and

(2) when the water contacts the hot investment, a violent reaction ensues. The

investment becomes soft and granular, and the casting is more easily cleaned.

Often the surface of the casting appears dark with oxides and tarnish. Such a surface

film can be removed by a process known as pickling, which consists of heating the

discolored casting in an acid. Probably the best pickling solution for gypsum-bonded

investments is a 50% hydrochloric acid solution. The hydrochlo ric acid aids in the

removal of any residual investment as well as of the oxide coating.

The disadvantage of the use of hydrochloric acid is that the fumes from the acid are

likely to corrode laboratory metal furnishings. In addition, these fumes are a health

hazard and should be vented via a fume hood. A solution of sulfuric acid is more

advantageous in this respect. Ultrasonic devices are also available for cleaning the

casting, as are commercial pickling solutions made of acid salts.

The best method for pickling is to place the casting in a test tube or dish and to pour

the acid over it. It may be necessary to heat the acid, but boiling should be avoided

because of the considerable amount of acid fumes involved. After pickling, the acid

is poured off and the casting is removed. The pickling solution should be renewed

frequently because it is likely to become contaminated with use.

Step 8: FINISHING AND POLISHING:

Finally the sprue is removed and the restoration may be stoned and polished on the

external surfaces except at the edges, in the laboratory. Edges are finished in the clinic

after cementing.

Finishing tools and polishers:

1. mandrels, abrasive disks.

2. Rubber cup polishers, bristle brushes.

3. Pumice

4. Wool mop

Casting procedure for chrome cobalt removable partial denture:

As usual impression of the jaw is made and the master model an dental stone is made.

This stone model is then duplicated to make a refractory cast of the casting

investment. Wax pattern is then made on this refractory cast.

-Wax sprue formers are are used and more than one are necessary because of the large

size of the pattern. Vents are made by attaching very thin sprues at the strategic areas

before the pattern is invested.

-The pattern is not removed from the model instead the whole model along with the

pattern and sprue formers is invested in a large ring or the casting flask.

The investment material is either silica bonded or phosphate bonded. This is

necessary for 2 reasons:

1. Investment must withstand the high temperature of melting chrome-cobalt

alloy that is above1250C.

2. investment must have sufficient expansion to compensate for the high casting

shrinkage of the metal.

Both of these investment give high thermal expansion of an average 1.5 to 2%.

Even then this value may be less considering the casting shrinkage of chrome cobalt

which is around 2.2 %. However other factors like shape of the casting, method of

spruing etc, also contribute to this and provide adequate compensation for the

shrinkage. Casting temperature of the investment to achieve this much thermal

expansion is between 800 to 1100C in any case above 1000C.

Chrome – cobalt alloy is melted using an oxy-acetylene gas flame or by an electric

source. As usual the centrifugal casting machine is used for the casting. The flask is

cooled slowly after casting and the casting is separated form the investment. The

surface of the appliance is smoothened by sand blasting and highly polished.

DEFECTIVE CASTINGS

Defects in castings can be classified under four I-leadings: (1) distortion; (2) surface

roughness and irregularities; (3) porosity; and (4) incomplete or missing detail. Some

of these factors have been discussed in connection with certain phases of the casting

techniques. The subject is summarized and analyzed in some detail in the following

sections.

Distortion: Any marked distortion of the casting is probably related to a distortion of

the wax pattern. This type of distortion can be minimized or prevented by proper

manipulation of the wax and handling of the pattern.

Unquestionably, some distortion of the wax pattern occurs as the investment hardens

around it. The setting and hygroscopic expansions of the investment may produce an

uneven movement of the walls of the pattern.

This type of distortion occurs in part from the uneven outward movement of the

proximal walls. The gingival margins are forced apart by the mold expansion,

whereas the solid occlusal bar of wax resists expansion during the early stages of

setting.

Surface Roughness, Irregularities, and Discoloration:

The surface of a dental casting should be an accurate reproduction of the surface of

the wax pattern from which it is made. Excessive roughness or irregularities on the

outer surface of the casting necessitate additional finishing and polishing whereas

irregularities on the cavity surface prevent a proper seating of an otherwise accurate

casting.

Causes of these surface defects:

Air Bubbles: Small nodules on a casting are caused by air bubbles that become

attached to the pattern during or subsequent to the investing procedure. Such nodules

can sometimes be removed if they are not in a critical area. However, for nodules on

margins or on internal surfaces, removal of these irregularities might alter the fit of

the casting.

Prevention:

-By vacuum investing

-using of mechanical mixer

Water Films: Wax is repellent to water, and if the investment becomes separated

from the wax pattern in some manner, a water film may form irregularly over the

surface. Occasionally, this type of surface irregularity appears as minute ridges or

veins on the surface.

Prevention:

-Use wetting agent on the pattern before investing.

Rapid Heating: It results in fins or spines on the casting or characteristic surface

roughness may be evident because of flaking of investment when the water or steam

pours into the mold. Furthermore, such a surge of steam or water may carry some of

the salts used as modifiers into the mold, which are left as deposits on the walls after

the water evaporates

Prevention:

As previously mentioned, the mold should be heated gradually; at least 60 minutes

should elapse during the heating of the investment-filled ring from room temperature

to 700º C. The greater the bulk of the investment, the more slowly it should be heated.

Underheating: Incomplete elimination of wax residues may occur if the heating -

time is too short or if insufficient air is available in the furnace. These factors are

particularly important with the low-temperature investment techniques. Voids or

porosity may occur in the casting from the gases formed when the hot alloy comes in

contact with the carbonaceous residues. Occasionally, the casting may be covered

with a tenacious carbon coating that is virtually impossible to remove by pickling.

Liquid:Powder Ratio: The amount of water and investment should be measured

accurately. The higher the L: P ratio, the rougher the casting. However, if too little

water is used, the investment may be unmanageably thick and cannot be properly

applied to the pattern. In vacuum investing, the air may not be sufficiently removed.

In either instance, a rough surface on the casting may result.

Prolonged Heating: When the high-heat casting technique is used, a prolonged -

heating of the mold at the casting temperature is likely to cause a disintegration of the

investment, and the walls of the mold are roughened as a result. Furthermore, the

products of decomposition are Sulfur compounds that may contaminate the alloy to

the extent that the Surface texture is affected. Such contamination may be the reason

that the surface of the casting sometimes does not respond to pickling. When the

thermal expansion technique is employed, the mold should be heated to the casting

temperature-never higher than 700º C – and the casting should be made immediately.

Temperature of the Alloy: If an alloy is heated to too high a temperature before

casting, the surface of the investment is likely to be attacked, and a surface roughness

of the type described in the previous section may result. As previously noted, in all

probability the alloy will not be overheated with a gas-air torch when used with the

gas supplied in most localities. If other fuel is used, special care should be observed

that the color emitted by the molten gold alloy, for example, is no lighter than a light

orange.

Casting Pressure: Too high a pressure during casting can produce a rough surface on

the casting. A gauge pressure of 0.10 to 0.14 MPa in an air pressure casting machine

or three to four turns of the spring in average type of centrifugal casting machine is

sufficient for small castings.

Composition of the Investment: The ratio of the binder to the quartz influences -the

surface texture of the casting. In addition, a coarse silica causes a surface roughness.

If the investment meets ADA Specification No. 2, the composition is probably not a

factor in the surface roughness.

Impact of Molten Alloy: The direction of the sprue former should be such that the

molten gold alloy does not strike a weak portion of the mold surface. Occasionally,

the molten alloy may fracture or abrade the mold surface on impact, regardless of its

bulk. Such a depression in the mold is reflected as a raised area on the casting, often

too slight to be noticed yet sufficiently large to prevent the seating of the casting.

Prevention:

This type of surface roughness or irregularity can be avoided by proper spruing so as

to prevent the direct impact of the molten metal at an angle of 90 degrees to the

investment surface.

Porosity:

Porosity may occur both within the interior region of a casting and on external

surface. The latter is a factor in surface roughness, but also it is generally a

manifestation of internal porosity. Not only does the internal porosity weaken the

casting but if it also extends to the surface, it may be a cause for discoloration. If

severe, it can produce leakage at the tooth-restoration interface, and secondary caries

may result. Although the porosity in a casting cannot be prevented entirely, it can be

minimized by use of proper techniques.

Porosities are classified as :

-Those caused by solidification shrinkage

-Localized shrinkage porosity

-Micro porosity

Those caused by gas

Pinhole porosity

-Gas inclusions

-Sub surface porosity

Those caused by air trapped in the mold(back pressure porosity)

Shrink spot or localized shrinkage porosity:

These are large irregular voids usually found near the sprue casting junction. It

occurs when the cooling sequence is incorrect and the sprue freezes before the rest of

the casting. During the correct cooling sequence the sprue should freeze last. This

allows more molten metal to flow into the mold to compensate for the shrinkage of

the casting as it solidifies. If the sprue solidifies before the rest of the casting no more

molten metal can be supplied from the button. The subsequent shrinkage produces

voids or pits known as shrink spot porosity.

Avoid by:

Using sprue of correct thickness

Attach sprue to the thickest portion of the pattern.

Flaring the sprue at the point of attachment or placing a reservoir close to the pattern.

Suck back porosity:

It is the variation of the shrink spot porosity. This an external void seen in the inside

of the crown opposite to the sprue. A hot spot is created by the hot metal impinging

on the mold wall near the sprue.

The hot spot causes this region to freeze last. Since the sprue has already solidified

no more molten material is available and the resulting shrinkage causes a peculiar

type of shrinkage called suck back porosity. It is avoided by reducing the temperature

difference between the mold and the molten alloy.

Microporosity: these are fine irregular voids within the casting. It is seen when the

casting freezes too rapidly. Rapid solidification occurs when the mold or casting

temperature is too low.

Pin hole porosity:

The voids are spherical and small in size. Gases like oxygen and hydrogen are

dissolved in the liquid metal. Then during solidification these gases will be expelled,

and cause pinpoint holes known as pin hiole porosity.

Gas inclusion porosity:

The voids are spherical but large in size. This is due to gas mechanically trapped by

the molten metal in the mold or carried in during the casting procedure.

Subsurface porosity:

This occurs just beneath the surface. This may be due to simultaneous nucleation of

the solid grains and gas bubbles at the first moment that the metal freezes at the mold

walls.

Prevention:

By controlling the rate at which the liquid metal enters the mold.

Back pressure porosity:

This is seen as surface irregularity on the fitting surface of the casting. But may also

be seen on the outside surface. Is due to inability of air in the mold to escape out due

to non-porous investment. Air in the mold must be eliminated first and then only the

liquid metal is made to enter. It is because no two things occupy the same space at one

and the same time.

This is also due to very low casting or mold temperature leading to solidification

before the entrapped air can escape.

Prevention:

Proper burnout.

Adequate proper mold and casting temperature.

Adequate casting pressure.

High w/p ratioMaking sure that the thickness of the investment between the tip of the

pattern and the end of the ring is not more than ¼”.

Incomplete or missing detail: Causes:

a. Due to inhibition of the entry of the liquid metal into the mold. This is in turn is

due to insufficient venting or due to high viscosity of the liquid metal.

Prevention:

There must be sufficient casting pressure and that pressure must be maintained at

least for few after casting. The metal must be heated to its correct fusion temperature

so that it is less viscous and flows readily into the mold. Since it takes less than a

second for the liquid metal to solidify, the casting must be done immediately done

when the metal is fused.

b. Due to incomplete elimination of the wax.

Prevention:

Proper time and temperature adapted during burnout.

Too large size casting is due to excessive mold expansion and this is prevented by

the use of correct type of investment and correct temperature.

Too small casting is due to, too little mold expansion and it is prevented by heating

the mold sufficiently.

REFERENCES:

1. Kennth J Anusavice, Philips science of dental materials 11th edition W B

Saunders publication 2003

2. Rossenstiel, Land, Fujimoto : Contemporary Fixed prosthodontics 3rd edition

Missouri Mosby 2001

3. Shilingburg, Herdert : Fundamentals of fixed prosthodontics ;3rd edition

Chicago