InvestmentIntroduction

The casting procedure by the lost wax technique which is now a

common practice, was not popular until 1907. When W.H. Taggart introduced

his technique and casting machine to the profession.

Modern dental practice involves a variety of casting operations, ranging

from the simplest inlay to all forms of cast crown, bridge structure and

removable partial dentures. Each of which makes use of the same fundamental

practices in forming the cast restorations.

Once the investment was set for an appropriate period approximately

one hour, for most gypsum and phosphate bonded investments it is ready for

burnout. The procedure for the two types of investments are similar. The

crucible former and any metal sprue former are carefully removed. Any debris

from the in gate area (funneled opening at the end of the ring) is cleaned with a

camel hair brush. If the invested ring is placed in a humidor at 100% humidity,

if at all possible, the investment should not be permitted to dry out.

Rehydration of set investment that has been stored or an extended period may

not replenish all of the lost water.

Definition

An investment can be described as a ceramic material which is suitable

for forming a mold into which a metal or alloy is appropriately cast. The

procedure for forming the mold is described as “investing” (wax pattern).

Depending on the melting range of the alloy and the preference of the

clinician, generally two types of investment:

1. Gypsum – bonded and2. Phosphate – bonded investment are employed.

- The gypsum based materials represent the type traditionally used for

conventional gold alloys.

- The phosphate based invest are designed purely for alloys used in the metal

ceramic restoration.

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Wax Elimination and Heating

The invested rings are placed in a room temperature furnace and heated

to the prescribed maximum temperature. For gypsum bonded investments the

temperature is either 468°C for hygroscopic technique or 650°C for the thermal

expansion technique.

With phosphate bonded investments, the maximum temperature setting

may range from 700°C to 870°C depending on the type of alloy selected.

The temperature setting is more critical with gypsum bonded

investments than with the phosphate type because the gypsum investments are

more prone to investment decomposition.

During burnout, some of the melted wax is absorbed by the investment

and residual carbon produced by ignition of the liquid wax becomes trapped in

the porous investments, it is also advisable to begin the burnout procedures

while the mold is still wet. Water trapped in the pores of the investment

reduces the absorption of the wax and as water vaporizes. It flushes wax from

the mold.

The formation of gases (CO2, H2O) depends however, on the presence of

a sufficient supply of oxygen, the relatively high temperature of the oven and

adequate time of heating of the ring.

If the amount of oxygen available to the wax in the mould cavity is not

sufficient, the temperature of the oven is not high enough or the wax pattern is

heated only for a short time, incomplete reaction between the wax and oxygen

may result (Craig).

One of the most satisfactory way of eliminating wax pattern is to place

the mold in the furnace with the hole down at first, so that the major portion of

the wax drains out and is eliminated as a liquid. The ring is then inverted with

sprue hole placed upward. In this way oxygen in the oven atmosphere can

circulate into the mold more readily.

Casting Procedure

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If more than one ring is placed in an oven a longer period of time is

required for wax elimination. The general rule is to add 5 minutes to the wax

elimination time for every ring placed in the oven at 500°C.

According to Skinner

This process is facilitated by placing the ring with the sprue hole down

over a slot in a ceramic tray in the burnout furnace, when the high heat

technique is used the mold temperature generates enough heat to convert

carbon to either carbon monoxide or dioxide, (cause for the discolouration of

alloy). These gases can then escape through the pores in the heated investment.

Hygroscopic (Low Heat) Technique

This technique obtains its compensation expansions from three sources.

1. 37°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 expansions.

This low heat technique offers the advantages of less mold degradation,

a cooler surface for smoother castings and the convenience of placing the mold

directly in the 500°C furnace, the last benefit makes it possible to keep one or

more furnaces at the burnout temperature, so that mold may be put in as they

are ready at various times.

This is particularly useful in large laboratories where molds are ready at

various times. Care must nevertheless be taken to allow sufficient burnout time

because the wax is more slowly oxidized at the low temperature.

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The mold should remain in the furnace at least 60 minutes, and they may

be held up to 5 hours longer with little damage.

Molds placed in the furnace at intervals lower the temperature of the

furnace. Extra time should be given to ensure complete wax elimination. Even

though the mold is held at this temperature for 60-90 minutes, sufficient

residual fine carbon may be retained to reduce the venting of the mold.

Because of this reduced venting back pressure porosity is a greater

hazard in the low heat technique. Since the investments generally employed

with low heat technique may be more dense.

Some times muffle furnaces may be so air tight that burnout takes place

in a reducing atmosphere, preventing complete oxidation of the wax residues.

Keeping the door open slightly permit air to enter and provides enough oxygen

for elimination of the wax. This is particularly important for the hygroscopic

expansion technique when a lower burnout temperature is used.

The standardized hygroscopic technique was developed for alloys with a

high gold content. There may be a need for slightly more expansion for the

newer noble alloys. This is obtained by making following changes.

1. Increasing the water bath temperature to 40°.

2. Using two layer of liner.

3. Increasing the burnout temperature to a range of 600°C to 650°C.

High Heat (Thermal Expansion) Technique

This approach depends almost entirely on high heat burnout to obtain

the required expansion while at the same time eliminating the wax pattern.

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Additional expansion results from the slow heating of gypsum

investments or 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.

Gypsum Investments

These are relatively fragile and require the use of a metal ring for

protection during heating. The molds are usually placed in a furnace of room

temperature and slowly heated to 650°C to 700°C in 60 minutes and held for

15 to 30 minutes at the upper temperature.

The rate of heating has some influence on the smoothness and some

instances overall dimensions. Initially, the rapid heating can generate steam

that can cause flacking or spalling of the mold walls.

Too many patterns in the same plane within the investment after cause

separation of a whole section of investment because the expanding wax creates

excessive pressure over a large area.

Too rapid a heating rate may also cause cracking of the investment. In

such a case the outside layer becomes heated before the weaker sections, the

outer layer starts to expand thermally, resulting in compressive stress in the

outside layer that counteracts tensile stresses in the middle regions of the mold

such a stress distribution causes the brittle investment to crack from the interior

outwardly in the form of radial cracks. These cracks produce a casting with fine

or spines.

This condition is especially likely to be present with a cristobalite

investment. Low inversion temperature of the cristobalite, rapid rate of

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expansion during the inversion makes it especially important to heat the

investment slowly.

The reduction of calcium sulfate by carbon takes place rapidly whenever

gypsum investments are heated above 700°C in the presence of carbon. Sulfur

dioxide as a product contaminates gold castings and makes them extremely

brittle. This emphasizes the need for complete elimination of wax and avoiding

burnout temperatures, above 700°C particularly if the investment contains

carbon.

After casting temperature has been reached, the casting should be made

immediately, maintaining high temperature for longer time may result in a

sulfur contamination of the casting also surface on the casting because of the

disintegration of the investment.

Some manufacturers who advocate much more rapid burnout procedure

suggest placing mold in a furnace at 15°C for 30 minutes and following with

very rapid heating to the final burnout temperature. A few are offering

investment that may be placed directly into a furnace at the final burnout

temperature held for 30 minutes and cast.

Because the design of the furnace, the proximity of the mold to the

heating element. The availability of air in the shuffle may affect size and

smoothness. It is advisable to examine these factors carefully before a casting

is made in this manner.

Phosphate Investments

Because the setting mechanism and reactions on heating are quite

different there are several differences to gypsum – bonded investments.

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Phosphate investment obtain their expansion from :

1. The expansion of the wax pattern – this is considerable because the

setting reaction raises the mold temperature considerably.

2. The setting expansion – this is usually higher than in gypsum – bonded

investments especially because special liquid are used to enhance such

expansion.

3. The thermal expansion is greater when taken to temperatures higher than

these used for gypsum – bonded investments.

Phosphate-Bonded

Phosphate investments are usually much harder and stronger than

gypsum investments. They are quite brittle and are subject to the same unequal

expansion of adjacent sections as phase changes occur during heating.

Phosphate Investments Require:

1. Higher burnout temperatures to ensure total elimination of wax.

2. The completion of chemical and physical changes.

3. Prevention of premature solidification of higher molding alloys, the usual

burnout temperatures range from 750 to 900°C. The heating rate is usually

slow to 315°C and is quite rapid thereafter reaching completion after a hold

at the upper temperature for 20 minutes.

For time saving, there are now some investments that can be subjected

to two stage heating more rapidly and placed directly in the furnace at the top

temperature, held for 20-30 minutes and then cast. To save more time, the use

of a ring and a liner is also eliminated the metal ring being replaced with a

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plastic ring that is tapered so that once the investment has set it can be washed

out of the ring, held for a specified time to complete setting, and then placed

directly into the hot furnace, obviously, the expansion on setting in different

than when a lined ring is used, so that changes in overall fit must be

considered. The required expansion may be adjusted by varying the liquid

concentration.

Time Allowable for Casting

The investment contracts thermally as it casts. When thermal expansion

or high heat technique is used, the investment uses heat after the ring is

removed from the furnace, and the mold contacts. Because of the liner and the

low thermal ductility of the investment, a short period can elapse before the

temperature of the mold is affected under average conditions of casting,

approximately 1 minute can pass without a noticeable loss in dimension.

In the low heat technique, the temperature gradient between the

investment mold and the root is not as great as with the high-heat technique.

Also the thermal expansion of the invest is not as important to the

shrinkage compensation. However the burnout temperature lies on a fairly

steep portion of the thermal expansion curve rather than on a plateau portion as

in high-heat technique. Therefore the alloy should also be cast soon after

removal of the ring from the oven.

Casting Mechanisms

Classified broadly into 2 types:

1. Centrifugal force type.

2. Air pressure to force the metal into the mold.

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Other alloys are melted in one of the three ways:

- The alloy is melted in a separate crucible by a torch flame and the

metal is cast into the mold by centrifugal force.

- The alloy is melted electrically by a resistance or induction furnace,

then cast into the mold centrifugally by motor or spring action.

- The alloy is melted as in the first two ways but it is cast by air

pressure, a vacuum or both.

Centrifugal Casting Machine

The casting machine is first wound from two to five turns (depending on

the particular machine and the speed of casting rotation desired).

The metal is melted by a torch flame in a glazed ceramic crucible

attached to the “broken arm” of the casting machine. The broken arm feature

accelerates the initial rotational speed of the crucible and casting ring, thus

increasing the linear speed of the liquid casting alloy as it moves into and

through the mold. Once the metal has reached the casting temperature and the

heated casting ring is in position, the machine is released and the spring

triggers the rotational motion.

As the metal fills the mold there is a hydrostatic pressure gradient from

the tip of the casting to the bottom surface is quite sharp and parabolic in form,

reaching at the button surface. (0.21-0.28MPa).

Because of this pressure gradient, there is a gradient in the heat transfer

rate, such that the greatest rate of heat transfer to the mold is at the high

pressures and of the gradient (i.e., the tip of the casting).

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Because this end 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.

A variety of centrifugal machine are available some designed to spin the

mold in plane to the table top, on which the machine is mounted and other to

rotate in a plane vertical to the table top.

Some are spring driven and others are operated by electric power.

- Attached to some machines is an electric heating unit to melt the

alloy before the mold is started spinning to force the metal into mold.

- Others have a simple refractors tray mounted on the machine in

which the alloy is placed to be melted by a blowtorch.

The advantage of the centrifugal machines and the simplicity of design

and operation with opportunity to cast both large and small castings on the

same machine.

When the air pressure type of machine to employ either compressed air

or some other gas, such as carbon dioxide or nitrogen can be used to force the

molten metal into the mold. The casting machine with an attached vacuum

system designed to assist the molten mold falling into the mold are available.

In some casting, addition of the vacuum may advantageous but in

general there is little evidence to indicate superiority in the quality of castings

produced by this addition.

One should keep certain objectives in mind at the time of making the

casting.

1. To heat the alloy no quickly as possible to a completely molten conditions.

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2. Prevent oxidation by heating the metal quickly with a well-adjusted torch or

other method and a small amount of flux on the metal surface.

3. Produce a sharp details by having adequate pressure applied to the hot-

melted metal to furnish it into the mold.

Electrical Resistance Heated Casting Machine

In this there is an automatic melting of the metal in a graphite crucible

within a furnace (rather than by use of a torch flame). This is an advantage for

alloys used for metal ceramic restorations which are alloyed with base metals

in true amount that had to oxidize on overheating.

Another advantages is that the:

3. Crucible in the furnace is located flush against the casting ring. Therefore

the metal button remain molten slightly longer, again ensuring that

solidification progresses completely from the tip of the casting to the button

surface.

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. Popular in the casting of jewellery

more commonly used for melting base metal alloys.

There is little practical differences in the properties or accuracy of

castings made with any of the three types of ceramic. The choice is a matter of

access and personal preference.

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Casting Crucibles

Generally, three types of casting crucibles are available : Clay, Carbon

and Quartz (Zircon alumina).

- Clay crucible are – many of crown bridge alloys such as high noble-

alloy.

- Carbon crucible – not only for high – noble crown bridge but also for

higher fusing gold based metal ceramic.

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

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

carbon contamination.

1. High palladium content.

2. Palladium silicon from metal ceramic copings.

3. Nickel-cobalt based.

Methods of Melting Alloys

The most common method of heating gold alloy for full cast metal

restorations have been the use of a gas-air blowtorch. A properly adjusted

blowtorch with develop a temperature that is adequate for melting distal gold

alloys, whose melting average is between 870°-1000°C.

Many descriptions of the proper flame for heating metals and alloys are

found in the literature one practical method of checking and interpreting the

flame condition is to apply the flame to a small piece of copper, placed on a

soldering block. The blowtorch is adjusted and is then directed upon the

copper. If the copper turns bright and clean as it is heated the flame and the

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blow torch manipulation are correct, if copper turns dark, dull red color

oxydation is occurring and the heating is ineffective.

Electric melting units of various designs are used in some laboratories to

melt the alloys. These units have the advantage that less skill may be required

by the operator to control such devices.

This is necessary for the use of blow torch however many of these

electric heating units have no limiting controls and as a result the operator is

required to exercise regarding the proper conditioning of the alloy.

Electric units are heated either by induction or by resistance heating

systems. These heated by induction melt alloy much faster than those by torch

melting, they can easily to overheated. An electronic monitor to induce the

proper temperature is very useful. Units employing resistance heating require a

longer time to complete the heating and casting operation when compared to

torch melting.

Melting Noble Metal Alloy

The alloy is best melted by placing it on the inner sidewall of the

crucible. In this position the operator can better observe the progress of the

melting opportunity for air gases in the flame to be reflected from the surface

of the metal rather than to be absorbed.

The fuel employed is a mixture of natural or artificial gas and air. - Gold

alloys cast restoration.

Oxygen air acetylene can also be used for cobalt-chromium based alloys

with high melting point.

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The temperature of the gas-air flame is operately influenced by the

nature of gas and the proportions of gas and air mixture. Care should be taken

to obtain a nonluminous brush flame, with the different combination zones

clearly differentiated.

If the air supply is excessive incomplete compaction and a lower

temperature results, roaring sounds accompanies this type of flame.

The parts of the flame:

- The first long cone eminating directly from the nozzle is the zone in

which air and gas are mixed before combustion. No heat is present in

this zone.

- The next cone, which is green and immediately surrounding the inner

cone and known as the combustion zone. Here the gas and air are

partially burned. This zone is oxidizing and should be kept away

from the molten metal during fusion.

- The next zone, deep blue, is the reducing zone it is the hottest part of

the flame and is just beyond the tip of the green combustion zone.

This area should be kept constantly on the metal during melting.

- The outer cone (oxidizing zone) is the area in which combustion

occurs with oxygen in the air. This portion of the flame be added to

melt the alloy, its temperature is lower than reducing zone also

oxidizes the metal.

The proper zone in contact with the metal can be readily detected by the

condition of the metal surface. With reducing zone – surface of gold alloys is

bright and mirrorlike.

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When oxidizing zone is in contact – there is a dull film of “dross”

developed over the surface.

The alloy first appears to be spongy, then small globules of fused metal

appears then the molten alloy soon assumes a spheroidal shape.

At the proper casting temperature, the molten alloy is slight orange and

tends to spin or follow the flame which moved slightly, at this point the metal

should be approximately 31°C to 66°C above its liquidous temperature. Casting

should be made immediately when the proper temperature is reached.

It is desirable to use the flux for gold alloys. The flux increases the

fluidity of the metal and the film of flux formed on the surface of the molten

alloy helps prevent oxidation.

Reducing fluxes containing charcoal are often used, small bits of carbon

may be carried into the mold. They are excellent for cleaning old metal.

A better flux is made from equal parts of fused borax powder ground

with boric acid powder. Boric acid aids in veterning borax on the surface. The

flux is added when the alloy is completely melted.

Cleaning the Casting

For gold crown and bridge alloys. After the casting has been completed,

the ring is removed and quenched in water as soon as the button exhibits a dull

red glow.

The disadvantages of quenching are the noble metal alloy is left in an

annealed condition for burnishing, polishing and finishing procedures. When

the water contacts the hot investments a violent reaction ensures. The

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

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Often the surface of the casting appears dark with oxide and tarnish such

a surface film can be removed by a process known as “pickling” which consists

of heating the discoloured casting in an acid. The best pickling solution for

gypsum-bonded investments is a 50% hydrochloric acid HCl acid aids in the

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

Disadvantage to the fumes from the air are likely to corrode laboratory metal

finishings and these fumes are health hazard. A solution of sulfuric acid is more

advantageous in this respect care should be taken not to over heat or margins of

the casting get distorted.

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 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

heating, the acid is poured off and the casting is removed, in no case should

casting be held with steel prongs so that both the casting and the tongs come

into contact with the pickling solutions as this may contaminates the casting.

The pickling solutions usually contains small amounts of copper dissolved

from previous casting when the steel prongs contact this electrolyte, a small

galvanic cell is created and copper is deposited on the casting at the point

where the prongs grip it. This copper deposition extends into the metal and is a

future success for discoloration in the area.

It is a common practice to heat the casting and then to drop it with the

pickling solution. The disadvantages is that a delicate margin may be melted in

flame or the casting may be distorted by the sudden thermal shock when placed

in the acid.

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Gold-based and palladium-based metal ceramic alloys and base metal

alloys are bench cooled to room temperature before the casting is removed

from the investment. Castings from these alloys are generally not pickled, and

when it is recommended for certain metal ceramic alloys it is only to

selectively remove specific surface oxides.

Neither the phosphate binder not the silica refractory is soluble in

hydrochloric acid or sulphuric acid. Hydrofloric acid dissolves the silica

refractory quite well without damage to a gold-palladium silver alloy but must

be used carefully with other alloys.

Base metal alloys require a light sandblasting usually with fine

aluminium oxide, chromium based partial dentures are usually sandblasted to

remove the investment. Acid should never be used for cleaning base metal

alloys.

Casting of Glass

A castable glass ceramics inlay or crown is prepared in a manner similar

to that of a metal casting, a wax pattern is made on a high strength stress die

and all sections of the pattern should be more than 1 mm thick with the

occlusal surfaces and marginal edges being 15 mm thick. Wax pattern is sprued

with 8 or 10 gauge sprues. The pattern is invested in a phosphate bonded

investment and allowed to set for 1 hour.

The invested pattern is placed in a room temperature oven and heated to

280°C and held at that temperature for 20 minutes, after which the temperature

is raised to 955°C for an additional 30 minutes.

A special centrifugal casting machine is used that has an electric furnace

and is motor driven. The glass is heated to 1360°C (2480) and then cast and

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spun for a sufficient time to allow the casting to cool. The casting is allowed a

set at room temperature for as minutes before diverting. At this point the

casting is transparent. After the sprue is cut off and the area finished it must be

cerammed (recrystalized) to produce a transparent crown. The restoration is

embedded in phosphate bonded investment prior to commonly for 6 hour at

1075°C. the cerammed restorations is one color and must be shaded with

ceramic stains to yield an esthetic restorations.

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A 3rd type is the ethyl silicate bonded invest used primarily in the casting

of R.P.D. with base metal alloys.

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

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. -hemihydrate of gypsum and

2. A form of silica.

-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

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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 contaminates the castings (with the sulfides of non habit allogest

elements such as silver and copper). Thus not to be heated above 700°C. -

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.

- -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.

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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.

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%.

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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).

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.

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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.

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.

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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

hygroscopic expansion with silica. 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.

Effect of Spatulation

- Mixing time is reduced hygroscopic expansion decreased.

- Older the investment 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

24

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

expansion of the setting expansion.

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.

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.

25

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.

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.

26

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.

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

27

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.

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,

28

in place of water colloidal silica liquid suspension freeze, should be assessed

before winter, freeze solid at 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 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

increasing the liquid : powder ratio also by decreasing the concentration of the

special liquid or by they may use a combination of these methods.

29

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.

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 a shrinkage. The soft gel is dried at a

temperature below 168°C. During the drying process, the gel losses alcohol and

30

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.

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.

The Sprue Former

The purpose of a sprue former or sprue pin is to provide a channel

through which molten alloy can reach the mold in an invested ring after the

wax has been eliminated.

With large restorations or prosthesis, such as removable partial denture

frame works and fixed partial dentures, the sprue former are made of wax. For

smaller casting metal pins can be used, plastic sprue forms are also available.

The diameter and length of the sprue former depends to a larger extent

on the 1. Type and size of the pattern, 2. The type of casting machine to be

used, 3. And the dimensions of the flask ring in which the casting is to be

made.

Prefabricated sprue formers are available in a wide range of gauges or

diameters sprue former gauge selection is often empirical, yet it is based on the

following five general principles (Skinner’s).

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 S.F. attached to a thin, delicate pattern could cause distortion.

31

However, if the sprue former diameter is too small, this area will

solidify before the casting itself and localized shrinkage porosity (“suck

back” porosity) may develop. Reservoir sprues are used to help

overcome this problem.

If possible, the sprue former should be attached to the portion of the

pattern with the largest cross-sectional areas, it is best for the molten

alloys to flow from a thick section to surrounding thin areas not the

reverse. This minimizes the risk for turbulence.

Porosity :

Also, the sprue former orientation should minimize the risk of metal

flow on to flat areas of the investment or small areas such as line angles.

The length of the sprue former should be long enough to be within 6 mm of

the trailing end and yet short enough so the molten alloy doesn’t solidify

before it fills the mold.

The type of sprue former selected influences the burnout technique used it

is advisable to use a two-stage burnout technique, whenever plastic sprue

former or pattern are involved, to ensure complete carbon elimination

because plastic sprues soften at temperature above the mounting point of

inlay wax.

Patterns may be sprued either directly or indirectly.

For direct spruing the sprue former provides a direct connection between

the pattern area and the sprue base or crucible former area.

32

In indirect spruing, a connector or reservoir bar is positioned between

the pattern and the crucible former – commonly used for multiple single units

and fixed partial dentures.

Also several single limits can be sprued with multiple direct sprue.

Reservoir should be added to a spruing network to prevent localized shrinkage

porosity.

When the molten alloy fills the heated casting ring, the pattern area

should solidify first and reservoir lost. Because of its large mass of alloy and

position in the heat centre of the ring, the reservoir remains molten to furnish

liquid alloy into the mold as it solidifies. Resulting solidification shrinkage

occurs in the reservoir bar and not in the restorations.

Sprue Former Attachment

The sprue former connection in the wax pattern is generally flared

(telescopic) for higher density gold alloy, but is often restricted for lower

density alloys.

Flaring act much in the same way as a reservoir, facilitating the entry of

the fluid alloy into the pattern area.

Sprue Former Position

sprue former attachment is often a matter of individual judgement, based

on the shape and form of the wax pattern.

Some prefer at the occlusal surface, others choose sites such as a

proximal wall or just below non functional cusp to minimize subsequent

grinding of occlusal anatomy and contact areas, as indicated earlier the ideal

area for the sprue former is the point of greatest bulk in the pattern to avoid

33

distorting this areas of wax during attachment, and to permit a smooth flow of

the alloy.

Sprue Former Direction

The sprue former should be directed away from thin or delicate parts of

the pattern, because the molten metal may abrade or fracture investment in this

area and result in a casting failure.

It should not be attached at a right angle to a broad flat surface, this will

lead to turbulence within the mold cavity and serve porosity in this region, if it

is sprued at a 45° angle to the proximal area a satisfactory casting can be

obtained.

Sprue Former Length

Length depends on the length of the casting ring, if sprue is short, the

resulting mould space may be far from the end of the casting ring that gases

can not be adequately vented to permit the molten alloy to fill the ring

completely (mould space), thus may result in the porosity.

For gypsum bonded should be within 6mm of the open end of the ring,

with the higher strong phosphate bonded investments position may be within 3-

4 mm of the top of the investment.

For reproducibility of casting accuracy, the pattern should be placed as

close to the center of the ring as possible.

34

Wax Pattern Removal

Sprue former should be attached with the pattern on the master die,

provided the pattern can be removed directly in line with its path of withdrawal

from the die.

Preparation of the Master Die

The most commonly used die materials are type-IV (dental stone, high

strength) and type-V (dental stone, high strength, high expansion).

Relatively, inexpensive, easy to use and generally compatible with all

impression materials. Type-IV stones have a setting expansion of 0.1% or less

whereas the harder type-V stones expand as 0.3% this greater expansion is

useful for compensation of the relatively large solidification shrinkage of base

metal alloys.

To increase the abrasion resistance several means including silver

plating, coating the surface with cyanoacrylate and adding a die hardner to the

gypsum. However each may also increase the die dimensions, thus reducing

accuracy.

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

35

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.

Die Stone – Investment Combination

In this technique (that has been developed) the die material and the

investing medium have a comparable composition. A commercial gypsum

bonded material, called divestments (whip mix corporation, Louisville KY) is

mixed with a colloidal silica liquid. The die is made from this mix and the wax

pattern constructed on it. Then the entire assembly (die and pattern) is inserted

in a mixture of divestment and water thereby eliminating the possibility of

distortion.

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.

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.

36

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 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.

37

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.

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

38

ring, with vacuum investing, the same equipment used to mix the investment is

employed to invest the pattern under vacuum.

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.

39

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.

Controlled Water – Added Technique

Another technique, in which the shrinkage compensation is controlled

by the addition of water during the setting of the investment.

Here the linear hygroscopic expansion increases directly with the

amount of water added until a maximal expansion is attained.

The compositions of investments in this technique ensure maximal

expansion during immersion in water. The amount of hygroscopic expansion

needed is then obtained by adding enough water to provide the desired

expansion. A soft, flexible rubber ring is employed instead of the usual

asbestos lined metal ring.

A specified amount of water is then added on the top of the investment

in the rubber ring and the investment is allowed to set, usually at room

temperature. This technique is rarely used, since the hygroscopic expansion

method described earlier provides adequate expansion in most cases.

40

Causes of Defective Casting

Introduction

In almost all instance, defects in casting can be avoided by strict

observance of procedures governed by certain fundamental rules and

principles. Seldom is a defect in a casting attributable to other factors than the

carelessness or ignorance of the operator.

Defect in casting can be classified under four headings:

1. Distortions

2. Surface roughness and irregularities

3. Porosity

4. Incomplete or missing detail

Distortion: Any marked distortion 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.

Some distortion of the wax pattern occurs as the investment hardens

around it, the setting and hygroscopic expansion of the investment may

produce uneven movement of the walls of the pattern. Eames W.B. O`Neal et

al (1978) established that die spacing was one of the most suitable methods to

compensate for casting variables and it ensured improved marginal adaptation

yet increasing retention by 25 percent.

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

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

41

expansion. Whereas the solid occlusal bar of wax resist expansion

during the early stages of setting.

The configuration of the pattern, the type of wax, thickness all influence

the distortion that occur. Distortion increase as the thickness of pattern

decreases, and the less the setting expansion of investment, the less is

distortion. There is probably not a great deal that can be done to control this

phenomena. However, Grajower R., Lewinstein (1985) found that shrinkage of

wax pattern on dies created marginal gap at shoulders and bevels which was

attributed to elastic stress in wax. Remodeling of pattern margins by heating

marginal wax with spatula was found to improve the adaptation of die. Surface

roughness, irregularities and discoloration.

Influence 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

necessitates additional finishing and polishing where as irregularities on the

cavity surface prevent a proper setting of an otherwise accurate casting.

Surface roughness is defined as relatively finely spaced surface

imperfections whose height width and direction establish the predominant

surface pattern. Surface irregularities refer to isolated imperfections such as

nodule, that do not characterize the total surface area.

The difference in the surface roughness of the casting and the wax

pattern from which it is made is probably related to the particle size of

investment and its ability to reproduce the wax pattern in microscopic detail.

42

Improper technique can lead to a marked increase in surface roughness

as well as to the formation of surface irregularities.

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

nodule can sometimes be removed if they are not in a critical area. The best

method to avoid air bubbles is to use the vacuum technique.

If manual method is used, various precautions can be observed. The use

of a mechanical mixture with vibration both before and after mixing should be

practiced routinely. A wetting agent may be useful in preventing the collection

of air bubbles on the surface of the pattern, but it is by no menas a certain

remedy. It is important that the wetting agent be applied in a thin layer. It is

best to air dry the wetting agent because any excess will dilute the investment ,

producing surface irregularities on the casting.

Water Films

Wax is repellant to water, and if the investment becomes separated from

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

surface. This type of surface irregularity appears as minute ridges or veins on

the surface.

If the pattern is moved slightly jarred or vibrated after investing or if the

painting procedure does not result in an intimate contact of the investment with

pattern, such a condition may result. A wetting agent is of aid in the prevention

of such irregularities. Too high a W : P ratio may also produce these

irregularities.

43

Too Rapid Heating

It results in fins or spicules on the casting. The mold should be heated

gradually ; at least 60 minutes should elapse during the heating from room

temperature to 700°C. The greater the bulk of the investment the more slowly

it should be heated.

Under Heating : Incomplete elimination of wax residues may occur if the

heating time is too short or if insufficient air is available in the furnace. It is

particularly important with the low-heat technique. 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 casting that is virtually impossible to remove by pickling.

Liquid Powder Ratio: 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 high heating casting technique is used, prolonged

heating is likely to cause disintegration of the investment and the walls of the

mold are roughned as a result. Further more the products of decomposition are

sulfer compounds that may contaminate the alloy to the extent that the surface

texture is affected. Such contamination sometimes doesn’t respond to pickling.

When thermal expansion technique is employed the mold should be heated to

the casting temperature, never higher than 700°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

44

surface roughness result. 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 produces a rough

surface on the casting. A gauge pressure of 0.10 to 0.14 Mpa [15 to 20 psi] in

an air pressure casting machine or three to four turns of the spring in an

average type of centrifugal casting machine is sufficient for small castings.

Composition of the Investment

The ratio of the binder to the quartz influence, 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.

Foreign Bodies : When foreign substances get into the mold, a surface

roughness may be produced. For example a rough crucible former with

investment clinging to it may roughen the investment on its removal so that bits

of investment carried into the mold with the molten alloy. Carelessness in the

removal of the sprue former may be a similar cause.

Any casting that shows sharp well defined deficiences indicates the

presence of some foreign particles in the mold such as pieces of investment or

bits of carbon from a flux. Bright appearing concavities may be the result of

flux being carried into the mold with the metal.

Impact of Molten Alloy

The molten alloy, should 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. Sometimes the abraded area is smooth so that it

can not be detected on the surface of the casting. Such a depression in the mold

45

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. This type can be avoided

by proper spruing so as to prevent impact at an angle of 90° to surface.

A glancing impact is likely to be less damaging and at the same time an

undesirable turbulence is avoided.

Pattern Position : If several pattern are invested in the same ring they should

not be too close together. Likewise too many patterns positioned in the same

place in the mold should be avoided, the extension of the wax is much greater

than that of the investment, causing breakdown or cracking of the investment if

the spacing between pattern is less than 3mm.

Carbon Inclusions : From a crucible, improperly adjusted torch or a carbon

containing investment can be absorbed by the alloy during casting.

- May lead to the formation of carbide or even a visible carbon inclusions.

Other Causes : There are certain surface discolorations and roughness that

may not be evident when the casting is completed but that may appear during

service various gold alloys, solders, bits of wire and mixture of different

casting alloys should never be melted together and reused. The resulting

mixture would not possess the proper physical properties form eutectic or

similar alloys with low corrosion resistance. Discoloration and corrosion may

also occur.

A source of discoloration often overlooked is the surface contamination

of a gold alloy restoration with mercury. Mercury penetrates rapidly into the

alloys and causes a marked loss in ductility and a greater susceptibility to

corrosion. Thus it is not a good practice to place a new amalgam restoration

46

adjacent to high noble alloy restoration, it also forms a galvanic circuit leading

to the breakdown of the anode i.e., amalgam.

Porosity

May occur within the interior region of a casting and on the external

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

manifestation of internal porosity.

Internal porosity weaken the casting and extends on the surface it may

because for discoloration. If severe it may produce leakage at the tooth

restoration interface and 54secondary caries may result. Although the porosity

in a casting cant not be prevented entirely, it can be minimized by use of proper

techniques.

Porosities in noble metal alloy castings may be classified as follows :

I. Solidification defects.

a. Localized shrinkage porosity.

b. Microporosity.

II. Trapped gases.

a. Pinhole porosity.

b. Gas inclusion porosity.

c. Subsurface porosity.

III. Residual air :

47

Localised shrinkage porosity is generally caused by incomplete feeding

of molten metal during solidification. The linear contraction of noble metal

alloys in changing from a liquid to a solid is at least 1.25%. therefore there

must be continual feeding of molten metal through the sprue to make up for the

shrinkage of feeding of molten metal through the sprue to make up for

shrinkage of metal volume during solidification. If the spure freezes in its cross

section before this feeding is completed to the casting proper, a localized

shrinkage void will occur in the last portion of the casting that solidifies.

The porosity in the pontic area is caused by the ability of the pontic to

retain heat because of its bulk and because of it is located in the heat centre of

the ring.

This problem can be solved by attaching one or more small gauge sprues

at the surface most distant from the main sprue attachment and extending the

sprue laterally within sprue of the edge of ring. These small (auxiliary) sprues

ensures that solidification begins within sprues and they act as cooling pins to

carry heat away from the pontic.

Localized shrinkage generally occur near sprue casting junction but it

may occur any where between dendrites where the last part of the casting that

solidified was in the low melting metal that remains as the dendrite branches

develop.

This type of void may also occur externally, usually in the interior of a

crown near the area of the sprue. If a hot spot has been created by the hot metal

impinging from the sprue channel on a point of the mold wall. This hot spot

causes the local regions to freeze last and result in what is called suckback

porosity.

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This often occurs at an occlusoaxial line angle or incisoaxial line angle

that is not well roudned. The entering metal impinge onto the mold surface at

this point and creates a higher localized mold temperature in this region that is

called a hot spot. A hot spot may retain a localized pool of molten metal after

other areas of the casting have solidified.

This in turn creates shrinkage void or suck back porosity. Suck back

porosity can be eliminated by flaring the point of the sprue attachment and by

reducing the mold melt temperature differential, that is lowering the casting

temperature by about 30°C.

Microporositiy also occur from solidification shrinkage but is generally

present in fine grain alloy castings when the solidification is too rapid for the

microvoids to segregate to the liquid pool. This premature solidification causes

the porosity in the form of small irregular voids.

Such phenomenon can occur from the rapid solidification of the mold or

casting temperature is too low. It is unfortunate that this type of defect is not

detectable unless the casting is sectioned. In many case it is generally not a

serious defect.

Both the pinhole and the gas inclusion porosities are related the

entrapment of gas during solidification. Both are characterized by a spherical

contour, but they are decidedly different in size. The gas inclusion porosities

are usually much larger than pinhole porosity. Many metals dissolve or occlude

gases while they are in molten state. For e.g. both copper and silver dissolve

oxygen in large amounts in the liquid state, molten platinum and palladium

have a strong affinity for H2 as well as oxygen.

On solidification the absorbed gases are expelled and the pinhole

porosity results.

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The larger void may also result from the same cause but it seems more

logical to assume that such voids may be caused by gas that is mechanically

trapped by the molten metal in the mold on that is incorporated during the

casting procedure.

All castings probably contain a certain amount of porosity. However, the

porosity should be kept to a minimum because it may adversely affect the

physical properties of the casting.

The porosity that extends to the surface is usually in the form of small

pinpoint holes, when the surface is polished other pinholes appear.

Larger spherical porosities can be caused by gas occluded from a poorly

adjusted torch flame, or the use of the mixing or oxidizing zones of the flame

rather than the reducing zone. These types of porosity can be minimized by

premelting the gold alloy on a charcoal or a graphite block if the alloy has been

used before and by correctly adjusting and positioning the torch flame during

melting.

Subsurface Porosity : Occurs due to simultaneous nucleation of solid grains

and gas bubbles at the first moment that the metal freezes at the mold walls.

This type of porosity can be eliminated by controlling the rate at which the

molten metal enter mold.

Entrapped Air Porosity

On the inner surface of the casting. Sometimes referred to as back

pressure porosity. Can produce large concave depressions.

This is caused by the inability of the air in the mold to escape through

the pores in the investment or by the pressure gradient that displaces the air

pocket toward the end of the investment via the molten sprue and button.

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The entrapment is frequently found in a pocket at the cavity surface of a

crown or mesio-occlusal distal casting. Occasionally it is found even on the

outside surface of the casting when the casting temperature or mold

temperature is so low that solidification occurs before the entrapped air can

escape.

The incidence of entrapped air can be increased by the dense modern

investments, an increase in mold density produced by vacuum investing and the

tendency for the mold to clog with residual carbon when the low heat technique

is used. Each of these factors tends to slow down the venting of gases from the

mold during casting.

Proper burnout an adequate mold and casting temperature, a sufficiently

high casting pressure and proper L:P ratio can help to eliminate this

phenomenon. Make sure that the thickness of investment between the tip of the

pattern and the end of the ring not be greater than 6mm.

Incomplete Casting

Occassionally a partially complete or perhaps no casting at all is found

because that the molten alloy has been prevented in some manner, from

completely filling the mold.

The two factors responsible are:

- Insufficient venting of the mold and

- High viscosity of the fused metal.

Insufficient venting is directly related to the back pressure exerted by the

air in the mold. If the air cannot be vented quickly, the molten alloy doesn’t fill

the mold before it solidifies. In such a case the magnitude of the pressure

should be suspected. The pressure should be applied at least 4 seconds. The

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mold is filled and solidified in 1 second or less yet it is quite soft during the

early stages.

These failures have castings with rounded incomplete margins. A

second common cause for an incomplete casting is incomplete elimination of

wax residues from the mold if too many products of combusion remains in the

mold the pores in the investment may become filled so that the air cant be

vented completely. If mixture or particles of air remain, the contact of the

molten alloy with these foreign substances produce an explosion that may

produce sufficient back pressure to prevent the mold from being filled.

The rounded margins are quite shiny in some cases because of the strong

reducing atmoshpere created by carbon monoxide left by the residual wax.

A lower L:P ratio of the investment has been associated with less

porosity. An increase in casting pressure during casting solves this problem.

Different alloy compositions probably exhibit varying viscosities in the

molten state, depending on composition and temperatuere. However, both the

surface tension and the viscosity of a molten alloy are decreased with an

increase in temperature. An incomplete casting resulting from too great a

viscosity of the casting metal can be attributed to insufficient heating.

Temperature of the alloy should be raised higher than its liquidus temperature

so that its viscosity and surface tension are lowered and its doesn’t solidify

prematurely as it enters the mold. Such premature solidification may amount

for the greater susceptibility of the white gold alloys to porosity because their

liquidus temperature are higher thus they are more difficult to melt with a gas

air flame.

To gain an understanding of dental materials we need a basic knowledge

of matter and its behaviour during handling.

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Assuming that the wax pattern is satisfactory, the procedure techniques

become a matter of enlarging the mold uniformly and sufficiently to

compensate for the casting shrinkage of the gold alloy. Theoretically, if the

shrinkage of the wax and the gold alloy are known, the mold can be expanded

an amount equal to such shrinkages and the problem is solved. There are

variables in the behaviour of the materials involved, especially the wax that

cannot be rigidly controlled.

Summary and Conclusion

The overall dimensional accuracy possible with current technique has

never been clearly defined.

Neither the allowable tolerance of accuracy in the fit of the casting nor

that obtainable during the procedure is known. (In the last analysis the

procedure is partially expired and a matter of routine procedure. The latter

should be rigidly followed.

There are however many steps in the procedure for which a considerable

number of facts are known and there are also certain variations in technique

described have produced equally satisfactory results. However, any technique

involves strict adherence to certain fundamental principles that are common to

all.

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Contents

Introduction

Classification

Gypsum-bonded investment

Phosphate-bonded investment

Ethyl silicate-bonded investment

Compensation for solidification shrinkage

The sprue former

Preparation of the master die

Casting ring liner

Investing procedure

Casting machines

Casting procedure

Causes of defective castings

Summary & Conclusion

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