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1. Investment
Introduction
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 and
2. 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.
1

2. 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
2

3. 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.
3

4. 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.
4

5. 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
5

6. 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.
6

7. 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
7

8. 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.
8

9. 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).
9

10. 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.
10

11. 1. To heat the alloy no quickly as possible to a completely molten conditions.
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.
11

12. 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.
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
12

13. 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
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.
13

14. Oxygen air acetylene can also be used for cobalt-chromium based alloys
with high melting point.
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.
14

15. 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.
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.
15

16. 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.
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.
16

17. 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.
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.
17

18. 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
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.
18

19. 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. α-hemihydrate of gypsum and
2. A form of silica.
α-hemihydrate gives greater strength to the material and acts as a binder
to hold the other ingredients together and provide rigidity. Although depends
on amount of binder – may contain 25% - 45% and is used for alloy with
melting ranges below 1000°C (i.e., gold-containing).
When heated to the required temperatures it shrinks considerably and
frequently fractures all form shrink considerably after dehydration between
19

20. 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. α-
hemihydrate requires less mixing water and shrinks less.
Silica
Added to provide a refractory during the heating of the investment and
to regulate the thermal expansion.
It exists in an allotrophic form.
1. Quartz
2. Tridymite
3. Cristobalite and
4. Fused quartz.
When heated a change in crystalline form occurs at a transition
temperatures, characteristics of the particular form of silica.
- When heated quartz invasion from a ‘low’ form α-quit to high form to
quartz at 570°C.
- Cristobalite undergoes – between 200°C-270°C from α –β cristobalite.
- Tridymite – 117°C to 163°C.
- α-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 α -changes to β -form resulting increase in volume.
20

21. 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%.
21

22. 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.
22

23. 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.
23

24. Effect of Composition
Proportional to the silica content of the investment the fine the particle
size of silica the greater hygroscopic expansion α-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
24

25. pattern at the water bath temperature permitting an increase in effective setting
expansion, softened conditions of wax reduces its confining effect on the
expansion of the setting expansion.
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.
25

26. The term hygroscopic is a misnomer, although the added water may be
drawn into the setting material by capillary action, the effect is not related to
hygroscopy.
On the basis of theory the hygroscopic expansion is a normal
phenomenon as that which occurs during normal set expansions the terms have
gained general acceptance by usage.
Thermal Expansion
The thermal expansions of a gypsum bonded investment is directly
related to the amount of silica present and to the type of silica employed, the
contraction of the gypsum is entirely balanced when the quartz content is
immersed to 75%. The thermal expansion curves of the quartz is influenced by
particle size of the quartz, the type of the gypsum binder and the resultant water
powder ratio necessary to provide a workable mix.
Much greater expansion occurs during the inversion of cristobalite, the
normal contraction of the gypsum during heating is easily eliminated. The
expansions occurs at a lower temperature because of the lower inversion
temperature.
Investments containing cristobalite expand earlier and to a greater extent
than those containing quartz.
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

27. 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

28. 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

29. 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

30. 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

31. 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

32.  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

33. 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

34. 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

35. 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

36. 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

37. 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

38. 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

39. 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

40. 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

41. 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

42. 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

43. 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

44. 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

45. 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

46. 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

47. 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

48. 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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49. 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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50. 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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51. 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
51

52. 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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53. 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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54. 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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