BASIC STEPS OF CASTING
SPRUE FORMER AND ITS ATTACHMENT
CASTING RINGS AND LINERS
CASITNG OF ALLOY INTO MOLD
MACHINES TO INDUCE CASTING
RECOVERY AND CLEANING OF
11th Century Theophilus Described
lost wax technique, which was a common
practice jewelry in 11th century
1558 - Benvenuto Cellini - claimed to have
attempted use of wax and clay for preparation
1884 - Agulihon de saran used 24K gold to
. 1897 - Phillibrook described a method
of casting metal filling
1907 -Taggart -devised a practically
useful casting machine.
Apart from this various studies
conducted on the properties of
investment materials and casting alloys
have lead a path for a better, practical
and useful processing methods.
CASTING:- Is defined as something that
has been cast in a mold, an object
formed by the solidification of a fluid that
has been poured or injected into a
FOR FIXED PARTIAL
The basic steps involved in the casting
of fixed partial denture are
1. Tooth preparation.
2. Impression and die preparation.
3. Wax pattern preparation.
7. Casting of molten alloy.
8. Recovery of casting, cleaning.
9. Finishing and polishing
Objectives of casting
1) To heat the alloy as quickly as possible to a
completely molten condition.
2) To prevent oxidation by heating the metal
with a well adjusted torch.
3) To produce a casting with sharp details by
having adequate pressure to the well melted
metal to force into the mold
The process of attaching a sprue
former/sprue pin to the wax pattern is
called as spruing
To provide a channel through which
molten alloy can reach the mold in an
invested ring after the wax has been
3 Basic requirements of sprue:-
1) Must allow the molten wax to escape
from the mould.
2) Sprue must enable the molten metal
to flow into the mould with as little
turbulence as possible.
3) Metal must remain molten slightly
longer than the alloy that has filled the
Type of sprue
Sprues made of different materials
1. Wax - used for casting of small and
large casting, which use single stage
used for castings of alloys which use 2
burn out with Po4 bonded investment.
Their main disadvantage is its softening
temperature, which is higher than wax
pattern. And may block escape of wax.
They may be used for casting FPD‘s
because of their high rigidity, which
minimizes distortion. Plastic sprues may be
completely solid (or) hallow plastic help in
3. Metal sprues
should be a non-rust metal to avoid
contamination of wax.
Hallow metallic sprue increase contact surface area
and strengthen the attachment between the sprue
They are removed from the investment at the same
time as the crucible former.
Care should be taken to examine for any fractured
investment material after metal sprue removal
Large diameter sprue:this improves the flow
of molten metal into the mould. The
diameter of sprue should be equal to the
thickest portion the wax pattern.
less diameter sprue:causes localized
A narrow sprue may be useful in air
pressure casting procedure where the metal
is melted on crucible former and narrow
sprue prevent the premature metal flow into
C) Sprue length
Very short sprue : porosity in casting at
the junction of sprue and pattern.
Very long sprue : sprue solidifies first
leading to casting shrinkage and incomplete
D) Sprue shape
The sprue former should be straight to
reduce chances of creating turbulence in
molten metal entering the mold.
High turbulence of alloy cause porosity
E) Number of sprue:
Usually a single sprue is used for
When two thick sections of a pattern are
connected by thin part of wax, 2 separate
sprues should be attached to each thick
The double sprue design is more
effective than the single sprue design in
decreasing the internal porosity
(jpd vol 78 no 4 oct 1997)
Attachment of sprue former and
Patterns may be sprued either directly or
For direct spruing ;-
the sprue former provides a direct
connection between the pattern area
and sprue base.
In indirect spruing -
a connector or reservoir bar is
positioned between the pattern and
It is common to use indirect spruing for
multiple single units and fixed partial
dentures, although several single units
can be sprued with multiple direct sprue
Principles of spruing
a) Location of attachment
b) Angulation of sprue
c) Attachment morphology
a) Location of attachment
The ideal location for attachment of sprue
is the thickest portion of wax pattern
The sprue should not be located where it
can obliterate centric occulsal contacts and
centric cusp tips and margins
Full veneer crown - sprue is attached to
Maxillary buccal and mandibular lingual
Partial veneer crown - sprue is attached to
cusp that encompasses the preparation.
If attached to cusp tips near margins of wax
pattern, distortion and restriction of flow of
molten metal into mold occurs.
b) Angle of sprue attachment
The sprue should be attached to pattern
such that it makes 45⁰ to the walls of
mold, which decreases the turbulence of
If the sprue is placed perpendicular to
the mold wall, it induces high turbulence
in molten alloy, leading to creation of a
hot spot on mold wall.
This results in localized shrinkage
c) Attachment morphology
The attachment of sprue former to the
wax pattern should be such that the
transition is smooth and do not posses
pits / irregularities into which investment
Irregularities produces tags of
investment which is prone for fracture by
molten alloy leading to casting failure.
Usually it is flared for high density gold alloys
but restricted for low density alloys. Flaring
acts as reservoir and facilitates the entry of
molted alloy into the mold area.
Tuccillo and Nielsen - flaring minimize
investment debris and aspirated air; allow
smooth flow of molten metal.
Nielsen and Shalita - flaring cause the spread
of heat over an increased region.
Indirect spruing uses the same basic
principles of spruing
But the only difference lies in attachment
of 3 running horizontal bars. The whole
indirect sprue complex consist of 3 parts.
The need for indirect spruing
as the ambient air is colder than molten alloy
the button solidifies earlier than molten alloy. So,
it can no longer serve as a reservoir to prevent
shrink spot porosity.
The use of horizontal runner bar will act as
reservoirs of molten alloy, which equalizes the
flow to all parts of F.P.D and stabilizes the pattern
against distortion during investment.
Reservoir is a small amount of additional
wax which is added to the sprue former
near the junction of wax pattern
It prevents localized shrinkage porosity as
the alloy in this part solidifies last after the
solidification of metal in mold
It is used in direct spruing.
The horizontal running bar of indirect
spacing provides the same function;
they are used when the distance
between the crucible and pattern is high.
The reservoir is present in prefabricated
plastic sprues also.
Small auxiliary sprues/vents are applied
to thin wax pattern to improve the quality
of casting. Usually 18- gauges sprues
are used. It is indicated with extremely
thin/thick casting to produce nonporous
They help in escape of gases during casting
and ensure beginning of solidification in critical
areas by acting as a heat sink.
It is attached to the wax pattern directly
opposite to larger sprue former. It terminates
either in investment attached to reservoir. As its
termination is near the outer wall of investment,
it solidifies first and induce solidification in main
The sprue is attached to crucible former
which constitutes the base of casting
relation with casting ring during
It also helps by holding sprue in desired
Crucible formers are basically of 2 types---
a) Steep-sided core: used with metal when
casted using centrifugal casting force. The tall
crucible formers allow the use of short sprue.
b) Shallow cone: used to cast metal using
They are available as----
Rubber crucible former
Metallic Crucible former
Plastic crucible former
They form a conical depression in
investment, which guides flow of molten
It should be clean and petroleum is applied
to prevent formation of rough investment
Molten sticky wax is applied on the apex of
cone portion of crucible former, which
contains a hole (for passing the sprue
Then the end of sprue former is passed
into the hole and held in position till the
molten wax sets.
The attachment area should be smooth
and without irregularities to prevent
creation of investment tags which are
prone to fracture when alloy is forced
Casting rings are used to confine the
fluid investment around the wax pattern
while the investment sets.
It also allow the hardened investment to
be safely handled during burnout and
They are available as---
1) Shapes - Round
I) Complete rings - Rigid
- Metal (stainless steel)
- Flexible - Rubber
II) Split rings - Metal
Considerations in selection of castings rings: -
1) The internal diameter of casting ring should
be 5-10mm greater than the widest
measurement of the pattern and about 6 mm
2) For single crown/inlay - small rings as
used. Diameter - 32 mm
3) For large fixed partial denture – 63mm
round/oval shaped casting ring are used
Ringless casting system
Plastic ring with rubber crucible formers are
used. The ring is conical in shape with tapering
walls. As the investment sets the investment is
tapped out of ring. Then burnout is done with out
casting ring, this causes greater expansion
Usually casting rings are rigid in nature.
Because of this the mold may become
smaller rather than larger due to the reverse
pressure resulting from confinement of the
setting expansion. To overcome this flexible
rings/ split rings are used.
But the most commonly used technique is to
provide expansion by lining the ring with a
Casting ring liner
They are commonly used to produce
expansion of mold. Various materials
used as ring liners ----
1. Asbestos liner
2. Cellulose (blotting paper) liner
3. Ceramic ring liner
4. Combination of ceramic and cellulose
5. Wax crinkled paper
Functions of Ring liner
1. Allow uniform setting expansion of
investment by decreasing the confinement of
rigid casting ring.
2. In case of wet liner technique ---The
absorbed water help in hygroscopic expansion.
The water in the setting investment and the
liner form a continuous phase.
The water in liner influences expansion of
mold at a distance of at least 25mm. Wetting
of the liner prevents absorption of water by
liner (if dry) from the setting investment.
3. Thickness of the liner should be < 1mm
4. The amount of expansion depends on the
number of liners used. The expansion seen with
2 liners is greater than one liner
1. Asbestos liner: Asbestos is refractory to
high temperature, they show a sufficient amount
of water absorption. There are 3 types of
White asbestos (least toxic) – this type is used
Blue asbestos (most toxic)
Brown asbestos (Intermediately toxic)
Asbestos is this no longer used in
dentistry. As produces 3 types of
2) Bronchogenic lung cancer
3) Mesothelioma – fatal tumour
The carcinogenicity is due to the
dimensions and durability of asbestos
which are longer than 4mm and
diameter less than 1.5mm & is known to
(JPD 1987; 57, 362-369)
2. Cellulose liner
This material shows adequate water
It is burnt during burnout procedure. So to
keep the investment in contact with ring after
The liner is kept 3mm short of ring ends.
This also restricts the longitudinal setting and
9.5 cm long cellulose liner is carefully adopted on
the walls of casting ring and is tucked in position
with sticky wax.
If wet liner technique is used, the lined ring is
immersed in water for some time. Then excess
water is shaken away.
squeezing of liner should be avoided.
The liner should end 3mm short of the casting ring
Ceramic ring liner
They are basically alumino-silicate fibrous
They do not absorb water to, but its network
of fibres can retain small amount of water on
its surface/wetting agents can be used to
increase the water sorption on surface.
They are refractory to high temperature. The
binders used in ceramic liner (Ex – neoprene-
latex) can contribute to toxicity (stimulate
fibrosis/ act as adsorbent surface for
They show potential for development of
Mesothelioma .They posses fibers of ---
Length 5.3-17.8 mm.
Diameter 0.2- 0.97 mm
Wax crinkled paper
They were used previously.
They are waxed to internal wall of
cylinder/held in position by paper clips
The wax pattern should be cleaned of
any debris, grease or oils.
For this we can use either:-
- A commercial wax pattern cleaner, or,
- A diluted synthetic detergent.
The pattern is left to air dry while the
investment is being prepared.
The thin film of cleanser left on the
pattern reduces the surface tension of
the wax & permits better ‗wetting‘ of the
investment to ensure complete
Wetting agent reduces the contact angle
of a liquid with wax surface.
Lower contact angle indicates that
treated wax surface has an affinity for
water, which results in investment being
able to spread more easily over wax.
Mixing of investment may be done
either by ----
i) Vacuum mixing
ii) Hand mixing
The incidence of bubble free casting
with different investing technique
– Open investing - 17%
– Vacuum investing - 95%
The incidence of nodules on casting is
more in hand mixing then vacuum
mixing. Application of surface tension
reducing agent decreased the nodules
(Johnston, IJP, 1992, 5; 424-433).
The best method is vacuum mix and
vacuum pour technique. But most
popular method vacuum mix and open
1. First, hand spatulate the mix
2. With the crucible former and pattern in
place attach the ring to the mixing bowl
3. Attach the vacuum hose and mix
according to the manufacturer‘s
4. Invert the bowl and fill the ring under
5. Remove the vacuum hose before setting
of the mixer
6. Remove the filled ring and crucible
former from the bowl
7. Immediately clean the bowl and mixing
blade under running water.
This technique is suited for Gypsum-bonded
investment because of their fluidity after
mixing. But with phosphate-bonded
investments because of greater viscosity it
can trap air in sharp corners of the pattern
II) Vacuum mixing and hand
investing (Brushing technique)
This technique is used in case of phosphate -
bonded investment to prevent air entrapment
and casting defects.
In this technique mixing is done on vac-u-
vester (vacuum mixing) then before filling the
ring, the wax pattern is painted with
investment material using brush.
Then pouring carefully around the
pattern fills the ring.
Brush technique: In this technique,
pattern is first painted with surface
tension reducer; the surface must be
A finger positioned under the crucible
former on the table of the vibrator
minimizes the risk of excessive vibration
and possible breaking of the pattern
from the sprue.
After the pattern has been completely
coated, the ring is immediately filled by
vibrating the remaining investment out of
When the investment reaches the level
of the pattern, tilt the ring several times
to cover and uncover the pattern,
thereby minimizing the possible
entrapment of air.
Investing must be performed quickly
within the working time of the
If the investment begins to set too soon,
rinse it off quickly with cold water.
After the ring is filled to the rim, allow the
investment to set.
If the hygroscopic technique is used, the
ring is placed in a 37⁰C (100⁰F) water
bath for 1 hour
Advantages of vacuum mixing-----
1) Remove air bubbles
2) Produce smooth castings
3) Increase tensile strength of investment
4) 95% of castings free of nodules.
5) Removes all the gaseous by products of
chemical reaction of investment material
Small ring: 1 package
Large ring: 2 packages
Hand mix for 15 seconds
Vacuum mix for 60 seconds
Working time: 2-3 minutes
• More investment liquid, less water =more
• Less investment liquid, more water =less
• Begin with a dry bowl Use a maximum of 27ml of
Using more liquid results in a weak
• For 100gms of investment:-Crowns/veneers:
5ml distilled water
• Inlays/Onlays: 16ml liquid, 11ml
• Follow instructions on investment
INVESTING OF GYPSUM BONDED
Require very specific W:P ratio‘s .
A variation of only 1ml of H2O can significantly alter
the setting expansion & the character of the casting
increasing W:P ratio makes investing process easier
but investment will lose strength,
cause cracks to occur during heating
surface of casting inferiors.
After the casting ring has been filled with investment
material, any excess should be removed before the
The filled ring is now set aside to allow
the investment material to complete its
setting reaction & the accompanying
Setting is complete in 30-40min.
Hygroscopic technique is used.
- Freshly filled investment ring is
immediately placed into water bath for
30min. & kept at 100ºF(38ºC).
INVESTING OF PHOSPHATE
Expansion of the mold cavity can be increased
1) increasing the no. of layers of asbestos or
fibrous ceramic lining the casting ring.
2) increasing the special liquid : water ratio.
3) increasing the total L:P ratio.
4) Placing the investment in contact with
water during setting.
5) Burning out the mold at a higher temp.
3mm on each end is left as it serves to
lock the investment within the ring &
equalize radial & axial expansion.
Residual, hardened investment in an unclean
mixing bowl will greatly accelerate the set of
newly mixed investment
Phosphate investment should not be mixed in
an apparatus that has been used for gypsum
investment. Residual gypsum will also
accelerate the set & will break down at temp.
above 2400ºF(1300ºC) liberating sulfurous
gases that can be detrimental to the casting
Ammonia gas is given off during mixing,
& it is important to hold the mixed
investment under the vacuum after
mixing ceases to dissipate some of this
gas & thereby reduce the incidence of
bubbles adhering to the wax pattern (
this additional holding time will vary from
Initial set of the phosphate bonded
investment is generally rapid with the
liberation of heat.
If burnout is not carried within 1-2hrs,
the ring should be stored in a humidor at
100% humidity, not soaked in water
since excessive hygroscopic expansion
Rapid steam release from a water
saturated ring can fracture the investment.
Carefully grinding or scraping the shiny
―skin‖ off the end of investment just prior to
burnout is advisable. This removes a
relatively impervious layer, opening the
pores of the investment & facilitating gas
release as the alloy is cast into the mold.
Once the investment has set for an appropriate
period 45min. it is ready for burnout.
A crucible former or any metal sprue former are
It is advisable to begin the burnout procedure
while the mold is still wet, because water trapped
in the pores of investment reduces the absorption
of wax & as water vaporizes, it flushes wax from
This burnout after 45min. determines with a
gradual increase in temp. with wax elimination
& phenomena of crystalline inversion that
accounts for volume increase on thermal
Temp. of investment must be increased in
successive stages & be well defined in terms
These time intervals bet. various successive
burnout temp. levels must be followed
Purpose of wax elimination. :-
1) To create a mold space – By wax elimination
2) To provide thermal expansion of investment.
3) To remove residual water in investment.
4) Heat soaking of investment - raises the
temperature of investment and eliminates the
temperature difference between the molten alloy
temp difference causes incomplete casting.
For expansion phenomena to take place
in the best possible conditions, it is
necessary that internal temp. of casting
ring gradually reach prescribed level.
The interval between successive temp.
level is in dispersible to permit the
external heat to reach the internal areas
of casting ring
Final burnout temp. of casting ring must
satisfy fundamental principles:-
1) Give a degree of expansion that is in
harmony with the shrinkage of alloy.
2) Maintain the viscosity of alloy at a level
necessary for complete filling of thinnest
area in mold.
3) Permit controlled cooling.
These investments are relatively fragile
& require the use of metal ring for
protection during heating.
So, the mould are usually placed in a
furnace at room temp. & slowly heated
to 650ºC-700ºC for 60min. & held for
15-30min. at the upper temp.
At 468⁰C for hygroscopic technique the
investment obtains its compensation
expansion from 3 sources:-
1) 37ºC water bath expands the wax pattern
2) Warm water entering the investment mould
from top adds some hygroscopic expansion.
3) Thermal expansion at this temp. provides the
1) Less mold degradation.
2) Cooler surface for smoother castings
3) Convenience of placing molds directly
Rate of heating has some influence on
the smoothness & in some instances on
Rapid heating can generate steam that
can cause flaking of the mould walls.
Too many patterns in the same plane
within the investment often cause
separation of a whole section of
investment because, expanding wax
creates excessive pressure over a large
Too rapid heating may also cause cracking
of the investment. In such case, outside
layer of the investment becomes heated
before the centre sections.
Outside layer starts to expand thermally,
resulting in compressive stress in the
outside layer that counteracts tensile
stresses in the middle regions of the mold.
Decomposition & alloy contamination is related to
a chemical reaction between residual carbon &
CaSO4 does not decompose unless it is heated
CaSO4 + 4C CaS + 4CO
3 CaSO4 + CaS 4CaO + 4SO2
This reaction takes place whenever gypsum
investments are heated above 700ºC in the
presence of carbon
reduction of CaSO4 by carbon takes place
rapidly above 700ºC.
Sulfur dioxide as a product of this reaction
contaminates gold castings & makes them
After casting temp. has reached, the casting
should be made immediately.
Maintaining a high temp. for a considerable
length of time may result in sulfur
contamination, rough surface
Methods for rapid burnout procedure are
- Placing the mold in a furnace at
315ºC for 30min. & then rapid heating.
- Directly place into a furnace at the final
burnout temp. held for 30min. & cast.
Burnout Procedure For
Phosphate bonded Investments
1) Higher burnout temp. for total elimination of
2) Completion of chemical & physical changes.
3) Prevention of premature solidification of
higher melting alloys. Usual burnout temp. range
Total expansion of 2% or more is
required for porcelain bonding alloys,
since gold & base metal alloys require
higher melting & solidification temp.
These investments- harder & stronger
Heating rate is usually slow to 315ºC &
is quite rapid thereafter, reaching
completion after a hold at upper temp.
- Quite brittle & are subject to the same
unequal expansion of adjacent sections
as phase changes occur during heating.
TIME ALLOWABLE FOR CASTING---
The investment contracts thermally as it cools.
When high heat technique is used, the
investment loses heat after the heated ring is
removed from the furnace & the mould
Because of the liner & low thermal conductivity
of the investment, a short period can elapse
before the temp. of the mould is appreciably
Under average condition approx. 1min. can
pass without a noticeable loss in dimensions.
In low- heat casting technique, temp. gradient
between the investment mould & the room is not
as great as that employed with high- heat
Casting of an alloy into the mold space
uses 2 basic requirements.
A) Heat source – to melt the alloy
B) Casting force – to force molten alloy into
mould casting force > surface tension of
+ resistance offered by gas in the mold
A) Heat Source: Different types of materials and
method are used as heat source to melt alloy. Two
basic modes are by using
Hydrogen oxygen generator
2) Electricity --
Electrical resistance melting
Electrical induction melting
Crucibles : The Melting of alloy requires
a crucible to act as a platform on which
the heat can be applied to the metal.
There are three types of casting
Clay crucibles are used with high noble
and noble metal alloys used for crown
and bridge alloy.
Quartz crucibles are recommended for
high-fusing alloys of any type of base
metal alloys and palladium alloys
Carbon crucibles –
for high noble crown and bridge and also
for higher fusing gold-based metal ceramic
Carbon crucibles should not be used in
melting of high palladium palladium
silver alloys (to be melted above 1504⁰c)
and also with nickel-chromium/cobalt
chromium base metal alloys
The crucibles used with noble metal
alloys should not be used for melting
base metal alloy
Copper –containing gold alloys and non-
copper gold alloys for use with porcelain
should not be melted in the same
Crucible should be discarded if it
contains large amount of oxides and
contaminants from previous metals
CLASSIFICATION OF DENTAL
1. ALLOY TYPES BY FUNCTIONS:
In 1927, the Bureau of Standard established
gold casting alloys, type I to type IV according
to dental function with hardness increasing from
type I to type IV.
Type I (Soft):
It is used for fabrication of small inlays,
class III and class V restorations which
are not subjected to great stress . These
alloys are easily burnishable
Type -II (Medium):
These are used for fabrication of inlays
subjected to moderate stress,
thick 3/4 crowns, abutments, pontics,
full crowns and soft saddles.
Type I and II are usually referred to as
Type -III (Hard):
It is used for fabrication of inlays subjected to
thin 3/4 crowns, thin cast backing abutments,
pontics, full crowns, denture bases and short
span FPDs .
Type III alloys can be age hardened.
Type-IV (Extra hard):
It is used for fabrication of inlays subjected to
high stress, denture bases, bars and clasps,
partial denture frameworks and long span
These alloys can be age hardened by heat
Type III and Type IV gold alloys are
generally called "Crown and Bridge
Alloys", although type IV alloy is used for
high stress applications such as RPD
Later, in 1960, metal ceramic alloys were
introduced and removable partial denture
alloys were added in this classification.
Metal ceramic alloys (hard and extra hard)---
It is suitable for veneering with dental
porcelain, copings, thin walled crowns, short
span FPDs and long span FPDs. These alloy
vary greatly in composition and may be gold,
palladium, nickel or cobalt based.
Removable partial denture alloys --
It is used for removable partial denture
frameworks. Now a days, light weight,
strong and less expensive nickel or cobalt
based have replaced type IV alloys
2. ALLOY TYPES BY DESCRIPTION:
By description, these alloys are classified into---
A)CROWN AND BRIDGE ALLOYS
This category of alloys include both noble and base
metal alloys that have been or potentially could be
used in the fabrication of full metal or partial
1. Noble metal alloys:
i) Gold based alloy - type III and type IV gold
alloys , low gold alloys
ii) Non-gold based alloy-Silver -palladium alloy
2. Base metal alloys:
i) Nickel-based alloys
ii)Cobalt based alloys
3. Other alloys:
i) Copper-zinc with Indium and nickel
ii)Silver-indium with palladium
B) METAL CERAMIC ALLOY
1.Noble metal alloys for porcelain
i) Gold-platinum -palladium alloy
ii) Gold-palladium-silver alloy
iii) Gold-palladium alloy
iv) Palladium silver alloy
v) High palladium alloy
2. Base metal alloys for porcelain
i) Nickel -chromium alloy
ii) Cobalt-chromium alloy
C) REMOVABLE PARTIAL DENTURE ALLOY
Although type-IV noble metal alloy may be used,
majority of removable partial framework are
made from base metal alloys---
1. Cobalt-chromium alloy
2. Nickel-chromium alloy
3. Cobalt-chromium-nickel alloy
4. Silver-palladium alloy
5. Aluminum -bronze alloy
3.ALLOY TYPE BY NOBILITY
High noble, noble, and predominantly base metal.
Alloy Classification of the American Dental Association
ALLOY TYPE TOTAL NOBLE METAL CONTENT
High noble metal > 60 wt% of the noble
Noble metal > 25 wt % of the noble metal
Predominantly base metal < 25 wt % of the noble metal
Amount of metal needed: -
Usually new gold alloys should be
used for castings in case of gold alloys and
other alloys if the remounts of castings are
They should be cleaned and at least
1/3rd of a new gold pellet by weight
must be used for each melting..
Sufficient mass of alloy must be present to
sustain adequate casting pressure---
High-density noble metal alloys.
For premolar and anterior castings- 6
For molor castings - 9 grams
For pontics - 12 grams
MELTING OF ALLOY
Different types of materials and method are used
as heat source to melt alloy. Two basic modes
are by using
Hydrogen oxygen generator
Expansion aids in enlarging the mold to
compensate for the casting shrinkage-
For gold alloy-
3 type of expansion may be seen
1)normal setting expansion
2)hygroscopic setting expansion
Hygroscopic low-heat technique
used with gypsum bonded investment
which are allowed to set under water.
They are used in casting gold alloys.
The temperature used in this technique
is 482⁰c for 60-90 mins
0.55% of expansion
High heat thermal expansion
. Gypsum bonded Investment : The
investment is slowly heated to 650⁰c -
700⁰c in 60mins. Then maintained for
For phosphate bonded:
This technique is used when investment
is allowed to set in open.
The temperature of 2nd stage in this
technique depends on type of
investment material used
After initial slow raise of temp to 315⁰c,
the temperature is rapidly raised to 750-
900⁰c and maintained for 30 mins.
The technique cause 1.33-1.58 % of
Normal setting expansion
Hygroscopic setting expansion
-if Hygroscopic setting
expansion is used then thermal expansion
will be 0.5-0.6%
If normal setting expansion then thermal
expansion should be 1-2%
For Phosphate bonded
1)wax pattern expansion;-the heat
during setting allows a significant
expansion of the wax pattern
Setting expansion-around 0.7 to 1%
Thermal expansion-around 1.33-1.58%
Casting shrinkage occurs in 3 stages
1)thermal contraction of the liquid metal
2)contraction of metal while changing
from liquid to solid state
3)thermal contraction of solid metal as it
cools to room temp.
Type 1- 1.56%
Type 4-(ni-cr based)-2.30%
Melting temp of pure gold –1063⁰c
Melting temp of gold alloy-924-960⁰c
Melting temp of base metal alloy-1155-
Melting of metal
Gas air torch: -Gas-air torch is used
to melt conventional noble metal alloys
(used for inlays, crown and bridge)
whose melting points less than 1000⁰c
Used to melt metal ceramic alloys of
higher temperature up to 1200⁰c
The tip of torch is available as single
the oxygen pressure is adjusted to
The flame is directed onto metal with
the nozzle of the torch about 1.5 cm away
from the metal.
Complete fluid should be obtained within
30 second at which point the
metal is poured into the
Oxy-acetylene torch :
The actual production of flame can be
done by adjusting the pressure and flow of
individual gases .
commonly advised pressure for acetylene
nozzle is 3.5 N/cm2 and oxygen nozzle 7-
The best results are obtained when
flame is used with a distance of 10cm
between the face of blow torch nozzle
and the base of crucible.
If distance is reduced to - 7.5 mm -slight
- 5 mm -increased porosity due to
occluded H2 gas
When the reducing zone is in contact,
the surface of the gold alloy is bright and
When the oxidizing portion of the flame
is in contact with alloy there is a dull film
of ―dross‖ developed over the surface
II Electrical source :-
A) Electrical resistance heated casting
It is used to melt ceramic alloys. Here
the alloy is automatically melted in
provides best means of temperature
control. It is quite convenient as
compared to blow torch.
B) Electrical arc melting:
is used to melt higher fusing alloys.
which is used to create a electrical arc at
the end of two electrodes
The apparatus requires a high electrical
A current is selected according to which
alloy is being melted
One electrode is attached to the
negative terminal and the other to the
positive terminal. Both electrodes are
placed about 5cm apart
A brilliant arc is produced around the
end of electrodes.
The arc is directed on to the alloy , the
end of electrodes being about 12mm
away from the alloy.
High fusing alloys show rounding off of
corners and signs of collapsing, at which
point they are thrust into the mould.
C) Induction casting :
- It is used to melt base metal alloys of
high melting temperature
. The centrifugal casting machine is
controlled by electricity.
Principle :- Induction casting is based on
electric currents in a metal core caused
by induction from a magnetic field.
When high density alternating current
(high frequency 1500 KHz) is passed
through the copper coil then it will
produce electric resistance
Because of electric resistance of coil
energy is transferred to thermal energy.
Magnetic field is produced by current
oscillating in that particular area.induces
an oscillating current in crucible.
Because crucible has some resistance
to the current,electric energy of current
is continuously transferd to thermal
alloy with the capability to have its
polarity changed, is placed inside the
the rapid change of polarity produced
causes the molecules of the alloy to
oscillate and their molecular bonds to
This high intensity of molecular activity
produces heat. The effect is that the
alloy becomes molten.
1) Water is circulated into the machine
under pressure of 20 psi. This travels
through the copper coil to keep it cool
during the melting process.
The metal is melted by an induction field
that develops within a crucible
surrounded by water cooled metal
In casting machines an electronic eye is
present above the crucible.
The starter switch is activated until the
metal becomes red in color.
In about 30 seconds the metal will begin
to sag and a circular shadow appears to
hour over the metal.
Eventually, the shadow will diminish in
size towards the center of crucible and
As the shadow disappears in the
direction the induction coil is lowered and
the casting machine is activated which
usually rotate at a speed of 600 RPM. This
causes the flow of molten alloy into the
the electricity supply to the machine
switched off and the water supply turned
If carbon crucible is used (for gold
alloys) the crucible itself becomes hot
and transfers heat to the alloy.
Device for forcing the molten alloy into
the mould under pressure after wax has
Alloy melted in crucible , followed by
application of air pressure (10-15 psi) to
force the material into the mould
Alloy is melted in separate crucible and the
metal is cast into the mould by centrifugal
Alloy melted electrically by resistance or
induction furnace & cast into the mould
by centrifugal force ( INDUCTION
Alloy melted electrically by resistance or
induction furnace, metal cast into mould
by air pressure or vaccum
positive force has to be applied..for molten
1) Vacuum force
2) Pneumatic (steam/Gas) Pressure
3) Centrifugal force
The casting of alloy is affected by 2
main factors :-
1) Amount of force
2) Time duration within which force is
The amount of force can be increased
by increasing the speed of rotation /
amount of pressure applied. The time
need to fill mold with pneumatic force is
greater than centrifugal force.
Casting by vacuum :-
Vacuum is applied to the external
surface of the investment mass, drawing
out the investment and mold gases,
allowing the melt to ingress.
It cannot work alone in filling the mold.
So, machines are used in combination
centifugal and gas pressure.
B) Gas pressure :-
The metal is melted in the investment
crucible. Then gas pressure is applied
on the molten metal.
different gases used are carbon
dioxide, carbon monoxide / nitrogen.
They apply a pressure of 10-15 Psi
by centrifugal force :-
This is the most feasible and commonly
used mode, for casting
. This machine utilize the centrifugal
force which is defined as a radical force
radiating outward from the center of
rotation of a body.
There are various types of machines which
use this principle and may be categorized
i) a) Spring operated
b) electrically operated
ii) a) Horizontally rotating
b) Vertically rotating
ADVANTAGE: Both small and large casting
on the same machine
Spring operated centrifugal
It consists two arms
one contains the casting assembly.
1) Cradle to seat the casting ring.
2) Bracket to place the crucible against
3) Head plate to prevent
displacement of casting
The other arm consists of appropriate
counter weight for proper rotation.
Loading pin prevents the rotation of arm.
Both arms pivot on this central spindle
Base consists of spring which rotates
As the metal fills the mould a hydrostatic
pressure gradient develops along the
length of casting
Ordinarily the pressure gradient at the
moment before solidification begins
reaches about 0.21 – 0.28 Mpa (30.40
Psi) at the tip of casting.
Because of this pressure gradient, there
is also a gradient in the heat transfer
rate such that the greatest rate of heat
transfer to the mold is at the high
pressure end of the gradient (i.e. tip of
the casting). Because this end also is
frequently the sharp edge of the margin
of a crown,
Recovery of a casting
quenched in water as soon as the button
exhibits a dull red glow.
Advantages of quenching:-
1) The noble metal alloy is left in an
annealed condition for burnishing,
polishing & similar procedures.
2) When water contact with hot
investment, a violent reaction ensues.
The investment becomes soft & granular
& the casting is more easily cleaned
A, Trimming is done from the button end
of the ring.
B, Investment is being pushed out of the
C, The mold is broken open.
D, Investment is removed from the
casting. Care must be taken to avoid
damaging the margin
The casting is held in a sandblasting
machine to clean the remaining
investment from its surface.
Surface of casting appears dark with
oxides & tarnish. Such surface film can
be removed by a process known as
Best method for Pickling is to place the
casting in a test tube or dish & pour the
acid over it.
May be necessary to heat the acid, but
boiling is avoided, because of
considerable amount of acid fumes
Pickling solution should be renewed
frequently, since it is likely to become
Palladium) can be soaked with
Nickel Chromium should never be
placed in acid because of high reactivity
The casting is trimmed , shaped and
smooth with a suitable burs or stones.
The sprue is sectioned off with a cutting
Minimum polishing is required if all the
procedures from the wax pattern to
casting are followed meticulously.
White stone ,rubber wheels, rubber
disks, and fine grit are included in the
finishing and polishing agents
Error in the procedure often results in
Dimensional inaccuracies &Distortion
Due to distortion of wax
Due to hygroscopic and setting
Surface roughness and
Surface roughness –
Cause :silica particle in investment
Inaccurate powder liquid ratio ,too rapid
Surface irregularities –nodules or fins
Cause :air bubble attached to pattern
,water film on pattern ,careless removal of
Prevention: correct powder liquid ratio,use
of mechanical mixer , use of wetting agent
High W/P ratio .
Prolonged heating of the mold cavity .
Overheating of the gold alloy .
Too high or too low casting pressure .
Composition of the investment .
Foreign body inclusion
May be internal or external .
External porosity causes discolouration .
Internal porosity weakens the restoration
Classification of porosity .
I .Those caused by solidification
a) Localised shrinkage porosity .
b) Suck back porosity .
c) Microporosity .
They are usually irregular in shape
II ) Those caused by gas :
a) Pin hole porosity .
b) Gas inclusions .
c) Subsurface porosity .
Usually they are spherical in shape
III ) Those caused by air trapped in the
Back pressure porosity .
Localised shrinkage porosity
Large irregular voids found near sprue
Suck back porosity
It is an external void seen in the inside
of a crown opposite the sprue .
Hot spot is created which freezes last .
It is avoided by :
Reducing the temp difference between
the mold & molten alloy
Fine irregular voids within the casting .
Occurs when casting freezes rapidly .
Also when mold or casting temp is too
Pin hole porosity :
Upon solidification the dissolved gases
are expelled from the metal causing tiny
Pt & Pd absorb Hydrogen .
Cu & Ag absorb oxygen
Gas inclusion porosities
Larger than pin hole porosities .
May be due to dissolved gases or due to
gases Carried in or trapped by molten
A poorly adjusted blow torech can also
Back pressure porosity
This is caused by inadequate venting of
the mold .
This can be prevented by :
- using adequate casting force .
-use investment of adequate porosity .
-place the pattern not more than 6-8 mm
away from tne end of the casting
This is due to :
- insufficient alloy .
-Alloy not able to enter thin parts of the
-When the mold is not heated to the
casting temp .
-Premature solidification of the alloy .
-sprues blocked with foreign bodies .
-Back pressure of gases .
-low casting pressure .
-Alloy not sufficiently molten
Small casting :
occurs when proper expansion is not
obtained & due to the shrinkage of the
Contamination of the casting
1) Due to overheating there is oxidation
of metal .
2) Use of oxidising zone of the flame .
3) Failure to use a flux .
4) Due to formation sulfur compounds .
It is due to :
1) Overheating of the investment .
2) Incomplete elimination of the wax
REVIEW OF LITERATURE
Thomas E.M (1952) conducted studies on
hygroscopic and setting expansion of
investment and find out that a confined
compensating expansion of at least 1.5 %
is necessary to compensate for the casting
shrinkage of the available inlay gold alloys.
Hygroscopic expansion when taking place
at 1000º F will compensate for the casting
shrinkage and shape of the wax pattern
has no influence on the amount of
Delgado et al (1953) studied hygroscopic
expansion of investment and
The use of mechanical spatulation or hand
spatulation does not affect the amount of
hygroscopic setting expansion, when a
water bath at mouth temperature is used.
Mechanical spatulation gives higher
expansion values for thick mixes than hand
spatulation when a water bath at room
temperature is used.
David (1963): investigated the influence of
factors on setting expansion and stated that:
Expansion of investment away from the wax
pattern is relatively small than the expansion
of the investment surrounded by the wax
Greased and dry asbestos liners tend to
decrease effective setting expansion whereas
loose and double asbestos tend to increase it.
Use of soft wax results in greater effective
setting expansion and Over spatulation and
thick mix increase setting expansion.
Robert Neiman and Atul Sarma in 1980
studied the setting and thermal reaction of
phosphate investment. They concluded that
The sequence of reaction based on
experimental findings were interpreted in
terms of chemical and structural presentation.
The simple chemical reaction has been shown
to be MgO + NH4 + H2PO4 =
NH4MgPO4.6H2O. However the setting
reaction is in reality a complex system of multi
molecular structure as described.
Alton M. lacy, Hisao Fukui et al in 1983 studied
factors affecting investment setting expansion
they studied the related effect of mixing rate,
ring liner position and storage. Their studies
-The rate and magnitude of of setting
expansion varied directly to the rate of mixing.
Although after 24 hrs the rapid mix investment
showed reversal of expansion. This was seen
with Gypsum bonded investment. No such
shrinkage was observed with phosphate
Lacy et al (1985) stated that machine
mixing under vaccum is more effective than
hand mixing in reducing the number of
bubbles from investment. They also stated
that increasing the mixing time had a little
effect on reducing the air bubbles but
decrease liquid powder ratio favors
reduction of incidence of air bubbles.
Debubblizer is effective in reducing air
bubble adhering to the surface
Papadopoulos T and Margrette in 1990
studied the heat rate in thermal expansion
of phosphate bonded investment. In their
study three heating rates were used one
too high (15c/min), too low (4c/min), and
one in middle (9c/min) from the results it
was concluded that rates that were too fast
must be avoided in heating procedure.
Optimum thermal expansion was found
when the heating rate was around 9c per
J.E Hutton and G.Marshel in 1995 studied
expansion of phosphate bonded
investment. The investment were mixed
with either distilled water or special liquid
and allowed a setting time of 1 or 24 hrs,
their study revealed that
1)1 hr is optimum time for achieving
complete expansion of investment material
2) Mixing the material with special liquid
increases the setting expansion
ASGAR. K - Further Investigation into the nature of
hygroscopic expansion of dental Casting Investment.
J. Prosthet. Dent 1958: 8;678.
DAVID B.M., BRUCEADY - Influence of factors on
setting expansion J.Prosthet.Dent 1963; 13:365.
DELGADO V.P. PYTOM F.A. - The hygroscopic
Setting Expansion of dental casting Investment. J.
Prosthet. Dent 1953; 3-423.
LACY. M. A and MORA. A. incidence of bubbles on
sample cast in phosphate bonded investment. J.
Prosthet. Dent 1985; 44, 367-369.
THOMAS E.M. - Resume of the
expansion required to compensation for
casting gold shrinkage. J. Prosthet. Dent
Stephen F. Rosenstiel. Contemporary
fixed prosthodontics III Ed. 1995.
Kenneth J. Anusavice. Phillips Science
of Dental materials. 11 th Ed. 2003.