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 mold.(GPT)
“Casting is the process by which a wax
pattern of a prepared tooth is fabricated
and converted to its metallic
Steps in casting
Preparing the wax pattern
Spruing the wax pattern
Attaching the sprue to crucible former
Investing the pattern in the casting ring
Burnout of the wax
Finishing and polishing
in the casting
Preparing the wax pattern
Prior to casting
margin of the wax pattern should be
pattern checked for smoothness, finish and
Sprue should be attached to the thickest
portion of the wax pattern
wax pattern can be removed from the die
surfactant should be applied on the wax to
obtain wetting of the investment
invested immediately to prevent distortion
SPRUING OF WAX PATTERN
Sprue is defined
as “the channel or
which plastic or
metal poured or
cast into gate or
reservoir and then
it must allow the molten wax
to escape from the mold
enable molten metal to flow
into the mold with minimal
metal within the sprue must
remain molten slightly longer
than the alloy that has filled
the mold this will act as a
reservoir to compensate the
Wax: preferred for most casting because they
melt at the same rate as the pattern and
allow for easy escape for molten wax
Plastic: resist distortion rigid,
may block the escape of wax,
hollow plastics are available
Metal: non rusting metal should be used,
removed before casting
should be larger than the thickest portion
of the wax pattern
2.6 mm can be used for most patterns
2.0mm for premolar partial veneer
narrow sprue are sufficient for casting to be
done on centrifugal machine
POSITIONING OF THE
Sprue should be attached to the point
of greatest bulk
45 angulation near the bulk of the
axial wall should have obtuse angle
This prevents air entrapment during
investing and suck back porosity after
attachment should be flared to
prevent turbulence during metal flow
Small auxiliary sprues or vent should
be placed to improve the casting
By allowing the gases to escape
The sprue is attached to a crucible
former, usually made of rubber,
which constitutes the base of the
casting ring during investing.
May be metal , plastic or rubber
The exact shape of the crucible
former depends on the type of
casting machine used.
With most modern machines, the
crucible former is tall to allow use of
a short sprue and allow the pattern
to be positioned near the end of the
also referred to as a sprue former
representation of a dental
A, Crucible former.
C, Cavity formed by wax
pattern after burnout. D,
F, Casting ring.
thickness of approximately 6
mm between the end of the
mold cavity and the end of
the invested ring to provide
pathways for sufficient gas
escape during casting.
Casting rings are used to confine the
fluid investment around the wax
pattern while the investment sets.
Considerations in selection of casting
They are available as:
1) Shapes - Round
2) Complete rings –
Rigid - Metal
Flexible - Rubber
3) Split rings - Metal
Ring less casting system:
- plastic rings which is conical in shape with
tapering walls are used.
- Used for traditional gold-base alloys.
Flexible rings Split casting rings
Casting ring liner
Materials used are:
- Asbestos liner
- Cellulose liner
- Ceramic liner
- Combination of ceramic
and cellulose liner
Function of casting ring
Allow uniform expantion.
In case of wet liner
Thickness of the liner
should be less than 1mm.
RINGLESS CASTING TECHNIQUE
With the use of higher-
bonded investments, the
ringless technique has
become quite popular.
The method uses a paper or
plastic casting ring and is
designed to allow
The process of covering or
enveloping an object such as a
denture, tooth, wax form,
crown, with a suitable
investment material before
processing, or casting
Investment materials are:
Gypsum bonded investment
Ethyl silicate bonded
-Thin film of cleaner on pattern
reduces surface tension of wax
better “wetting” of wax pattern
by the investment.
-Some of the commercially available
debubblizing agents can be used.
•The wax pattern should
not stand for more than
before being invested.
So, it is best to invest
the wax pattern as soon
1. Hand mixing
2. Vacuum mixing
After mixing the
investment is poured in to
the casting ring up to its
Wax elimination or burn out
It is advisable to begin the burnout procedure while
the mould is still wet.
Water trapped in the pores of the investment reduces
the absorption of wax.
As the water vaporizes it flushes wax from the mold.
Purpose of burn out: to flush out wax pattern and
create a hollow cavity
Heat application: Heating of ring should be done
Hygroscopic low-heat technique
High-heat thermal expansion technique
Hygroscopic low-heat technique
The temperature used in this technique is 500°c for 60-90
Obtain compensation expansion from three sources:
Immersion of investment in 37°c water bath.
The warm water entering the investment mold from the top
adds some of the expansion.
The thermal expansion at 500°c
This technique causes 0.55% of expansion.
HIGH-HEAT THERMAL EXPANSION
The investment is slowly heated to 650°c - 700°c in
60mins. Then maintained for 15-30 mins at this
Above 700°c sulfur dioxide
- Contaminates gold castings and makes them
ACCELERATED CASTING METHOD
To reduce the total time, Alternative Accelerated
casting technique is proposed
Uses phosphate bonded investment which sets in 15
mins and then 15 min burn out is done at 815°c.
This method is used for preparing post and core
Gas air torch: melt conventional
noble metal alloys (used for
inlays, crown and bridge) whose
melting points less than 1000°c
Gas oxygen torch: Used to melt
metal ceramic alloys of higher
temperature up to 1200°c.
Oxy acetylene torch: One
volume of acetylene and two
and half volume of oxygen are
Hydrogen oxygen generator
Two type of torch tips:
Zones of the blow torch flame:
Zone 1 - colorless zone /Non combustion zone
Zone 2 – Combustion zone
Zone 3 – Reducing zone
Zone 4 - oxidizing zone
CHANGES SEEN IN METAL DURING
Initially appear spongy
later small globules of fused metal appear
molten metal flows assuming a spherical shape
At proper casting temperature the molten alloy is light orange and
tend to spin or follow.
At this stage the temperature of molten alloy is 38°c above its liquidus
During melting of the gold alloys flux may be added
To increase fusing of metal
Commonly used fluxes are fused borax powder ground with boric acid
power or Charcoal www.facebook.com/notesdental
1. Air pressure casting
Alloy is melted in situ in
crucible hollow of the
ring, followed by applied
2. Centrifugal casting
Alloy is melted in a
crucible, and forced in to
mold by centrifugal
ELECTRICAL RESISTANCE - HEATED
It is used to melt ceramic alloys.
Here the alloy is automatically
melted in graphite crucible.
Direct-current arc melting machine
Produce between two electrodes:
the alloys and the water cooled
> 4000°C – alloy melts very quickly.
High risk of over heating of the
Metal is melted by an
induction field that
developed with in the
crucible surrounded by
water- cooled metal
It is more commonly used
for melting base metal
not been used for noble
alloy casting as much as
Force required to overcome the surface tension of alloy +
Resistance offered by gas in the mold.
This can be done by use of following different type of force
Air or Gas Pressure
Sufficient mass of alloy must be present to sustain
adequate casting pressure
6g is typically adequate for premolar and anterior casting
10g is adequate for molar casting
12 g is adequate for pontic
They are of 3 types:
• Clay Crucibles
• Carbon Crucibles
• Quartz Crucibles
Cleaning of the casting
Consider the gold crown & bridge alloys.
After casting has been completed, ring
is removed & quenched in water.
1. Noble metal is left in an annealed
condition for burnishing & Polishing.
2. When water contacts hot investment,
violent reaction ensues. Investment
becomes soft, granular & casting is
more easily cleaned.
Surface of the casting appears dark
with oxides and tarnish. Such a surface
film can be removed by a process called
Best method for pickling is to place a
casting in a dish & pour acid over it.
Heat the acid but don't boil it.
Gold and palladium based metal ceramic
alloys and base metals, these alloys are
not generally pickled.
An unsuccessful casting results in
considerable trouble and loss of time.
With present techniques, casting
failures should be the exception, not
It can be classified under four headings
surface roughness and irregularities;
Incomplete or missing detail
It is usually due to distortion of wax
minimized or prevented by proper
manipulation of the wax and handling of the
Some distortion of the wax pattern
occurs as the investment hardens
Setting and hygroscopic expansions of the
Produce a non-uniform expansion of the
walls of the patternwww.facebook.com/notesdental
SURFACE ROUGHNESS, IRREGULARITIES,
Accurate reproduction of the surface of the wax pattern
from which it is made.
Surface roughness and irregularity should not confused,
both are different terms
These depends on size of investment particles.
Various other causes
composition of the investment
impact of molten alloy on the mold wall
Porosity or void fraction is a measure
of the void (i.e., "empty") spaces in a
material, and is a fraction of the volume
of voids over the total volume.
It can be either internal or external
What it does to the casting
Localized shrinkage porosity
Caused by premature termination of the
flow of molten metal during solidification.
Generally occurs near the sprue-casting
junction, but it may occur anywhere
It also occurs due to solidification shrinkage.
generally present in fine-grain alloy castings.
solidification is too rapid for the microvoids to
to the liquid pool.
causes the formation of small, irregular voids.
mold or casting temperature is too low.
Not detectable unless the casting is sectioned
and is not a serious defect
localized shrinkage porosity,
interior of a crown near the area of the sprue
Occlusoaxial line angle or incisoaxial line angle
that is not well rounded.
entering metal impinges onto the mold surface at
this point – hot spot
This spot may retain a localized pool of molten
metal after other areas of the casting have
Creates a shrinkage void or suck-back porosity.
Pinhole and gas inclusion
Entrapment of gas during
gas inclusion porosities are usually
much larger than pinhole porosity,
Many metals dissolve or occlude
gases while they are molten
Copper and silver: oxygen
Platinum and palladium: hydrogen as
well as oxygen.
simultaneous nucleation of solid
grains and gas bubbles.
diminished by controlling the rate at
which the molten metal enters the
Entrapped air bubbles on the inner
surface of the casting.
large concave depressions.
caused by the inability of the air in
the mold to escape through the pores
in the investment
Occasionally only a partially complete
casting or perhaps no casting at all is
The obvious cause is that the molten
alloy has been prevented in some
manner from completely filling the mold.
insufficient venting of the mold and high
viscosity of the fused metal.
incomplete elimination of wax residues from
Magnitude of the casting
pressure should be
applied for at least
lower L/P ratio is
associated with less
porosity of the
elimination of wax
Pores in the investment
may become filled